Ichthyosaurs (Ancient Greek for "fish lizard" – and ) are large extinct marine reptiles. Ichthyosaurs belong to the order known as Ichthyosauria or Ichthyopterygia ('fish flippers' – a designation introduced by Sir

Richard Owen Sir Richard Owen (20 July 1804 – 18 December 1892) was an English biologist, comparative anatomist and paleontologist. Owen is generally considered to have been an outstanding naturalist with a remarkable gift for interpreting fossils. ...

in 1842, although the term is now used more for the parent clade of the Ichthyosauria). Ichthyosaurs thrived during much of the Mesozoic era; based on fossil evidence, they first appeared around 250 million years ago ( Ma) and at least one species survived until about 90 million years ago, into the Late Cretaceous. During the Early Triassic epoch, ichthyosaurs and other

ichthyosauromorphs The Ichthyosauromorpha are an extinct clade of marine reptiles consisting of the Ichthyosauriformes and the Hupehsuchia, living during the Mesozoic. The node clade Ichthyosauromorpha was first defined by Ryosuke Motani ''et al.'' in 2014 as t ...

evolved from a group of unidentified land reptiles that returned to the sea, in a development similar to how the mammalian land-dwelling ancestors of modern-day dolphins and whales returned to the sea millions of years later, which they gradually came to resemble in a case of convergent evolution. Ichthyosaurs were particularly abundant in the Late Triassic and

Early Jurassic The Early Jurassic Epoch (in chronostratigraphy corresponding to the Lower Jurassic Series) is the earliest of three epochs of the Jurassic Period. The Early Jurassic starts immediately after the Triassic-Jurassic extinction event, 201.3 Ma&nb ...

periods, until they were replaced as the top aquatic predators by another marine reptilian group, the

Plesiosauria The Plesiosauria (; Greek: πλησίος, ''plesios'', meaning "near to" and ''sauros'', meaning "lizard") or plesiosaurs are an order or clade of extinct Mesozoic marine reptiles, belonging to the Sauropterygia. Plesiosaurs first appeared ...

, in the later Jurassic and Early Cretaceous, though previous views of ichthyosaur decline during this period are probably overstated. Ichthyosaurs diversity declined due to environmental volatility caused by climatic upheavals in the early Late Cretaceous, becoming extinct around the Cenomanian-Turonian boundary approximately 90 million years ago. Science became aware of the existence of ichthyosaurs during the early nineteenth century, when the first complete skeletons were found in England. In 1834, the order Ichthyosauria was named. Later that century, many excellently preserved ichthyosaur fossils were discovered in Germany, including soft-tissue remains. Since the late twentieth century, there has been a revived interest in the group, leading to an increased number of named ichthyosaurs from all continents, with over fifty valid genera being now known. Ichthyosaur species varied from in length. Ichthyosaurs resembled both modern fish and dolphins. Their limbs had been fully transformed into flippers, which sometimes contained a very large number of digits and phalanges. At least some species possessed a dorsal fin. Their heads were pointed, and the jaws often were equipped with conical teeth to catch smaller prey. Some species had larger, bladed teeth to attack large animals. The eyes were very large, for deep diving. The neck was short, and later species had a rather stiff trunk. These also had a more vertical tail fin, used for a powerful propulsive stroke. The vertebral column, made of simplified disc-like vertebrae, continued into the lower lobe of the tail fin. Ichthyosaurs were air-breathing, warm-blooded, and bore live young. They may have had a layer of

blubber Blubber is a thick layer of vascularized adipose tissue under the skin of all cetaceans, pinnipeds, penguins, and sirenians. Description Lipid-rich, collagen fiber-laced blubber comprises the hypodermis and covers the whole body, except for ...

for insulation.

History of discoveries

Early finds

The first known illustrations of ichthyosaur bones, vertebrae, and limb elements were published by the Welshman

Edward Lhuyd Edward Lhuyd FRS (; occasionally written Llwyd in line with modern Welsh orthography, 1660 – 30 June 1709) was a Welsh naturalist, botanist, linguist, geographer and antiquary. He is also named in a Latinate form as Eduardus Luidius. Lif ...

in his ''Lithophylacii Brittannici Ichnographia'' of 1699. Lhuyd thought that they represented fish remains. In 1708, the Swiss naturalist

Johann Jakob Scheuchzer Johann Jakob Scheuchzer (2 August 1672 – 23 June 1733) was a Swiss scholar born at Zürich. Herbarium deluvianum Zürich, Zwingli-Platz ( Grossmünster) : Former home of Konrad von Mure († 1280) and the house, where Johann Jakob Scheuc ...

described two ichthyosaur vertebrae assuming they belonged to a man drowned in the Universal Deluge. In 1766, an ichthyosaur jaw with teeth was found at

Weston Weston may refer to: Places Australia * Weston, Australian Capital Territory, a suburb of Canberra * Weston, New South Wales * Weston Creek, a residential district of Canberra * Weston Park, Canberra, a park Canada * Weston, Nova Scotia ...

near

Bath Bath may refer to: * Bathing, immersion in a fluid ** Bathtub, a large open container for water, in which a person may wash their body ** Public bathing, a public place where people bathe * Thermae, ancient Roman public bathing facilities Plac ...

. In 1783, this piece was exhibited by the

Society for Promoting Natural History A society is a group of individuals involved in persistent social interaction, or a large social group sharing the same spatial or social territory, typically subject to the same political authority and dominant cultural expectations. Societ ...

as those of a crocodilian. In 1779, ichthyosaur bones were illustrated in

John Walcott John is a common English name and surname: * John (given name) * John (surname) John may also refer to: New Testament Works * Gospel of John, a title often shortened to John * First Epistle of John, often shortened to 1 John * Second E ...

's ''Descriptions and Figures of Petrifications''. Towards the end of the eighteenth century, British fossil collections quickly increased in size. Those of the naturalists

Ashton Lever Sir Ashton Lever FRS (5 March 1729 – 28 January 1788) was an English collector of natural objects, in particular the Leverian collection.John Hunter were acquired in their totality by museums; later, it was established that they contained dozens of ichthyosaur bones and teeth. The bones had typically been labelled as belonging to fish, dolphins, or crocodiles; the teeth had been seen as those of sea lions. The demand by collectors led to more intense commercial digging activities. In the early nineteenth century, this resulted in the discovery of more complete skeletons. In 1804,

Edward Donovan Edward Donovan (1768 – 1 February 1837) was an Anglo-Irish writer, natural history illustrator, and amateur zoologist. He did not travel, but collected, described and illustrated many species based on the collections of other naturalists. Hi ...

at St Donats uncovered a ichthyosaur specimen containing a jaw, vertebrae, ribs, and a shoulder girdle. It was considered to be a giant lizard. In October 1805, a newspaper article reported the find of two additional skeletons, one discovered at Weston by Jacob Wilkinson, the other, at the same village, by Reverend Peter Hawker. In 1807, the last specimen was described by the latter's cousin, Joseph Hawker. This specimen thus gained some fame among geologists as 'Hawker's Crocodile'. In 1810, near

Stratford-upon-Avon Stratford-upon-Avon (), commonly known as just Stratford, is a market town and civil parish in the Stratford-on-Avon district, in the county of Warwickshire, in the West Midlands region of England. It is situated on the River Avon, north-we ...

, an ichthyosaur jaw was found that was combined with plesiosaur bones to obtain a more complete specimen, indicating that the distinctive nature of ichthyosaurs was not yet understood, awaiting the discovery of far better fossils.

The first complete skeletons

In 1811, in

Lyme Regis Lyme Regis is a town in west Dorset, England, west of Dorchester and east of Exeter. Sometimes dubbed the "Pearl of Dorset", it lies by the English Channel at the Dorset– Devon border. It has noted fossils in cliffs and beaches on the He ...

, along what is now called the Jurassic Coast of Dorset, the first complete ichthyosaur skull was found by Joseph Anning, the brother of Mary Anning, who in 1812 while still a young girl, secured the torso of the same specimen. Their mother, Molly Anning, sold the combined piece to squire Henry Henley for £23. Henley lent the fossil to the London Museum of Natural History of William Bullock. When this museum was closed, the

British Museum The British Museum is a public museum dedicated to human history, art and culture located in the Bloomsbury area of London. Its permanent collection of eight million works is among the largest and most comprehensive in existence. It documen ...

bought the fossil for a price of £47.5s; it still belongs to the collection of the now independent Natural History Museum and has the inventory number BMNH R.1158. It has been identified as a specimen of ''

Temnodontosaurus ''Temnodontosaurus'' (Greek for "cutting-tooth lizard"temno, meaning "to cut", odont meaning "tooth" and sauros meaning "lizard") is an extinct genus of ichthyosaur from the Early Jurassic period. They lived between 200 and 175 million years a ...

platyodon''. Anning 1st ichthyosaur skeleton

In 1814, the Annings' specimen was described by Professor

Everard Home Sir Everard Home, 1st Baronet, FRS (6 May 1756, in Kingston upon Hull – 31 August 1832, in London) was a British surgeon. Home was born in Kingston-upon-Hull and educated at Westminster School. He gained a scholarship to Trinity College, C ...

, in the first scientific publication dedicated to an ichthyosaur. Intrigued by the strange animal, Home tried to locate additional specimens in existing collections. In 1816, he described ichthyosaur fossils owned by William Buckland and James Johnson. In 1818, Home published data obtained by corresponding with naturalists all over Britain. In 1819, he wrote two articles about specimens found by

Henry Thomas De la Beche Sir Henry Thomas De la Beche KCB, FRS (10 February 179613 April 1855) was an English geologist and palaeontologist, the first director of the Geological Survey of Great Britain, who helped pioneer early geological survey methods. He was the f ...

and Thomas James Birch. A last publication of 1820 was dedicated to a discovery by Birch at Lyme Regis. The series of articles by Home covered the entire anatomy of ichthyosaurs, but highlighted details only; a systematic description was still lacking. Conybeare Ichthyosaur 1824

Home felt very uncertain how the animal should be classified. Though most individual skeletal elements looked very reptilian, the anatomy as a whole resembled that of a fish, so he initially assigned the creature to the fishes, as seemed to be confirmed by the flat shape of the vertebrae. At the same time, he considered it a transitional form in-between fishes and crocodiles, not in an evolutionary sense, but as regarded its place in the ''

scala naturae The great chain of being is a hierarchical structure of all matter and life, thought by medieval Christianity to have been decreed by God. The chain begins with God and descends through angels, humans, animals and plants to minerals. The great c ...

'', the "Chain of Being" hierarchically connecting all living creatures. In 1818, Home noted some coincidental similarities between the coracoid of ichthyosaurians and the sternum of the platypus. This induced him to emphasize its status as a transitional form, combining, like the platypus, traits of several larger groups. In 1819, he considered it a form between newts, like

Olm The olm or proteus (''Proteus anguinus'') is an aquatic salamander in the family Proteidae, the only exclusively cave-dwelling chordate species found in Europe. In contrast to most amphibians, it is entirely aquatic, eating, sleeping, and br ...

, and lizards; he now gave a formal generic name: ''Proteo-Saurus''. However, in 1817, Karl Dietrich Eberhard Koenig had already referred to the animal as ''Ichthyosaurus'', "fish saurian" from Greek ἰχθύς, ''ichthys'', "fish". This name at the time was an invalid '' nomen nudum'' and was only published by Koenig in 1825, but was adopted by De la Beche in 1819 in a lecture where he named three ''Ichthyosaurus'' species. This text would only be published in 1822, just after De la Beche's friend William Conybeare published a description of these species, together with a fourth one. The type species was ''Ichthyosaurus communis'', based on a now lost skeleton. Conybeare considered that ''Ichthyosaurus'' had priority relative to ''Proteosaurus''. Although this is incorrect by present standards, the latter name became a "forgotten" '' nomen oblitum''. In 1821, De la Beche and Conybeare provided the first systematic description of ichthyosaurs, comparing them to another newly identified marine reptile group, the

. Much of this description reflected the insights of their friend, the anatomist Joseph Pentland. In 1835, the order Ichthyosauria was named by

Henri Marie Ducrotay de Blainville Henri Marie Ducrotay de Blainville (; 12 September 1777 – 1 May 1850) was a French zoologist and anatomist. Life Blainville was born at Arques, near Dieppe. As a young man he went to Paris to study art, but ultimately devoted himself to natu ...

. In 1840,

named an order

Ichthyopterygia Ichthyopterygia ("fish flippers") was a designation introduced by Sir Richard Owen in 1840 to designate the Jurassic ichthyosaurs that were known at the time, but the term is now used more often for both true Ichthyosauria and their more primitiv ...

as an alternative concept.

Popularisation during the 19th century

The discovery of a hitherto unsuspected extinct group of large marine reptiles generated much publicity, capturing the imagination of both scientists and the public at large. People were fascinated by the strange build of the animals, especially the large

scleral ring Sclerotic rings are rings of bone found in the eyes of many animals in several groups of vertebrates, except for mammals and crocodilians. They can be made up of single bones or multiple segments and take their name from the sclera. They are beli ...

s in the eye sockets, of which it was sometimes erroneously assumed these would have been visible on the living animal. Their bizarre form induced a feeling of alienation, allowing people to realise the immense span of time passed since the era in which the ichthyosaur swam the oceans. Not all were convinced that ichthyosaurs had gone extinct: Reverend George Young found a skeleton in 1819 at Whitby; in his 1821 description, he expressed the hope that living specimens could still be found. Geologist

Charles Lyell Sir Charles Lyell, 1st Baronet, (14 November 1797 – 22 February 1875) was a Scottish geologist who demonstrated the power of known natural causes in explaining the earth's history. He is best known as the author of ''Principles of Geolo ...

to the contrary, assumed that the Earth was eternal so that in the course of time the ichthyosaur might likely reappear, a possibility lampooned in a famous caricature by De la Beche. Temnodontosaurus

Public awareness was increased by the works of the eccentric collector Thomas Hawkins, a

pre-Adamite The pre-Adamite hypothesis or pre-Adamism is the theological belief that humans (or intelligent yet non-human creatures) existed before the biblical character Adam. Pre-Adamism is therefore distinct from the conventional Abrahamic belief that Ada ...

believing that ichthyosaurs were monstrous creations by the devil: ''Memoirs of Ichthyosauri and Plesiosauri'' of 1834 and ''The Book of the Great Sea-Dragons'' of 1840. The first work was illustrated by

mezzotint Mezzotint is a monochrome printmaking process of the '' intaglio'' family. It was the first printing process that yielded half-tones without using line- or dot-based techniques like hatching, cross-hatching or stipple. Mezzotint achieves tonalit ...

s by John Samuelson Templeton. These publications also contained scientific descriptions and represented the first textbooks of the subject. In the summer of 1834, Hawkins, after a taxation by William Buckland and Gideon Mantell, sold his extensive collection, then the largest of its kind in the world, to the British Museum. However, curator Koenig quickly discovered that the fossils had been heavily restored with plaster, applied by an Italian artist from

Lucca Lucca ( , ) is a city and ''comune'' in Tuscany, Central Italy, on the Serchio River, in a fertile plain near the Ligurian Sea. The city has a population of about 89,000, while its province has a population of 383,957. Lucca is known as one ...

; of the most attractive piece, an ''Ichthyosaurus'' specimen, almost the entire tail was fake. It turned out that Professor Buckland had been aware of this beforehand, and the museum was forced to reach a settlement with Hawkins, and gave the fake parts a lighter colour to differentiate them from the authentic skeletal elements. Crystal Palace Ichthyosaurus

Ichthyosaurs became even more popular in 1854 by the rebuilding at

Sydenham Hill Sydenham Hill forms part of a longer ridge and is an affluent locality in southeast London. It is also the name of a road which runs along the northeastern part of the ridge, demarcating the London Boroughs of Southwark, Bromley, and Lewisham ...

the Crystal Palace The Crystal Palace was a cast iron and plate glass structure, originally built in Hyde Park, London, to house the Great Exhibition of 1851. The exhibition took place from 1 May to 15 October 1851, and more than 14,000 exhibitors from around th ...

, originally erected at the world exhibition of 1851. In the surrounding park, life-sized, painted, concrete statues of extinct animals were placed, which were designed by

Benjamin Waterhouse Hawkins Benjamin Waterhouse Hawkins (8 February 1807 – 27 January 1894) was an English sculptor and natural history artist renowned for his work on the life-size models of dinosaurs in the Crystal Palace Park in south London. The models, accurately ...

under the direction of Richard Owen. Among them were three models of an ichthyosaur. Although it was known that ichthyosaurs had been animals of the open seas, they were shown basking on the shore, a convention followed by many nineteenth century illustrations with the aim, as Conybeare once explained, of better exposing their build. This led to the misunderstanding that they really had an amphibious lifestyle. The pools in the park were at the time subjected to tidal changes, so that fluctuations in the water level at intervals submerged the ichthyosaur statues, adding a certain realism. Remarkably, internal skeletal structures, such as the scleral rings and the many phalanges of the flippers, were shown at the outside.

Later 19th-century finds

During the nineteenth century, the number of described ichthyosaur genera gradually increased. New finds allowed for a better understanding of their anatomy. Owen had noted that many fossils showed a downward bend in the rear tail. At first, he explained this as a ''post mortem'' effect, a tendon pulling the tail end downwards after death. However, after an article on the subject by Philip Grey Egerton, Owen considered the possibility that the oblique section could have supported the lower lobe of a tail fin. This hypothesis was confirmed by new finds from Germany. In the

Posidonia Shale The Posidonia Shale (german: Posidonienschiefer, also called Schistes Bitumineux in Luxembourg) geologically known as the Sachrang Formation, is an Early Jurassic (Toarcian) geological formation of southwestern and northeast Germany, northern Swi ...

Holzmaden Holzmaden is a town in Baden-Württemberg, Germany that lies between Stuttgart and Ulm. Holzmaden is 4 km south-east from Kirchheim unter Teck and 19 km south-east of Esslingen am Neckar. The A 8 runs south from Holzmaden. The town a ...

, dating from the early Jurassic, already in the early nineteenth century, the first ichthyosaur skeletons had been found. During the latter half of the century, the rate of discovery quickly increased to a few hundred each year. Ultimately, over four thousand were uncovered, forming the bulk of ichthyosaur specimens displayed today. The sites were also a '' Konservat-Lagerstätte'', meaning not only the quantity, but also the quality was exceptional. The skeletons were very complete and often preserved soft tissues, including tail and dorsal fins. Additionally, female individuals were discovered with embryos.

20th century

In the early twentieth century, ichthyosaur research was dominated by the German paleontologist Friedrich von Huene, who wrote an extensive series of articles, taking advantage of an easy access to the many specimens found in his country. The amount of anatomical data was hereby vastly increased. Von Huene also travelled widely abroad, describing many fossils from locations outside of Europe. During the 20th century, North America became an important source of new fossils. In 1905, the Saurian Expedition led by

John Campbell Merriam John Campbell Merriam (October 20, 1869 – October 30, 1945) was an American paleontologist, educator, and conservationist. The first vertebrate paleontologist on the West Coast of the United States, he is best known for his taxonomy of ver ...

and financed by

Annie Montague Alexander Annie Montague Alexander (29 December 1867 - 10 September 1950) was an explorer, naturalist, paleontological collector, and philanthropist. She founded the University of California Museum of Paleontology (UCMP) and the Museum of Vertebrate Zoolo ...

, found twenty-five specimens in central Nevada, which were under a shallow ocean during the Triassic. Several of these are now in the collection of the University of California Museum of Paleontology. After a slack during the middle of the century, with no new genera being named between the 1930s and the 1970s, the rate of discoveries picked up towards its end. Other specimens are embedded in the rock and visible at Berlin–Ichthyosaur State Park in

Nye County Nye County is a county in the U.S. state of Nevada. As of the 2020 census, the population was 51,591. Its county seat is Tonopah. At , Nye is Nevada's largest county by area and the third-largest county in the contiguous United States, be ...

. In 1977 the Triassic ichthyosaur ''Shonisaurus'' became the

state fossil Most American states have made a state fossil designation, in many cases during the 1980s. It is common to designate one species in which fossilization has occurred, rather than a single specimen, or a category of fossils not limited to a single ...

of Nevada. About half of the ichthyosaur genera today seen as valid were described after 1990. In 1992 Canadian paleontologist

Elizabeth Nicholls Elizabeth (Betsy) Laura Nicholls (January 31, 1946 – October 18, 2004) was an American-Canadian paleontologist who specialized in Triassic marine reptiles. She was a paleontologist at the Royal Tyrrell Museum in Alberta, Canada. Early lif ...

uncovered the largest known specimen, a '' Shastasaurus''. The new finds have allowed a gradual improvement in knowledge about the anatomy and physiology of what had already been seen as rather advanced "Mesozoic dolphins". Christopher McGowan published a larger number of articles and also brought the group to the attention of the general public. The new method of

cladistics Cladistics (; ) is an approach to biological classification in which organisms are categorized in groups ("clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is typically shared derived cha ...

provided a means to exactly calculate the relationships between groups of animals, and in 1999, Ryosuke Motani published the first extensive study on ichthyosaur phylogenetics. In 2003, McGowan and Motani published the first modern textbook on the Ichthyosauria and their closest relatives.

Evolutionary history

Origin

The origin of the ichthyosaurs is contentious. Until recently, clear transitional forms with land-dwelling vertebrate groups had not yet been found, the earliest known species of the ichthyosaur lineage being already fully aquatic. In 2014, a small basal ichthyosauriform from the upper Lower Triassic was described that had been discovered in China with characteristics suggesting an amphibious lifestyle. In 1937, Friedrich von Huene even hypothesised that ichthyosaurs were not reptiles, but instead represented a lineage separately developed from amphibians. Today, this notion has been discarded and a consensus exists that ichthyosaurs are

amniote Amniotes are a clade of tetrapod vertebrates that comprises sauropsids (including all reptiles and birds, and extinct parareptiles and non-avian dinosaurs) and synapsids (including pelycosaurs and therapsids such as mammals). They are distin ...

tetrapod Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids (reptiles, including dinosaurs and therefore birds) and synapsids (pelycosaurs, extinct therapsid ...

s, having descended from terrestrial egg-laying amniotes during the late Permian or the earliest Triassic. However, establishing their position within the amniote evolutionary tree has proven difficult, due to their heavily

derived Derive may refer to: *Derive (computer algebra system), a commercial system made by Texas Instruments * ''Dérive'' (magazine), an Austrian science magazine on urbanism *Dérive, a psychogeographical concept See also * *Derivation (disambiguation ...

morphology obscuring their ancestry. Several conflicting hypotheses have been posited on the subject. In the second half of the 20th century, ichthyosaurs were usually assumed to be of the

Anapsida An anapsid is an amniote whose skull lacks one or more skull openings (fenestra, or fossae) near the temples. Traditionally, the Anapsida are the most primitive subclass of amniotes, the ancestral stock from which Synapsida and Diapsida evolved, ...

, seen as an early branch of "primitive" reptiles. This would explain the early appearance of ichthyosaurs in the fossil record, and also their lack of clear affinities with other reptile groups, as anapsids were supposed to be little specialised. This hypothesis has become unpopular for being inherently vague because Anapsida is an unnatural, paraphyletic group. Modern exact quantitative cladistic analyses consistently indicate that ichthyosaurs are members of the clade

Diapsida Diapsids ("two arches") are a clade of sauropsids, distinguished from more primitive eureptiles by the presence of two holes, known as temporal fenestrae, in each side of their skulls. The group first appeared about three hundred million years ag ...

. Some studies showed a basal, or low, position in the diapsid tree. More analyses result in their being

Neodiapsida Neodiapsida is a clade, or major branch, of the reptilian family tree, typically defined as including all diapsids apart from some early primitive types known as the araeoscelidians. Modern reptiles and birds belong to the neodiapsid subclade S ...

, a derived diapsid subgroup. Until the 1980s, a close relationship was assumed between the Ichthyosauria and the Sauropterygia, another marine reptile group, within an overarching

Euryapsida __NOTOC__ Euryapsida is a polyphyletic (unnatural, as the various members are not closely related) group of sauropsids that are distinguished by a single temporal fenestra, an opening behind the orbit, under which the post-orbital and squamosal bo ...

. Today, however, this is usually rejected, with the Euryapsida being seen as an unnatural polyphyletic assemblage of reptiles that happen to share some adaptations to a swimming lifestyle. An exception is a study in 1997 by John Merck, which showed monophyletic archosauromorph euryapsids.

Affinity with the Hupehsuchia

HupehsuchusNanchangensis-PaleozoologicalMuseumOfChina-May23-08

Since 1959, a second enigmatic group of ancient sea reptiles is known, the Hupehsuchia. Like the Ichthyopterygia, the Hupehsuchia have pointed snouts and show

polydactyly Polydactyly or polydactylism (), also known as hyperdactyly, is an anomaly in humans and animals resulting in supernumerary fingers and/or toes. Polydactyly is the opposite of oligodactyly (fewer fingers or toes). Signs and symptoms In hum ...

, the possession of more than five fingers or toes. Their limbs more resemble those of land animals, making them appear as a transitional form between these and ichthyosaurs. Initially, this possibility was largely neglected because the Hupehsuchia have a fundamentally different form of propulsion, with an extremely stiffened trunk. The similarities were explained as a case of convergent evolution. Furthermore, the descent of the Hupehsuchia is no less obscure, meaning a possible close relationship would hardly clarify the general evolutionary position of the ichthyosaurs. In 2014, ''

Cartorhynchus ''Cartorhynchus'' (meaning "shortened snout") is an extinct genus of early ichthyosauriform marine reptile that lived during the Early Triassic epoch, about 248 million years ago. The genus contains a single species, ''Cartorhynchus lenticarpu ...

'' was announced, a small species with a short snout, large flippers, and a stiff trunk. Its lifestyle might have been amphibious. Motani found it to be more basal than the Ichthyopterygia and named an encompassing clade

Ichthyosauriformes The Ichthyosauriformes are a group of marine reptiles, belonging to the Ichthyosauromorpha, that lived during the Mesozoic. The stem clade Ichthyosauriformes was in 2014 defined by Ryosuke Motani and colleagues as the group consisting of all ic ...

. The latter group was combined with the Hupesuchia into the

Ichthyosauromorpha The Ichthyosauromorpha are an extinct clade of marine reptiles consisting of the Ichthyosauriformes and the Hupehsuchia, living during the Mesozoic. The node clade Ichthyosauromorpha was first defined by Ryosuke Motani ''et al.'' in 2014 as t ...

. The ichthyosauromorphs were found to be diapsids. The proposed relationships are shown by this cladogram:

Early Ichthyopterygia

The earliest ichthyosaurs are known from the Early and Early-Middle (

Olenekian In the geologic timescale, the Olenekian is an age in the Early Triassic epoch; in chronostratigraphy, it is a stage in the Lower Triassic series. It spans the time between Ma and Ma (million years ago). The Olenekian is sometimes divided int ...

and

Anisian In the geologic timescale, the Anisian is the lower stage or earliest age of the Middle Triassic series or epoch and lasted from million years ago until million years ago. The Anisian Age succeeds the Olenekian Age (part of the Lower Triassic ...

) Triassic strata of Canada, China, Japan, and Spitsbergen in Norway, being up to 246 million years old. These first forms included the genera '' Chaohusaurus'', ''

Grippia ''Grippia'' is a genus of early ichthyopterygian, an extinct group of reptiles that resembled dolphins. Its only species is ''Grippia longirostris''. It was a relatively small ichthyopterygian, measuring long and weighing . Fossil remains from ...

'', and ''

Utatsusaurus ''Utatsusaurus hataii'' is the earliest-known ichthyopterygian which lived in the Early Triassic period (c. 245–250 million years ago). It was nearly long with a slender body. The first specimen was found in Utatsu-cho (now part of Minamisan ...

''. This diversity suggests an even earlier origin. They more resembled finned lizards than the fishes or dolphins to which the later, more familiar species were similar. Their bodies were elongated and they probably used an

anguilliform Fish locomotion is the various types of animal locomotion used by fish, principally by swimming. This is achieved in different groups of fish by a variety of mechanisms of propulsion, most often by wave-like lateral flexions of the fish's body a ...

locomotion, swimming by undulations of the entire trunk. Like land animals, their pectoral girdles and pelves were robustly built, and their vertebrae still possessed the usual interlocking processes to support the body against the force of gravity. However, they were already rather advanced in having limbs that had been completely transformed into flippers. They also were probably warm-blooded and viviparous. These very early "proto-ichthyosaurs" had such a distinctive build compared to "ichthyosaurs proper" that Motani excluded them from the Ichthyosauria and placed them in a basal position in a larger clade, the

. However, this solution was not adopted by all researchers.

Later Triassic forms

The basal forms quickly gave rise to ichthyosaurs in the narrow sense sometime around the boundary between the Early Triassic and

Middle Triassic In the geologic timescale, the Middle Triassic is the second of three epochs of the Triassic period or the middle of three series in which the Triassic system is divided in chronostratigraphy. The Middle Triassic spans the time between Ma an ...

; the earliest Ichthyosauria in the sense Motani gave to the concept, appear about 245 million years ago. These later diversified into a variety of forms, including the still

sea serpent A sea serpent or sea dragon is a type of dragon sea monster described in various mythologies, most notably Mesopotamian ( Tiamat), Judaeo-Christian (Leviathan), Greek (Cetus, Echidna, Hydra, Scylla), and Norse ( Jörmungandr). Mythology and f ...

-like '' Cymbospondylus'', a problematic form which reached ten metres in length, and smaller, more typical forms like ''

Mixosaurus ''Mixosaurus'' is an extinct genus of Middle Triassic (Anisian to Ladinian, about 250-240 Mya) ichthyosaur. Its fossils have been found near the Italy–Switzerland border and in South China. The genus was named in 1887 by George H. Baur. The ...

''. The

Mixosauria The Mixosauria were an early group of ichthyosaurs, living between 247.2 and 235 million years ago, during the Triassic period. Fossils of mixosaurs have been found all over the world: China, Timor, Indonesia, Italy, Germany, Spitsbergen, Switz ...

were already very fish-like with a pointed skull, a shorter trunk, a more vertical tail fin, a dorsal fin, and short flippers containing many phalanges. The sister group of the Mixosauria were the more advanced Merriamosauria. By the Late Triassic, merriamosaurs consisted of both the large, classic Shastasauria and more advanced, "dolphin-like" Euichthyosauria. Experts disagree over whether these represent an evolutionary continuum, with the less specialised shastosaurs a paraphyletic grade that was evolving into the more advanced forms, or whether the two were separate clades that evolved from a common ancestor earlier on. Euichthyosauria possessed more narrow front flippers, with a reduced number of fingers. Basal euichthyosaurs were '' Californosaurus'' and '' Toretocnemus''. A more derived branch were the

Parvipelvia Parvipelvia (Latin for "little pelvis" - ''parvus'' meaning "little" and ''pelvis'' meaning "pelvis") is an extinct clade of euichthyosaur ichthyosaurs that existed from the Late Triassic to the early Late Cretaceous (middle Norian to Cenomanian ...

, with a reduced pelvis, basal forms of which are '' Hudsonelpidia'' and '' Macgowania''. Shonisaurus BW 2

During the

Carnian The Carnian (less commonly, Karnian) is the lowermost stage of the Upper Triassic Series (or earliest age of the Late Triassic Epoch). It lasted from 237 to 227 million years ago (Ma). The Carnian is preceded by the Ladinian and is followed by ...

and

Norian The Norian is a division of the Triassic Period. It has the rank of an age (geochronology) or stage (chronostratigraphy). It lasted from ~227 to million years ago. It was preceded by the Carnian and succeeded by the Rhaetian. Stratigraphic ...

, Shastosauria reached huge sizes. ''

Shonisaurus ''Shonisaurus'' is a very large genus of ichthyosaur. At least 37 incomplete fossil specimens of the marine reptile have been found in the Luning Formation of Nevada, USA. This formation dates to the late Carnian age of the late Triassic period ...

popularis'', known from a number of specimens from the Carnian of Nevada, was long. Norian Shonisauridae are known from both sides of the Pacific. '' Himalayasaurus tibetensis'' and '' Tibetosaurus'' (probably a synonym) have been found in Tibet. These large (10- to 15-m-long) ichthyosaurs have by some been placed into the genus ''Shonisaurus''. The gigantic ''Shonisaurus sikanniensis'' (considered as a shastasaurus between 2011 and 2013) whose remains were found in the Pardonet Formation of British Columbia by

, has been estimated to be as much as in length—if correct, the largest marine reptile known to date. In the Late Triassic, ichthyosaurs attained the peak of their diversity. They occupied many ecological niches. Some were

apex predator An apex predator, also known as a top predator, is a predator at the top of a food chain, without natural predators of its own. Apex predators are usually defined in terms of trophic dynamics, meaning that they occupy the highest trophic lev ...

s; others were hunters of small prey. Several species perhaps specialised in

suction feeding Aquatic feeding mechanisms face a special difficulty as compared to feeding on land, because the density of water is about the same as that of the prey, so the prey tends to be pushed away when the mouth is closed. This problem was first identifi ...

or were ram feeders; also,

durophagous Durophagy is the eating behavior of animals that consume hard-shelled or exoskeleton bearing organisms, such as corals, shelled mollusks, or crabs. It is mostly used to describe fish, but is also used when describing reptiles, including fossil t ...

forms are known. Towards the end of the Late Triassic, a decline of variability seems to have occurred. The giant species seemed to have disappeared at the end of the Norian. Rhaetian (latest Triassic) ichthyosaurs are known from England, and these are very similar to those of the

. A possible explanation is an increased competition by sharks, Teleostei, and the first

. Like the dinosaurs, the ichthyosaurs and their contemporaries, the plesiosaurs, survived the

Triassic–Jurassic extinction event The Triassic–Jurassic (Tr-J) extinction event, often called the end-Triassic extinction, marks the boundary between the Triassic and Jurassic periods, , and is one of the top five major extinction events of the Phanerozoic eon, profoundly affec ...

, and quickly diversified again to fill the vacant ecological niches of the early Jurassic.

Jurassic

During the Early Jurassic, the ichthyosaurs still showed a large variety of species, ranging from in length. From this epoch, the original British discoveries were made, so that among ichthyosaurs, their names are the ones most familiar to the general public. Genera include '' Eurhinosaurus'', ''

Ichthyosaurus ''Ichthyosaurus'' (derived from Greek ' () meaning 'fish' and ' () meaning 'lizard') is a genus of ichthyosaurs from the Early Jurassic (Hettangian - Pliensbachian), with possible Late Triassic record, from Europe (Belgium, England, Germany, Swi ...

'', '' Leptonectes'', '' Stenopterygius'', and the large predator ''

'', along with the basal parvipelvian ''

Suevoleviathan ''Suevoleviathan'' is an extinct genus of primitive ichthyosaur found in the Early Jurassic (Toarcian) of Holzmaden, Germany. Taxonomy The genus was named in 1998 by Michael Maisch for ''Leptopterygius disinteger'' and ''Ichthyosaurus integ ...

'', which was little changed from its Norian ancestors. The general morphological variability had been strongly reduced, however. Giant forms, suction feeders and durophagous species were absent. All of these animals were streamlined, dolphin-like forms, although the more basal animals were perhaps more elongated than the advanced and compact ''Stenopterygius'' and ''Ichthyosaurus''. The latter belonged to a parvipelvian subgroup, the

Thunnosauria Thunnosauria (Greek for "tuna lizard" – ''thunnos'' meaning "tuna" and ''sauros'' meaning "lizard") is an extinct clade of parvipelvian ichthyosaurs from the Early Jurassic to the early Late Cretaceous (Hettangian– Cenomanian) of Asia, Aus ...

. These were fully adapted to an efficient thunniform locomotion, propelling themselves with the end of the tail only, equipped with a vertical tail fin. Their front flippers had more than five fingers. Another parvipelvian branch was the Eurhinosauria such as ''Leptonectes'' and ''Eurhinosaurus'', which were specialised forms having very elongated and pointy snouts. Few ichthyosaur fossils are known from the Middle Jurassic. This might be a result of the poor

fossil record A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in ...

in general of this epoch. The strata of the Late Jurassic seem to indicate that a further decrease in diversity had taken place. From the Middle Jurassic onwards, almost all ichthyosaurs belonged to the thunnosaurian clade

Ophthalmosauridae Ophthalmosauridae is an extinct family of thunnosaur ichthyosaurs from the Middle Jurassic to the early Late Cretaceous (Bajocian - Cenomanian) worldwide. Almost all ichthyosaurs from the Middle Jurassic onwards belong to the family, until the ex ...

. Represented by the ''

Ophthalmosaurus ''Ophthalmosaurus'' (meaning "eye lizard" in Greek) is an ichthyosaur of the Jurassic period (165–150 million years ago). Possible remains from the Cretaceous, around 145 million years ago, are also known. It was a relatively medium-sized ichth ...

'' and related genera, they were very similar in general build to ''Ichthyosaurus''. The eyes of ''Ophthalmosaurus'' were huge, and these animals likely hunted in dim and deep water. However, new finds from the Cretaceous indicate that ichthyosaur diversity in the Late Jurassic must have been underestimated.

Cretaceous

Traditionally, ichthyosaurs were seen as decreasing in diversity even further with the Cretaceous, though they had a worldwide distribution. All fossils from this period were referred to a single genus: '' Platypterygius''. This last ichthyosaur genus was thought to have become extinct early in the late Cretaceous, during the Cenomanian about 95 million years ago. The ichthyosaurs thus would have disappeared much earlier than other large Mesozoic reptile groups that generally survived until the end of the Cretaceous. Two major explanations were given for this. Firstly, it could have been a matter of chance. The second explanation had the extinction as caused by competition. Less hydrodynamically efficient animals, such as the Mosasauridae and long-necked plesiosaurs, flourished. The ichthyosaurian overspecialisation could be a contributing factor to their extinction, possibly being unable to 'keep up' with the fast-swimming and highly evasive new teleost fish, which had become dominant at this time, against which the sit-and-wait ambush strategies of the mosasaurids proved superior. This model thus emphasised evolutionary stagnation, the only innovation shown by ''Platypterygius'' being its ten fingers. Recent studies, however, show that ichthyosaurs were actually far more diverse in the Cretaceous than previously thought. Fragments referred to ''Platypterygius'' in fact represented diverse species. In 2012, at least eight lineages spanned the Jurassic-Cretaceous boundary, among them '' Acamptonectes'', '' Sveltonectes'', '' Caypullisaurus'', and '' Maiaspondylus''. In 2013, a Cretaceous basal thunnosaurian was revealed: '' Malawania''. Indeed, likely a radiation during the Early Cretaceous occurred due to an increase of coastlines when the continents further broke up. The demise of the ichthyosaurs has recently been described as a two-step process. A first extinction event in the beginning of the Cenomanian eliminated two of the three ichthyosaur feeding guilds then present, the 'soft-prey specialists' and the 'generalists', leaving only an apex predator group, which probably was not particularly specialized. The second major ichthyosaur extinction took place during the

Cenomanian-Turonian boundary event The Cenomanian-Turonian boundary event, also known as the Cenomanian-Turonian extinction, Cenomanian-Turonian oceanic anoxic event ( OAE 2), and referred to also as the Bonarelli event, was one of two anoxic extinction events in the Cretaceous p ...

, an '

anoxic event Oceanic anoxic events or anoxic events ( anoxia conditions) describe periods wherein large expanses of Earth's oceans were depleted of dissolved oxygen (O2), creating toxic, euxinic (anoxic and sulfidic) waters. Although anoxic events have not ...

', eliminating the apex predators, after which just a single lineage survived, ''Platypterygius hercynicus'', which disappeared shortly afterwards, about 93 million years ago. Ichthyosaur extinction was thus a pair of abrupt events rather than a long decline, probably related to the environmental upheavals and climatic changes in the Cenomanian and Turonian. If so, it was not caused by mosasaurid competition; large mosasaurs did not appear until 3 million years after the ichthyosaur extinction, likely to fill the resulting ecological void. Plesiosaurian polycoltylids perhaps also filled some of the niches previously occupied by ichthyosaurs, although they had coexisted for 19 million years. The extinction was most likely the result of ecological change and volatility that caused changes in migration, food availability, and birthing grounds. This part of the Cretaceous was one in which many other marine extinctions occurred, including those of some types of microplankton, ammonites,

belemnite Belemnitida (or the belemnite) is an extinct order of squid-like cephalopods that existed from the Late Triassic to Late Cretaceous. Unlike squid, belemnites had an internal skeleton that made up the cone. The parts are, from the arms-most ...

s, and reef-building

bivalves Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, biv ...

Taxonomy

This taxonomy was presented by Michael Werner Maisch in his 2010 review of ichthyosaur classification. * Clade Ichthyopterygia (''sensu'' Motani (1999)) ** Genus '' Chaohusaurus'' ** Family Grippiidae ** Family

Parvinatatoridae ''Parvinatator'', from Latin, “parvus” little and “natator” swimmer, is an extinct genus of small ichthyopterygian marine reptile that lived during the Early to Middle Triassic. Its fossils have been found in British Columbia, Canada ...

** Family

Thaisauridae ''Thaisaurus'' is an extinct genus of ichthyopterygian marine reptile that lived during the Spathian (late Olenekian, Early Triassic). Fossils have been found in Thailand.New Material of ''Qianichtyosaurus'' Li, 1999 (Reptilia, Ichthyosauria) fr ...

** Family

Utatsusauridae ''Utatsusaurus hataii'' is the earliest-known ichthyopterygian which lived in the Early Triassic period (c. 245–250 million years ago). It was nearly long with a slender body. The first specimen was found in Utatsu-cho (now part of Minamisa ...

** ? Family Omphalosauridae ** ? Genus '' Isfjordosaurus'' ** Order Ichthyosauria (''sensu'' Motani (1999)) *** Family Quasianosteosauridae *** Parvorder Hueneosauria **** Nanorder

***** Family Wimaniidae ***** Family Mixosauridae **** Nanorder Longipinnati ***** Family Toretocnemidae ***** Family Cymbospondylidae ***** Hyporder Merriamosauria ****** Family Merriamosauridae ****** Family Besanosauridae ****** Family Shastasauridae ****** Family Shonisauridae ****** Family Californosauridae ****** Minorder

******* Family Hudsonelpidiidae ******* Family Macgowaniidae ******* Suborder Neoichthyosauria ******** Family Temnodontosauridae ******** Family Leptonectidae ******** Family Suevoleviathanidae ******** Infraorder

********* Family Ichthyosauridae ********* Family Stenopterygiidae ********* Family

Phylogeny

In modern phylogeny, clades are defined that contain all species forming a certain branch of the evolutionary tree. This also allows one to clearly indicate all relationships between the several subgroups in a

cladogram A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to ...

. In 1999, a node clade Ichthyopterygia was defined by Motani as the group consisting of the

last common ancestor In biology and genetic genealogy, the most recent common ancestor (MRCA), also known as the last common ancestor (LCA) or concestor, of a set of organisms is the most recent individual from which all the organisms of the set are descended. The ...

of ''Ichthyosaurus communis'', ''Utatsusaurus hataii'' and ''Parvinatator wapitiensis''; and all its descendants. Within Motani's phylogeny, the Ichthyopterygia were the larger parent clade of a smaller stem clade Ichthyosauria that was defined as the group consisting of ''Ichthyosaurus communis'' and all species more closely related to ''Ichthyosaurus'' than to ''Grippia longirostris''. Motani's concept of the Ichthyosauria was thus more limited than the traditional one that also contained basal forms, such as ''Grippia'', ''Utatsusaurus'', and ''Parvinatator''. The following

is based on Motani (1999): An alternative terminology was proposed by Maisch & Matzke in 2000, trying to preserve the traditional, more encompassing content of the concept Ichthyosauria. They defined a node clade Ichthyosauria as the group consisting of the last common ancestor of '' Thaisaurus chonglakmanii'', ''Utatsusaurus hataii'', and ''Ophthalmosaurus icenicus'', and all its descendants. Ichthyosauria ''sensu'' Motani might materially be identical to a clade that Maisch & Matzke in 2000 called Hueneosauria, depending on the actual relationships. Cladogram based on Maisch and Matzke (2000) and Maisch and Matzke (2003) with clade names following Maisch (2010):

Description

Size

Ichthyosaurs averaged about in length. Some individual specimens were as short as ; some species were much larger: the Triassic '' Shonisaurus popularis'' was about long and in 2004 '' Shonisaurus sikanniensis'' (classified as a shastasaurus between 2011 and 2013) was estimated to have been in length. Fragmentary finds suggest the presence of a form in the early Jurassic. In 2018, lower jaw fragments from England were reported indicating a length of between 20 and 25 m (66 to 82 ft). According to weight estimates by Ryosuke Motani a ''Stenopterygius'' weighed around , whilst a ''

icenicus'' weighed .

General build

While the earliest known members of the ichthyosaur lineage were more eel-like in build, later ichthyosaurs resembled more typical fishes or dolphins, having a

porpoise Porpoises are a group of fully aquatic marine mammals, all of which are classified under the family Phocoenidae, parvorder Odontoceti (toothed whales). Although similar in appearance to dolphins, they are more closely related to narwhals a ...

-like head with a short neck and a long snout. Ichthyosaur fore and hind limbs had been fully transformed into flippers. Some species had a fin on their backs and a more or less vertical fin at the rear of a rather short tail. Although ichthyosaurs looked like fish, they were not. Ichthyosaur vs dolphin

Evolutionary biologist Stephen Jay Gould said that the ichthyosaur was his favourite example of convergent evolution, where similarities of structure are

analogous Analogy (from Greek ''analogia'', "proportion", from ''ana-'' "upon, according to" lso "against", "anew"+ ''logos'' "ratio" lso "word, speech, reckoning" is a cognitive process of transferring information or meaning from a particular subject (t ...

, not homologous, thus not caused by a common descent, but by a similar adaptation to an identical environment:

This sea-going reptile with terrestrial ancestors converged so strongly on fishes that it actually evolved a dorsal fin and tail in just the right place and with just the right hydrological design. These structures are all the more remarkable because they evolved from nothing—the ancestral terrestrial reptile had no hump on its back or blade on its tail to serve as a precursor.

Diagnostic traits

Derived ichthyosaurs in the narrow sense, as defined by Motani in 1999, differ from their closest basal

ichthyopterygia Ichthyopterygia ("fish flippers") was a designation introduced by Sir Richard Owen in 1840 to designate the Jurassic ichthyosaurs that were known at the time, but the term is now used more often for both true Ichthyosauria and their more primitiv ...

n relatives in certain traits. Motani listed a number of these. The external nostril is located on the side of the skull, and is hardly visible from above. The upper rim of the eye socket consists of a bone bar formed by the prefrontal and the postfrontal bones. The postorbital in side view is excluded from the supratemporal fenestra. The opening for the parietal eye is located on the border of the parietal and the frontal bone. The lateral wing of the pterygoid is incompletely and variably ossified. The ulna lacks the part behind the original shaft axis. The rear dorsal vertebrae are disc-shaped.

Skeleton

Skull

Basal Ichthyopterygia already had elongated, triangular skulls. With ichthyosaurs in the narrow sense, their snouts became very pointy. The snout is formed by the

premaxilla The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammal has ...

. The maxilla behind it is usually shorter and sometimes excluded from the external nostril by the rear branch of the premaxilla. Accordingly, the number of premaxillary teeth is high, while the maxillary teeth are fewer in number or even completely absent. The rear top of the snout is formed by the

nasal bones The nasal bones are two small oblong bones, varying in size and form in different individuals; they are placed side by side at the middle and upper part of the face and by their junction, form the bridge of the upper one third of the nose. Ea ...

. Derived species have a foramen internasale, a midline opening separating the rear of the nasal bones. The nasal bone usually forms the top and front rim of the bony nostril, itself often placed just in front of the eye socket. However, with some Triassic species, the premaxilla is so strongly extended at its back that it even excludes the nasal from the nostril. The rear of the skull is dominated by a large

eye socket In anatomy, the orbit is the cavity or socket of the skull in which the eye and its appendages are situated. "Orbit" can refer to the bony socket, or it can also be used to imply the contents. In the adult human, the volume of the orbit is , of ...

, often covering the major part of the rear side surface. In the socket, a large

is present; this is a circular structure of small, overlapping bone segments protecting the eye against the water pressure. Both in the relative and absolute senses, ichthyosaurs have the largest eye sockets of all known vertebrates. The other rear skull elements are typically so compressed and fused that they are difficult to identify. The top rear element of the skull was usually assumed to be the

supratemporal bone The supratemporal bone is a paired cranial bone present in many tetrapods and tetrapodomorph fish. It is part of the temporal region (the portion of the skull roof behind the eyes), usually lying medial (inwards) relative to the squamosal and later ...

, while the

squamosal The squamosal is a skull bone found in most reptiles, amphibians, and birds. In fishes, it is also called the pterotic bone. In most tetrapods, the squamosal and quadratojugal bones form the cheek series of the skull. The bone forms an ancestral co ...

and

quadratojugal The quadratojugal is a skull bone present in many vertebrates, including some living reptiles and amphibians. Anatomy and function In animals with a quadratojugal bone, it is typically found connected to the jugal (cheek) bone from the front and ...

were sometimes fused. However, in 1968,

Alfred Sherwood Romer Alfred Sherwood Romer (December 28, 1894 – November 5, 1973) was an American paleontologist and biologist and a specialist in vertebrate evolution. Biography Alfred Romer was born in White Plains, New York, the son of Harry Houston Romer an ...

stated that the presumed supratemporal was in fact the squamosal, an interpretation which was supported by McGowan in 1973. In 1990, though, John Steve Massare convinced most researchers that the original identification had been the correct one after all. The supratemporal forms the rear rim of the supratemporal opening; a lower temporal opening at the side is lacking. The front rim of the supratemporal opening is typically formed by the

postfrontal The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, th ...

; only with the very basal ''

'' the

postorbital The ''postorbital'' is one of the bones in vertebrate skulls which forms a portion of the dermal skull roof and, sometimes, a ring about the orbit. Generally, it is located behind the postfrontal and posteriorly to the orbital fenestra. In some ve ...

and the squamosal still reach the edge. Between the paired supratemporal openings, the skull roof is narrow; some species have a longitudinal crest on it as an attachment for the jaw muscles. Basal Ichthyopterygia have a parietal eye opening between the paired parietal bones. With ichthyosaurs proper, this opening moves to the front, first to the border between the parietals and the frontals and ultimately between the frontals, a condition shown by derived species.

Postparietal Postparietals are cranial bones present in fish and many tetrapods. Although initially a pair of bones, many lineages possess postparietals which were fused into a single bone. The postparietals were dermal bones situated along the midline of the ...

and tabular bones are lacking. Often, the bones of the back of the skull and the palate are incompletely ossified, apparently having partly remained

cartilage Cartilage is a resilient and smooth type of connective tissue. In tetrapods, it covers and protects the ends of long bones at the joints as articular cartilage, and is a structural component of many body parts including the rib cage, the neck an ...

. The

occipital condyle The occipital condyles are undersurface protuberances of the occipital bone in vertebrates, which function in articulation with the superior facets of the atlas vertebra. The condyles are oval or reniform (kidney-shaped) in shape, and their anteri ...

is typically very convex. The stapes, the bone transmitting sound waves from the eardrum to the

middle ear The middle ear is the portion of the ear medial to the eardrum, and distal to the oval window of the cochlea (of the inner ear). The mammalian middle ear contains three ossicles, which transfer the vibrations of the eardrum into waves in the ...

, is elongated and not pierced by a foramen. Pterygoid teeth are typically lacking.

Lower jaws

Like the snout, the lower jaws are elongated. However, in some species, such as '' Eurhinosaurus'' and '' Excalibosaurus'', the front of the snout far protrudes beyond the lower jaws. While the front of the lower jaw is typically low, its rear depth is very variable. The greater part of the lower jaw is formed by the front

dentary In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...

, the tooth-bearing bone. At its inner side the dentary is covered by a splenial that extends forwards until the symphysis, the common contact surface where both lower jaws are grown together. The jaw joints do not allow a horizontal chewing movement: they function as simple hinges to vertically open or close the jaws.

Teeth

Ichthyosaur teeth are typically conical. Fish-eating species have long and slender tooth crowns that are slightly recurved. Forms specialised in catching larger prey have shorter, broader, and straighter teeth; sometimes, cutting edges are present. '' Thalattoarchon'', an apex predator, had larger teeth formed like flattened blades.

Durophagous Durophagy is the eating behavior of animals that consume hard-shelled or exoskeleton bearing organisms, such as corals, shelled mollusks, or crabs. It is mostly used to describe fish, but is also used when describing reptiles, including fossil t ...

species that ate

shellfish Shellfish is a colloquial and fisheries term for exoskeleton-bearing aquatic invertebrates used as food, including various species of molluscs, crustaceans, and echinoderms. Although most kinds of shellfish are harvested from saltwater enviro ...

have low, convex teeth that are closely packed. Many ichthyosaur dentitions are

heterodont In anatomy, a heterodont (from Greek, meaning 'different teeth') is an animal which possesses more than a single tooth morphology. In vertebrates, heterodont pertains to animals where teeth are differentiated into different forms. For example, ...

, combining several tooth shapes, e.g. small teeth in the front and larger teeth at the rear. The teeth are usually placed in

tooth socket Dental alveoli (singular ''alveolus'') are sockets in the jaws in which the roots of teeth are held in the alveolar process with the periodontal ligament. The lay term for dental alveoli is tooth sockets. A joint that connects the roots of the ...

s; derived species possess a common tooth groove. In the latter case, adult individuals sometimes become toothless. Teeth in tooth sockets sometimes fuse with the jawbone. With ichthyosaur teeth, the dentine shows prominent vertical wrinkles. Durophagous forms have teeth with deep vertical grooves and wrinkles in the enamel.

Postcrania

=Vertebral column

Basal Ichthyopterygia, like their land-dwelling ancestors, still had

vertebra The spinal column, a defining synapomorphy shared by nearly all vertebrates,Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic ...

e that possessed a full set of processes that allowed them to interlock and articulate, forming a vertebral column supporting the weight of the body. As ichthyosaurs were fully aquatic, their bodies were supported by the Archimedes force exerted by the water; in other words, they were

buoyant Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the pr ...

. Therefore, the vertebral processes had lost much of their function. Early ichthyosaurs proper had rear dorsal vertebrae that had become disc-shaped, like those of typical fishes. With more derived species, the front dorsals also became discs. Gradually, most processes were lost, including those for rib attachment. The vertebral bodies became much shorter. The front and rear sides of the discs were hollowed out, resulting in a so-called amphicoelous condition. A transverse cross-section of such a vertebra has an hourglass shape. This morphology is unique within the

Amniota Amniotes are a clade of tetrapod vertebrates that comprises sauropsids (including all reptiles and birds, and extinct parareptiles and non-avian dinosaurs) and synapsids (including pelycosaurs and therapsids such as mammals). They are distingu ...

and makes discerning ichthyosaur vertebrae from those of other marine reptiles easy. The only process that kept its function was the spine at the top, serving as an attachment for the dorsal muscles. However, even the spine became a simple structure. The neural arch, of which it was an outgrowth, typically no longer fused to the vertebral centre. The neck is short, and derived species show a reduction in the number of

cervical vertebra In tetrapods, cervical vertebrae (singular: vertebra) are the vertebrae of the neck, immediately below the skull. Truncal vertebrae (divided into thoracic and lumbar vertebrae in mammals) lie caudal (toward the tail) of cervical vertebrae. In sau ...

e. The short neck positions the skull close to the trunk, usually in a slight oblique elevation to it. Derived species usually also have a reduced number of dorsals, the total of presacral vertebrae totalling about forty to fifty. The vertebral column is little differentiated. Basal Ichthyopterygia still have two sacral vertebrae, but these are not fused. Early Triassic forms have a transversely flattened tail base with high spines for an undulating tail movement. Derived forms have a shorter tail with the characteristic kink at the end; a section of wedge-shaped vertebrae, itself supporting the fleshy upper tail fin lobe, forced the tail end into the lower fin lobe. As derived species no longer have transversal processes on their vertebrae—again a condition unique in the Amniota—the parapophyseal and diapophysael rib joints have been reduced to flat facets, at least one of which is located on the vertebral body. The number of facets can be one or two; their profile can be circular or oval. Their shape often differs according to the position of the vertebra within the column. The presence of two facets per side does not imply that the rib itself is double-headed: often, even in that case, it has a single head. The ribs typically are very thin and possess a longitudinal groove on both the inner and the outer sides. The lower side of the chest is formed by

gastralia Gastralia (singular gastralium) are dermal bones found in the ventral body wall of modern crocodilians and tuatara, and many prehistoric tetrapods. They are found between the sternum and pelvis, and do not articulate with the vertebrae. In ...

. These belly ribs have a single centre segment and one or two outer segments per side. They are not fused into a real plastron. Usually two gastralia are present per dorsal rib.

=Appendicular skeleton

= The

shoulder girdle The shoulder girdle or pectoral girdle is the set of bones in the appendicular skeleton which connects to the arm on each side. In humans it consists of the clavicle and scapula; in those species with three bones in the shoulder, it consists of ...

of ichthyosaurs is not much modified from its original condition. Some basal forms show a hatchet- or crescent-shaped

shoulder blade The scapula (plural scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on either ...

scapula The scapula (plural scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on eithe ...

; derived forms have an elongated blade positioned on a broader base. The scapula is not fused with the

coracoid A coracoid (from Greek κόραξ, ''koraks'', raven) is a paired bone which is part of the shoulder assembly in all vertebrates except therian mammals (marsupials and placentals). In therian mammals (including humans), a coracoid process is prese ...

into a scapulocoracoid, indicating that the forces exerted on the shoulder girdle were moderate. The shoulder joint is positioned on the border between the scapula and the coracoid. Both coracoids are fused on their common midline. The coracoid shape is very variable, but usually it is rather low. The upper part of the shoulder girdle is formed by two long and slender

clavicle The clavicle, or collarbone, is a slender, S-shaped long bone approximately 6 inches (15 cm) long that serves as a strut between the shoulder blade and the sternum (breastbone). There are two clavicles, one on the left and one on the right ...

s, crowned by a central interclavicular bone that is large and triangular with basal forms, small and T-shaped in Jurassic species.

Breast bone The sternum or breastbone is a long flat bone located in the central part of the chest. It connects to the ribs via cartilage and forms the front of the rib cage, thus helping to protect the heart, lungs, and major blood vessels from injury. Sha ...

s or

sterna ''Sterna'' is a genus of terns in the bird family Laridae. The genus used to encompass most "white" terns indiscriminately, but mtDNA sequence comparisons have recently determined that this arrangement is paraphyletic. It is now restricted to ...

are absent. Ichthyosaur paddle 1

Basal forms have a forelimb that is still functionally differentiated, in some details resembling the arm of their land-dwelling forebears; the

ulna The ulna (''pl''. ulnae or ulnas) is a long bone found in the forearm that stretches from the elbow to the smallest finger, and when in anatomical position, is found on the medial side of the forearm. That is, the ulna is on the same side of t ...

and radius are elongated and somewhat separated; the

carpal The carpal bones are the eight small bones that make up the wrist (or carpus) that connects the hand to the forearm. The term "carpus" is derived from the Latin carpus and the Greek καρπός (karpós), meaning "wrist". In human anatomy, the ...

s are rounded, allowing the wrist to rotate; the number of phalanges is within the range shown by land animals. Ichthyosaurs proper, to the contrary, have a forelimb that is fully adapted to its function as a flipper. However, the adaptations are very variable. Triassic species typically have a very derived

humerus The humerus (; ) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius and ulna, and consists of three sections. The humeral upper extremity consists of a round ...

, changed into a disc. Jurassic species tend to have a more elongated humeral form with a rounded head, narrow shaft, and expanded lower end. The radius and ulna are always strongly flattened, but can be circular, with or without notch, or have a waist. Notches can be homologous to the original shafts, but also be newly formed. Jurassic forms no longer have a space, the ''spatium interosseum'', between the radius and ulna. Often, the latter bones gradually merge into lower, disc-shaped elements - the up to four carpals which again differ little in form from the up to five metacarpals. Hul - Ophthalmosaurus icenius 3

A strongly derived condition show the phalanges, small, disc-shaped elements positioned in long rows. Sometimes, the number of fingers is reduced, to as low as two. This is a rather common phenomenon within the Tetrapoda. Unique, however, for derived tetrapods, is the fact that some species show nonpathological

, the number of fingers being higher than five. Some species have ten fingers per hand. These fingers, again, can have an increased number of phalanges, up to thirty, a phenomenon called

hyperphalangy The phalanges (singular: ''phalanx'' ) are digital bones in the hands and feet of most vertebrates. In primates, the thumbs and big toes have two phalanges while the other digits have three phalanges. The phalanges are classed as long bones. ...

, also known from the Plesiosauria, mosasaurs, and the

Cetacea Cetacea (; , ) is an infraorder of aquatic mammals that includes whales, dolphins, and porpoises. Key characteristics are their fully aquatic lifestyle, streamlined body shape, often large size and exclusively carnivorous diet. They propel the ...

. The high number of elements allows the flipper to be shaped as a

hydrofoil A hydrofoil is a lifting surface, or foil, that operates in water. They are similar in appearance and purpose to aerofoils used by aeroplanes. Boats that use hydrofoil technology are also simply termed hydrofoils. As a hydrofoil craft gains sp ...

. When a high number of fingers is present, their identity is difficult to determine. It is usually assumed that fingers were added at both the front and at the rear, perhaps to a core of four original fingers. If fingers are added, often the number of metacarpals and carpals is also increased; sometimes even an extra lower arm element is present. Earlier, ichthyosaurs were commonly divided into "longipinnate" and "latipinnate" forms, according to the long or wide shape of the front flippers, but recent research has shown that these are not natural groups; ichthyosaur clades often contain species with and without elongated forelimbs. The ichthyosaur pelvis is typically rather reduced. The three pelvic bones: the ilium, the ischium, and the

pubic bone In vertebrates, the pubic region ( la, pubis) is the most forward-facing (ventral and anterior) of the three main regions making up the coxal bone. The left and right pubic regions are each made up of three sections, a superior ramus, inferior r ...

, are not fused and often do not even touch each other. Also, the left and right pelvic sides no longer touch; only basal forms still have sacral ribs connecting the ilia to the vertebral column. The hip joint is not closed on the inside. The pubic bone typically does not connect to the ischium behind it; the space in between is by some workers identified as the ''fenestra thyreoidea''; other researchers deny that the term is applicable given the general loose structure of the pelvis. Some later species have a connected pubic bone and ischium, but in this case, the femoral head no longer articulates with the hip joint. Triassic species have plate-like pubic bones and ischia; in later species these elements become elongated with a narrow shaft and can form a single rod. Typically, the hindlimbs are shorter than the forelimbs, possessing a lesser number of elements. Often, the rear flipper is only half the length of the front flipper. The thighbone is short and broad, often with a narrow waist and an expanded lower end. The tibia, fibula and metatarsals are merged into a mosaic of bone discs supporting the hydrofoil. Three to six toes are present. The toe phalanges also show hyperphalangy; exceptionally, ''

'' shows a reduced number of phalanges.

Soft tissue

Ichthyosaur - Naturmuseum Senckenberg - DSC02173

The earliest reconstructions of ichthyosaurs all omitted dorsal fins and caudal (tail) flukes, which were not supported by any hard skeletal structure, so were not preserved in many fossils. Only the lower tail lobe is supported by the vertebral column. In the early 1880s, the first body outlines of ichthyosaurs were discovered. In 1881, Richard Owen reported ichthyosaur body outlines showing tail flukes from Lower Jurassic rocks in Barrow-upon-Soar, England. Other well-preserved specimens have since shown that in some more primitive ichthyosaurs, like a specimen of '' Chaohusaurus geishanensis'', the tail fluke was weakly developed and only had a dorsal tail lobe, making the tail more paddle-like. Over the years, the visibility of the tail lobe has faded away in this specimen. The presence of dorsal fins in ichthyosaurs has been controversial. Finely preserved specimens from the Holzmaden ''

Lagerstätte A Lagerstätte (, from '' Lager'' 'storage, lair' '' Stätte'' 'place'; plural ''Lagerstätten'') is a sedimentary deposit that exhibits extraordinary fossils with exceptional preservation—sometimes including preserved soft tissues. These f ...

n'' in Germany found in the late 19th century revealed additional traces, usually preserved in black, of the outline of the entire body, including the first evidence of dorsal fins in ichthyosaurs. Unique conditions permitted the preservation of these outlines, which probably consist of bacterial mats, not the remains of the original tissues themselves. In 1987, David Martill argued that, given the indirect method of conservation by bacteria, these outlines were unlikely to have been reliably preserved in any fine detail. He concluded that no authentic dorsal fins had been discovered. After displaced skins flaps from the body would have initially been misinterpreted as fins, fossil preparators later came to expect such fins to be present, and would have identified any discolouration in the appropriate position as a dorsal fin or even have falsified such structures. The lack of a dorsal fin would also explain why ichthyosaurs, contrary to porpoises, retained hind flippers, as these were needed for stability. Other researchers noted that, while the outlines might have been sharpened and smoothed by preparators because fossil bacterial mats usually have indistinct edges, many of the preserved dorsal fins were probably authentic and at least somewhat close to the true body outline. At least one specimen, R158 (in the collections of the Paleontologiska Museet,

Uppsala University Uppsala University ( sv, Uppsala universitet) is a public research university in Uppsala, Sweden. Founded in 1477, it is the oldest university in Sweden and the Nordic countries still in operation. The university rose to significance during ...

), shows the expected faded edges of a bacterial mat, so it has not been altered by preparators, yet still preserves a generally tuna-like body outline including a dorsal fin. In 1993, Martill admitted that at least some dorsal fin specimens are authentic. The fossil specimens that preserved dorsal fins also showed that the flippers were pointy and often far wider than the underlying bones would suggest. The fins were supported by fibrous tissue. In some specimens, four layers of collagen are visible, the fibres of the covering layers crossing those of the collagen below. In 2017, from the German Posidonia Shale the discovery was reported of 182.7-million-year-old vertebrae of ''Stenopterygius'' in a carbonate nodule, still containing collagen fibers, cholesterol, platelets, and red and white blood cells. The structures would not have been petrified, but represent the original organic tissues of which the biomolecules could be identified. The exceptional preservation was explained by the protective environment offered by the nodule. The red blood cells found, were one-fourth to one fifth the size of those of modern mammals. This would have been an adaptation for an improved oxygen absorption, also in view of the low oxygen levels during the Toarcian. The cholesterol had a high-carbon-13 isotope component which might indicate a higher position in the

food chain A food chain is a linear network of links in a food web starting from producer organisms (such as grass or algae which produce their own food via photosynthesis) and ending at an apex predator species (like grizzly bears or killer whales), det ...

and a diet of fish and

cephalopod A cephalopod is any member of the molluscan class Cephalopoda (Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, a ...

s. In 2018, evidence of

was discovered with '' Stenopterygius''.

Skin and colouration

Typically, fossils that preserve it suggest that the skin of ichthyosaurs was smooth and elastic, lacking scales. However, these remains are not impressions ''per se'', but outlines formed from bacterial growth. In one case, a true impression of the skin was reported from a specimen of '' Aegirosaurus'' found in the Solnhofen Plattenkalk, rocks which were capable of preserving even the finest detail. Minuscule scales seemed to be visible in this specimen. The colouration of ichthyosaurs is difficult to determine. In 1956, Mary Whitear reported finding

melanocyte Melanocytes are melanin-producing neural crest-derived cells located in the bottom layer (the stratum basale) of the skin's epidermis, the middle layer of the eye (the uvea), the inner ear, vaginal epithelium, meninges, bones, and heart. M ...

s, pigment cells in which reddish-brown pigment granules would still be present, in a skin specimen of a British fossil, R 509. Ichthyosaurs are traditionally assumed to have employed

countershading Countershading, or Thayer's law, is a method of camouflage in which an animal's coloration is darker on the top or upper side and lighter on the underside of the body. This pattern is found in many species of mammals, reptiles, birds, fish, and ...

(dark on top, light at the bottom) like sharks, penguins, and other modern animals, serving as camouflage during hunting. This was contradicted in 2014 by the discovery of melanosomes, black

melanin Melanin (; from el, μέλας, melas, black, dark) is a broad term for a group of natural pigments found in most organisms. Eumelanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the amino ...

-bearing structures, in the skin of ichthyosaur specimen YORYM 1993.338 by Johan Lindgren of

Lund University , motto = Ad utrumque , mottoeng = Prepared for both , established = , type = Public research university , budget = SEK 9 billion thermoregulation and to camouflage them in deep water while hunting. This is in contrast to mosasaurids and prehistoric leatherback turtles, which were found to be countershaded. However, a 2015 study doubted Lindgren's interpretation. This study noted that a basal layer of melanosomes in the skin is ubiquitous in reptile coloration, but does not necessarily correspond to a dark appearance. Other

chromatophore Chromatophores are cells that produce color, of which many types are pigment-containing cells, or groups of cells, found in a wide range of animals including amphibians, fish, reptiles, crustaceans and cephalopods. Mammals and birds, in contrast ...

structures (such as iridiophores, xanthophores, and erythrophores) affect coloration in extant reptiles but are rarely preserved or identified in fossils. Thus, due to the unknown presence of these chromatophores, YORYM 1993.338, could have been countershaded, green, or various other colors or patterns.

Gastroliths

Gastroliths, stomach stones that might have assisted digestion or regulated buoyancy, have only on a few occasions been found associated with ichthyosaur skeletons, once with a specimen of ''

Nannopterygius ''Nannopterygius'' (meaning "small wing/flipper" in Greek) is an extinct genus of ophthalmosaurid ichthyosaur that lived during the Middle Jurassic to the Early Cretaceous (Callovian to Berriasian stages). Fossils are known from England, Kazakhs ...

'' and a second time in a '' Panjiangsaurus'' fossil. Ichthyosaur coproliths, petrified

faeces Feces ( or faeces), known colloquially and in slang as poo and poop, are the solid or semi-solid remains of food that was not digested in the small intestine, and has been broken down by bacteria in the large intestine. Feces contain a relati ...

, are very common, though, already being sold by Mary Anning.

Paleobiology

Ecology

Apart from the obvious similarities to fish, ichthyosaurs also shared parallel developmental features with dolphins,

lamnid shark The Lamnidae are the family of mackerel sharks known as white sharks. They are large, fast-swimming predatory fish found in oceans worldwide, though prefer environments with colder water. The name of the family is formed from the Greek word ''la ...

s, and tuna. This gave them a broadly similar appearance, possibly implied similar activity levels (including thermoregulation), and presumably placed them broadly in a similar ecological niche. Ichthyosaurs were not primarily coastal animals; they also inhabited the open ocean. They have been found in all Mesozoic oceans. This is even true of the earliest Ichthyopterygia, making identification of a certain area as their place of origin impossible.

Feeding

Ichthyosaurs were carnivorous; they ranged so widely in size, and survived for so long, that they are likely to have had a wide range of prey. Species with pointed snouts were adapted to grab smaller animals. McGowan speculated that forms with protruding upper jaws, in the Eurhinosauria, would have used their pointy snouts to slash prey, as has been assumed for swordfish. The most commonly preserved gut contents in ichthyosaurs are the remains of

s. Less commonly, they fed on fish and other vertebrates, including smaller ichthyosaurs. The large Triassic form '' Thalattoarchon'' had large, bladed teeth and was probably a macropredator, capable of killing prey its own size, and '' Himalayasaurus'' and several species of ''

'' also shared adaptations for killing very large prey.C. McGowan. 1974. A revision of the longipinnate ichthyosaurs of the Lower Jurassic of England, with descriptions of two new species (Reptilia: Ichthyosauria). Life Sciences Contribution of the Royal Ontario Museum 97. These food preferences have been confirmed by coproliths which indeed contain the remains of fishes and cephalopods. Another confirmation is provided by fossilised stomach contents. Buckland in 1835 described the presence in a specimen of a large mass of partly digested fishes, recognisable by their scales. Subsequent research in 1968 determined that these belonged to the fish genus ''

Pholidophorus ''Pholidophorus'' (from el, φολῐ́ς , 'horny scale' and el, φέρω , 'to bear') is an extinct genus of stem- teleost fish. Numerous species were assigned to this genus in the past, but only the type species ''Pholidophorus latiusculu ...

'', but also that cephalopod beaks and sucker hooks were present. Such hard food particles apparently were retained by the stomach and regularly regurgitated. Carcasses of drowned animals were eaten as well: in 2003 a specimen of '' Platypterygius longmani'' was reported having besides fishes and a turtle the bones of a land bird in its stomach. Caypullisaurus Dak1

Some early ichthyosaurs were

and had flat convex teeth adapted for crushing

. They thus ate

benthos Benthos (), also known as benthon, is the community of organisms that live on, in, or near the bottom of a sea, river, lake, or stream, also known as the benthic zone.suction feeder Aquatic feeding mechanisms face a special difficulty as compared to feeding on land, because the density of water is about the same as that of the prey, so the prey tends to be pushed away when the mouth is closed. This problem was first identifi ...

s, sucking animals into their mouths by quickly opening their relatively short jaws. This was first assumed for ''Shonisaurus'', which giant by this means might have secured a constant food supply for its huge body, and in 2011 for the short-snouted '' Guanlingsaurus liangae''. However, in 2013 a study concluded that the

hyoid bone The hyoid bone (lingual bone or tongue-bone) () is a horseshoe-shaped bone situated in the anterior midline of the neck between the chin and the thyroid cartilage. At rest, it lies between the base of the mandible and the third cervical ver ...

of ichthyosaurs, at the tongue base, was insufficiently ossified to support a suction feeding movement and suggested the alternative that such species were ram feeders, gathering food by constantly swimming forwards with a wide-open mouth. Typical ichthyosaurs had very large eyes, protected within a bony ring, suggesting that they may have hunted at night or at great depths; the only extant animals with similarly large eyes are the giant and

colossal squid The colossal squid (''Mesonychoteuthis hamiltoni'') is part of the family Cranchiidae. It is sometimes called the Antarctic squid or giant cranch squid and is believed to be the largest squid species in terms of mass. It is the only recognize ...

s. Sight thus seems to have been one of the main senses employed while hunting. Hearing might have been poor, given the very robust form of the stapes. Grooves in the palate however, suggest that smell might have been acute or even that electro-sensory organs might have been present. Ichthyosaurs themselves served as food for other animals. During the Triassic their natural predators mainly consisted of sharks and other ichthyosaurs; in the Jurassic these were joined by large

and marine

Crocodylomorpha Crocodylomorpha is a group of pseudosuchian archosaurs that includes the crocodilians and their extinct relatives. They were the only members of Pseudosuchia to survive the end-Triassic extinction. During Mesozoic and early Cenozoic times, cro ...

. This is again confirmed by stomach contents: in 2009 e.g., a plesiosaur specimen was reported with an ichthyosaur embryo in its gut.

Locomotion

In ichthyosaurs, the main propulsion was provided by a lateral movement of the body. Early forms employed an

or eel-like movement, with undulations of the entire trunk and tail. This is usually considered rather inefficient. Later forms, like the

, has a shorter trunk and tail and probably used a more efficient carangiform or even thunniform movement, in which the last third of the body, respectively, the tail end, is flexed only. The trunk in such species is rather stiff. The tail was bi-lobed, with the lower lobe being supported by the caudal vertebral column, which was "kinked" ventrally to follow the contours of the ventral lobe. Basal species had a rather asymmetric or "heterocercal" tail fin. The asymmetry differed from that of sharks in that the lower lobe was largest, instead of the upper lobe. More derived forms had a nearly vertical symmetric tail fin. Sharks use their asymmetric tail fin to compensate for the fact that they are negatively buoyant, heavier than water, by making the downward pressure exerted by the tail force the body as a whole in an ascending angle. This way, swimming forwards will generate enough lift to equal the sinking force caused by their weight. In 1973, McGowan concluded that, because ichthyosaurs have a reversed tail fin asymmetry compared to sharks, they were apparently positively buoyant, lighter than water, which would be confirmed by their lack of gastroliths and of

pachyostosis Pachyostosis is a non-pathological condition in vertebrate animals in which the bones experience a thickening, generally caused by extra layers of lamellar bone. It often occurs together with bone densification (osteosclerosis), reducing inner ca ...

or dense bone. The tail would have served to keep the body in a descending angle. The front flippers would be used to push the front of the body further downwards and control pitch. In 1987 however, Michael A. Taylor suggested an alternative hypothesis: as ichthyosaurs could vary their lung content, contrary to sharks (which lack a swimming bladder), they could also regulate their buoyancy. The tail thus mainly served for a neutral propulsion, while small variations in buoyancy were stabilised by slight changes in the flipper angles. In 1992, McGowan accepted this view, pointing out that shark tails are not a good analogy of derived ichthyosaur tails that have more narrow lobes, and are more vertical and symmetric. Derived ichthyosaur tail fins are more like those of tuna fish and indicate a comparable capacity to sustain a high cruising speed. A comparative study by Motani in 2002 concluded that, in extant animals, small tail fin lobes positively correlate with a high beat frequency. Modern researchers generally concur that ichthyosaurs were negatively buoyant. Inia-2

In 1994, Judy Massare concluded that ichthyosaurs had been the fastest marine reptiles. Their length/depth ratio was between three and five, the optimal number to minimise water resistance or drag. Their smooth skin and streamlined bodies prevented excessive turbulence. Their hydrodynamic efficiency, the degree to which energy is converted into a forward movement, would approach that of dolphins and measure about 0.8. Ichthyosaurs would be a fifth faster than plesiosaurs, though half of the difference was explained by assuming a 30% higher metabolism for ichthyosaurs. Together, within Massare's model these effects resulted in a cruising speed of slightly less than five kilometres per hour. However, in 2002, Motani corrected certain mistakes in Massare's formulae and revised the estimated cruising speed to less than two kilometres per hour, somewhat below that of modern

. However, as the speeds estimated for plesiosaurs and mosasaurids were also revised downwards, ichthyosaurs maintained their relative position. Ichthyosaurs had fin-like limbs of varying relative length. The standard interpretation is that these, together with the dorsal fin and tail fin, were used as control surfaces for directional stability, controlling yaw, and for stabilising pitch and roll, rather than propulsion. However, during the 1980s the German paleontologist Jürgen Riess proposed an alternative model. After having studied the flying movement made by the forelimbs of plesiosaurs, he suggested that at least those ichthyosaurs that had long flippers used them for a powerful propulsive stroke, moving them up and down. This would explain the non-degenerated shoulder girdle and the evolution of the hand bones, whose perfect hydrofoil profile would have been useless if it was not functionally employed. He thought to have discovered modern analogues in the

Queensland lungfish The Australian lungfish (''Neoceratodus forsteri''), also known as the Queensland lungfish, Burnett salmon and barramunda, is the only surviving member of the family Neoceratodontidae. It is one of only six extant lungfish species in the world. ...

and the

Amazon river dolphin The Amazon river dolphin (''Inia geoffrensis''), also known as the boto, bufeo or pink river dolphin, is a species of toothed whale classified in the family Iniidae. Three subspecies are currently recognized: ''I. g. geoffrensis'' (Amazon rive ...

, which he presumed also used their long fins for propulsion. Riess expounded upon this hypothesis in a series of articles. This alternative interpretation was generally not adopted by other workers. In 1998, Darren Naish pointed out that the lungfish and the river dolphin actually do not use their fins in this way and that e.g. the modern

humpback whale The humpback whale (''Megaptera novaeangliae'') is a species of baleen whale. It is a rorqual (a member of the family Balaenopteridae) and is the only species in the genus ''Megaptera''. Adults range in length from and weigh up to . The hu ...

has very long front flippers, supported by a mosaic of bones, but that these nevertheless mainly serve as rudders. In 2013, a study concluded that broad ichthyosaur flippers, like those of ''Platyptergygius'', were not used for propulsion but as a control surface.

Diving

Many extant lung-breathing marine vertebrates are capable of deep diving. There are some indications about the diving capacity of ichthyosaurs. Quickly ascending from a greater depth can cause decompression sickness. The resulting bone necrosis has been well documented with Jurassic and Cretaceous ichthyosaurs, where it is present in 15% and 18% of specimens, respectively, but is rare in Triassic species. This could be a sign that basal forms did not dive as deeply, but might also be explained by a greater predation pressure during the later epochs, more often necessitating a fast flight to the surface. However, this last possibility is contradicted by the fact that, with modern animals, damage is not caused by a limited number of rapid ascension incidents, but by a gradual accumulation of non-invalidating degeneration during normal diving behaviour. Additional evidence is provided by the eyes of ichthyosaurs that among vertebrates are both relatively and absolutely the largest known. Modern

leopard seal The leopard seal (''Hydrurga leptonyx''), also referred to as the sea leopard, is the second largest species of seal in the Antarctic (after the southern elephant seal). Its only natural predator is the orca. It feeds on a wide range of prey inc ...

s can dive to up to hunting on sight. Motani suggested that ichthyosaurs, with their relatively much larger eye sockets, should have been able to reach even greater depths. ''Temnodontosaurus'', with eyes that had a diameter of twenty-five centimetres, could probably still see at a depth of 1,600 metres. At these depths, such eyes would have been especially useful to see large objects. Later species, such as ''

'', had relatively larger eyes, again an indication that diving capacity was better in late Jurassic and Cretaceous forms.

Metabolism

Similar to modern

cetacea Cetacea (; , ) is an infraorder of aquatic mammals that includes whales, dolphins, and porpoises. Key characteristics are their fully aquatic lifestyle, streamlined body shape, often large size and exclusively carnivorous diet. They propel the ...

ns, such as whales and dolphins, ichthyosaurs were air-breathing. Whales and dolphins are mammals and warm-blooded. Of ichthyosaurs it was traditionally assumed that they were cold-blooded, being reptiles. However, since the 1970s many dominant reptile groups of the Mesozoic, such as theropod dinosaurs,

pterosaur Pterosaurs (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") is an extinct clade of flying reptiles in the order, Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 to ...

s and plesiosaurs, have been considered warm-blooded, as this offers an elegant explanation of their dominance. Some direct evidence is available that ichthyosaurs too might have been endothermic. In 1990, Vivian de Buffrénil published a

histological Histology, also known as microscopic anatomy or microanatomy, is the branch of biology which studies the microscopic anatomy of biological tissues. Histology is the microscopic counterpart to gross anatomy, which looks at larger structures vis ...

study, indicating that ichthyosaurs possessed a fibrolamellar bone structure, as with warm-blooded animals in general, typified by fast growth and a strong

vascularisation Angiogenesis is the physiological process through which new blood vessels form from pre-existing vessels, formed in the earlier stage of vasculogenesis. Angiogenesis continues the growth of the vasculature by processes of sprouting and splittin ...

. Early Triassic species already show these traits. In 2012, it was reported that even the very basal form ''Utatsusaurus'' had this bone type, indicating that the ancestors of ichthyosaurs were already warm-blooded. Additional direct proof for a high metabolism is the isotopes of oxygen ratio in the teeth, which indicates a body temperature of between 35 and 39 °C, about 20° higher than the surrounding seawater. Blubber is consistent with warm-bloodedness as the insulating qualities require the animal to generate its own heat. Indirect evidence for endothermy is provided by the body shape of derived ichthyosaurs, which with its short tail and vertical tail fin seems optimised for a high cruising speed that can only be sustained by a high metabolism: all extant animals swimming this way are either fully warm-blooded or, like sharks and tuna, maintain a high temperature in their body core. This argument does not cover basal forms with a more eel-like body and undulating swimming movement. In 1996, Richard Cowen, while accepting endothermy for the group, presumed that ichthyosaurs would have been subject to ''

Carrier's constraint Carrier's constraint is the observation that air-breathing vertebrates which have two lungs and flex their bodies sideways during locomotion find it very difficult to move and breathe at the same time, because the sideways flexing expands one lung ...

'', a limitation to reptilian respiration pointed out in 1987 by

David Carrier David Carrier (; born 1944) is an American philosopher of art and cultural critic. Education Carrier received a Ph.D. in philosophy from Columbia University, where he was a student of Arthur Danto, in 1972. He was a Getty Scholar (1999–2000), ...

: their undulated locomotion forces the air out of the lungs and thus prevents them from taking breath while moving. Cowen hypothesised that ichthyosaurs would have overcome this problem by porpoising: constantly jumping out of the water would have allowed them to take a gulp of fresh air during each jump. Other researchers have tended to assume that for at least derived ichthyosaurs Carrier's constraint did not apply, because of their stiff bodies, which seems to be confirmed by their good diving capacity, implying an effective respiration and oxygen storage system. For these species porpoising was not a necessity. Nevertheless, ichthyosaurs would have often surfaced to breathe, probably tilting their heads slightly to take in air, because of the lower position of the nostrils compared to that of dolphins.

Reproduction

Ichthyosaurs were viviparous, i.e. bore live young instead of laying eggs. Although they were reptiles and descended from egg-laying,

oviparous Oviparous animals are animals that lay their eggs, with little or no other embryonic development within the mother. This is the reproductive method of most fish, amphibians, most reptiles, and all pterosaurs, dinosaurs (including birds), and ...

, ancestors, viviparity is not as unexpected as it first appears. Air-breathing marine creatures must either come ashore to lay

egg An egg is an organic vessel grown by an animal to carry a possibly fertilized egg cell (a zygote) and to incubate from it an embryo within the egg until the embryo has become an animal fetus that can survive on its own, at which point the a ...

s, like turtles and some

sea snake Sea snakes, or coral reef snakes, are elapid snakes that inhabit marine environments for most or all of their lives. They belong to two subfamilies, Hydrophiinae and Laticaudinae. Hydrophiinae also includes Australasian terrestrial snakes, w ...

s, or else give birth to live young in surface waters, like whales and dolphins. Given their

streamlined Streamlines, streaklines and pathlines are field lines in a fluid flow. They differ only when the flow changes with time, that is, when the flow is not steady. Considering a velocity vector field in three-dimensional space in the framework of ...

and transversely flattened bodies, heavily adapted for fast swimming, it would have been difficult, if not impossible, for ichthyosaurs to move far enough on land to lay eggs. This was confirmed as early as 9 December 1845 when naturalist

Joseph Chaning Pearce Joseph is a common male given name, derived from the Hebrew Yosef (יוֹסֵף). "Joseph" is used, along with "Josef", mostly in English, French and partially German languages. This spelling is also found as a variant in the languages of the mo ...

reported a small embryo in a fossil of ''Ichthyosaurus communis''. The embryo, with a length of eleven centimetres, was positioned in the birth canal of its two-and-a-half metre long mother, with its head pointed to the rear. Pearce concluded from the fossil that ichthyosaurs had to have been viviparous. Maternal Chaohusaurus

Later, from the Holzmaden deposits numerous adult fossils were found containing fetuses. In 1880,

Harry Govier Seeley Harry Govier Seeley (18 February 1839 – 8 January 1909) was a British paleontologist. Early life Seeley was born in London on 18 February 1839, the second son of Richard Hovill Seeley, a goldsmith, and his second wife Mary Govier. When his fat ...

, heading a special British paleontological committee studying the problem of ichthyosaur reproduction, concluded that birth was given in the water and that fossils containing fetuses in the birth canal probably represented cases of premature death of the juvenile, causing the demise of the mother animal as well. A comparison has been made with dolphins and whales, whose young need to be born tail-first to prevent drowning; if the juvenile is born head-first, it dies and the mother with it if the corpse gets stuck in the birth canal. However, an alternative explanation is that such fossils actually represent females that had died for other reasons while pregnant, after which the decomposition gasses drove out the fetuses head-first. In 2014, a study reported the find of a fossilized '' Chaohusaurus'' female that had died while giving birth to three neonates. Two had already been expelled while a third was present in the birth canal. The fossil also documented that early ichthyosaurs were also born head first, perhaps opposed to later genera. As ''Chaohusaurus'' is a very basal ichthypterygian—previously, the most basal genus of which fetuses were known, had been ''

''—this discovery suggests that the earliest land-dwelling ancestors of ichthyosaurs had already been viviparous. Ichthyosauria offspring in Vienna

Compared with placental mammals or plesiosaurs, ichthyosaur fetuses tend to be very small and their number per litter is often high. In one female of ''Stenopterygius'' seven have been identified, in another eleven. The fetuses have at most a quarter of the length of the mother animal. The juveniles have about the same body proportions as adult individuals. The main ontogenetical changes during growth consist in the fusion and greater robustness of the skeletal elements.

Crocodile Crocodiles (family Crocodylidae) or true crocodiles are large semiaquatic reptiles that live throughout the tropics in Africa, Asia, the Americas and Australia. The term crocodile is sometimes used even more loosely to include all extant mem ...

s, most sea turtles and some

lizard Lizards are a widespread group of squamate reptiles, with over 7,000 species, ranging across all continents except Antarctica, as well as most oceanic island chains. The group is paraphyletic since it excludes the snakes and Amphisbaenia althou ...

s determine the sex of their offspring by manipulating the temperature of the developing eggs' environment; i.e. they do not have distinct sex chromosomes. Live-bearing reptiles do not regulate sex through incubation temperature. A study in 2009, which examined 94 living species of reptiles, birds and mammals, found that the genetic control of sex appears to be crucial to live birth. It was concluded that with marine reptiles such control predated viviparity and was an adaptation to the stable sea-climate in coastal regions. Genetics likely controlled sex in ichthyosaurs, mosasaurs and plesiosaurs.

Social behaviour and intelligence

Ichthyosaurs are often assumed to have lived in herds or hunting groups. Little evidence is available about the nature of ichthyosaur

social behaviour Social behavior is behavior among two or more organisms within the same species, and encompasses any behavior in which one member affects the other. This is due to an interaction among those members. Social behavior can be seen as similar to ...

. Some indications exist that a level of sexual dimorphism was present. Skeletons of ''Eurhinosaurus'' and ''Shastasaurus'' show two morphotypes. Individuals with a longer snout, larger eyes, a longer trunk, a shorter tail, and longer flippers with additional phalanges, could have represented the females; the longer trunk may have provided room for the embryos. Generally, the brain shows the limited size and elongated shape of that of modern cold-blooded reptiles. However, in 1973, McGowan, while studying the natural

endocast An endocast is the internal cast of a hollow object, often referring to the cranial vault in the study of brain development in humans and other organisms. Endocasts can be artificially made for examining the properties of a hollow, inaccessible sp ...

of a well-preserved specimen, pointed out that the

telencephalon The cerebrum, telencephalon or endbrain is the largest part of the brain containing the cerebral cortex (of the two cerebral hemispheres), as well as several subcortical structures, including the hippocampus, basal ganglia, and olfactory bulb. ...

was not very small. The visual lobes were large, as could be expected from the eye size. The olfactory lobes were, though not especially large, well-differentiated; the same was true of the

cerebellum The cerebellum (Latin for "little brain") is a major feature of the hindbrain of all vertebrates. Although usually smaller than the cerebrum, in some animals such as the mormyrid fishes it may be as large as or even larger. In humans, the cerebe ...

Pathologies

Though fossils revealing ichthyosaur behavior remain rare, one ichthyosaur fossil is known to have sustained bites to the snout region. Discovered in Australia, and analyzed by Benjamin Kear ''et alii'' in 2011, measurements of the wounds reveal that the bite marks were inflicted by another ichthyosaur, likely of the same species, a probable case of face biting during a conflict. The wounds show signs of healing in the form of bone growth, meaning that the victim survived the attack. Another, very large ichthyosaur close to nine metres in length was found in Svalbard; it was nearly complete save for its tail. Scrutiny of the find revealed that while hunting ammonites (as evidenced by an ammonite shell in the throat region), the ichthyosaur was ambushed and attacked, likely by a

pliosaurid Pliosauridae is a family of plesiosaurian marine reptiles from the Latest Triassic to the early Late Cretaceous (Rhaetian to Turonian stages) of Australia, Europe, North America and South America. The family is more inclusive than the archety ...

(known from the same habitat), which severed its tail. The ichthyosaur then sank to the depths, drowning and eventually becoming fossilized in the deep water. The find was revealed to the public in the National Geographic special ''Death of a Sea Monster''.James Woods, 2011, ''Death of a Sea Monster'', DVD-R, National Geographic Channel, 45 m.

Geological formations

The following is a list of

geological formations A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics (lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock expo ...

in which ichthyosaur fossils have been found:

References

Sources

* Ellis, Richard, (2003) ''Sea Dragons – Predators of the Prehistoric Oceans''. University Press of Kansas. . * * McGowan, Christopher (1992). ''Dinosaurs, Spitfires and Sea Dragons''. Harvard University Press. . * McGowan, Christopher & Motani, Ryosuke (2003). "Ichthyopterygia, ''Handbook of Paleoherpetology'', Part 8, Verlag Dr. Friedrich Pfeil. * Motani, R. (1997). "Temporal and spatial distribution of tooth implantation in ichthyosaurs", in JM Callaway & EL Nicholls (eds.), ''Ancient Marine Reptiles''.

Academic Press Academic Press (AP) is an academic book publisher founded in 1941. It was acquired by Harcourt, Brace & World in 1969. Reed Elsevier bought Harcourt in 2000, and Academic Press is now an imprint of Elsevier. Academic Press publishes referen ...

. pp. 81–103. *

External links

USMP Berkeley's ichthyosaur introductionRyosuke Motani's detailed Ichthyosaur homepage, with vivid graphics
{{Authority control 01 Extinct reptiles Mesozoic reptiles Triassic reptiles Jurassic reptiles Cretaceous reptiles Olenekian first appearances Turonian extinctions Taxa named by Henri Marie Ducrotay de Blainville