Acamptonectes
   HOME

TheInfoList



OR:

''Acamptonectes'' is a
genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...
of
ophthalmosaurid 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 e ...
ichthyosaur Ichthyosauria is an order of large extinct marine reptiles sometimes referred to as "ichthyosaurs", although the term is also used for wider clades in which the order resides. Ichthyosaurians thrived during much of the Mesozoic era; based on fo ...
s, a type of dolphin-like
marine reptile Marine reptiles are reptiles which have become secondarily adapted for an aquatic or semiaquatic life in a marine environment. Only about 100 of the 12,000 extant reptile species and subspecies are classed as marine reptiles, including mari ...
s, that lived during the
Early Cretaceous The Early Cretaceous (geochronology, geochronological name) or the Lower Cretaceous (chronostratigraphy, chronostratigraphic name) is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 143.1 ...
around 130 million years ago. The first specimen, a partial adult skeleton, was discovered in
Speeton Speeton is a village in the civil parish of Reighton, in North Yorkshire, England. It lies near the edge of the coastal cliffs midway between Filey and Bridlington. It is North Yorkshire's easternmost settlement, but historically lay in the Ea ...
, England, in 1958, but was not formally described until 2012 by Valentin Fischer and colleagues. They also recognised a partial
subadult A juvenile is an individual organism (especially an animal) that has not yet reached its adult form, sexual maturity or size. Juveniles can look very different from the adult form, particularly in colour, and may not fill the same niche as t ...
skeleton belonging to the genus from
Cremlingen Cremlingen is a municipality in the district of Wolfenbüttel, Lower Saxony, Germany. It borders Lehre, Königslutter, Sickte, and Braunschweig. Parts of it are adjacent to the Elm hills. History From May 8, 1944 until April 12, 1945 a concent ...
, Germany, and specimens from other localities in England. The genus contains the single
species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...
''Acamptonectes densus''; the generic name means "rigid swimmer" and the specific name means "compact" or "tightly packed". A small ichthyosaur, ''Acamptonectes'' is estimated to have been long. The generic name refers to unusual adaptations in the body of ''Acamptonectes'' that made its trunk rigid, including tightly fitting bones in the
occiput The occipital bone () is a cranial dermal bone and the main bone of the occiput (back and lower part of the skull). It is trapezoidal in shape and curved on itself like a shallow dish. The occipital bone lies over the occipital lobes of the ...
(back and lower part of the skull) and interlocking vertebral centra ("bodies" of the vertebrae), which were likely adaptations that enabled it to swim at high speeds with a tuna-like form of locomotion. Other distinguishing characteristics include an extremely slender snout and unique ridges on the basioccipital bone of the braincase. As an ichthyosaur, ''Acamptonectes'' had large
eye sockets In anatomy, the orbit is the cavity or socket/hole 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 ...
and a
tail fluke A fin is a thin component or appendage attached to a larger body or structure. Fins typically function as foil (fluid mechanics), foils that produce lift (force), lift or thrust, or provide the ability to steer or stabilize motion while travelin ...
. ''Acamptonectes'' was similar in
morphology Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines *Morphology (archaeology), study of the shapes or forms of artifacts *Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies, ...
to the related but earlier ophthalmosaurines ''
Ophthalmosaurus ''Ophthalmosaurus'' (Greek ὀφθάλμος ''ophthalmos'' 'eye' and σαῦρος ''sauros'' 'lizard') is a genus of ichthyosaur known from the Middle-Late Jurassic. Possible remains from the earliest Cretaceous, around 145 million years ago, a ...
'' and ''
Mollesaurus ''Mollesaurus'' is an extinct genus of large ophthalmosaurine ichthyosaur known from northwestern Patagonia of Argentina. Etymology ''Mollesaurus'' was named by Marta S. Fernández in 1999 and the type species is ''Mollesaurus periallus''. T ...
''. The discovery of ''Acamptonectes'' had significant implications for the evolutionary history of ichthyosaurs. The
generalised A generalization is a form of abstraction whereby common properties of specific instances are formulated as general concepts or claims. Generalizations posit the existence of a domain or set of elements, as well as one or more common characteri ...
platypterygiine Platypterygiinae is an extinct subfamily of ophthalmosaurid thunnosaur ichthyosaurs from the early Late Jurassic to the early Late Cretaceous (Kimmeridgian - Cenomanian) of Asia, Australia, Europe, North America and South America. Currently, the ...
ophthalmosaurids were long believed to be the only lineage of ichthyosaurs that survived into the Early Cretaceous following a
mass extinction An extinction event (also known as a mass extinction or biotic crisis) is a widespread and rapid decrease in the biodiversity on Earth. Such an event is identified by a sharp fall in the diversity and abundance of multicellular organisms. It occ ...
of ichthyosaurs across the
Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...
boundary. As one of the first-known ophthalmosaurine ophthalmosaurids from the Early Cretaceous, the discovery of ''Acamptonectes'' provided evidence against such a mass extinction. Although the larger eyes of ''Acamptonectes'' would have made it better adapted than platypterygiines to depth diving, it was probably a generalist predator. Its teeth, which were slender and textured with longitudinal ridges, were adapted for impaling prey, which suggests it likely fed on soft, fleshy prey such as fish and squid.


History of discovery

Over a series of weekends in
1958 Events January * January 1 – The European Economic Community (EEC) comes into being. * January 3 – The West Indies Federation is formed. * January 4 ** Edmund Hillary's Commonwealth Trans-Antarctic Expedition completes the thir ...
, four students and a technician from
Hull University The University of Hull is a public research university in Kingston upon Hull, a city in the East Riding of Yorkshire, England. It was founded in 1927 as University College Hull. The main university campus is located in Hull and is home to the Hu ...
's geology department collected a fossil specimen from the
Speeton Clay Formation The Speeton Clay Formation (SpC)Speeton Clay Formation
- Speeton Speeton is a village in the civil parish of Reighton, in North Yorkshire, England. It lies near the edge of the coastal cliffs midway between Filey and Bridlington. It is North Yorkshire's easternmost settlement, but historically lay in the Ea ...
in northern England. The fossil belonged to an
ichthyosaur Ichthyosauria is an order of large extinct marine reptiles sometimes referred to as "ichthyosaurs", although the term is also used for wider clades in which the order resides. Ichthyosaurians thrived during much of the Mesozoic era; based on fo ...
or "fish lizard", a
Mesozoic The Mesozoic Era is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...
group of marine reptiles; it consists of a partial adult skeleton that includes a fragmentary
skull roof The skull roof or the roofing bones of the skull are a set of bones covering the brain, eyes and nostrils in bony fishes, including land-living vertebrates. The bones are derived from dermal bone and are part of the dermatocranium. In com ...
, a
mandible In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone i ...
,
vertebra Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spina ...
e,
rib In vertebrate anatomy, ribs () are the long curved bones which form the rib cage, part of the axial skeleton. In most tetrapods, ribs surround the thoracic cavity, enabling the lungs to expand and thus facilitate breathing by expanding the ...
s, and the scapular girdle (the shoulder area). In 1991, it was transferred to the
Hunterian Museum The Hunterian is a complex of museums located in and operated by the University of Glasgow in Glasgow, Scotland. It is the oldest museum in Scotland. It covers the Hunterian Museum, the Hunterian Art Gallery, the Mackintosh House, the Zoology M ...
of the
University of Glasgow The University of Glasgow (abbreviated as ''Glas.'' in Post-nominal letters, post-nominals; ) is a Public university, public research university in Glasgow, Scotland. Founded by papal bull in , it is the List of oldest universities in continuous ...
(GLAHM) when the geology department of Hull University was closed. It was catalogued under the specimen number GLAHM 132855, and was also known as the "Speeton Clay ichthyosaur". Palaeontologist Robert M. Appleby described the specimen and assigned it to the genus ''
Platypterygius ''Platypterygius'' is a historically paraphyletic genus of platypterygiine ichthyosaur from the Cretaceous period. It was historically used as a wastebasket taxon, and most species within ''Platypterygius'' likely are undiagnostic at the genus or ...
'' as the species "''P. speetoni''" (which he considered primitive within that genus), in a
monograph A monograph is generally a long-form work on one (usually scholarly) subject, or one aspect of a subject, typically created by a single author or artist (or, sometimes, by two or more authors). Traditionally it is in written form and published a ...
that remained unpublished at the time of his death in 2003. A second specimen of the species was found in
1985 The year 1985 was designated as the International Youth Year by the United Nations. Events January * January 1 ** The Internet's Domain Name System is created. ** Greenland withdraws from the European Economic Community as a result of a n ...
, also in the Speeton Clay, and is catalogued as NHMUK R11185 at the
Natural History Museum, London The Natural History Museum in London is a museum that exhibits a vast range of specimens from various segments of natural history. It is one of three major museums on Exhibition Road in South Kensington, the others being the Science Museum (Lo ...
(NHMUK). This specimen consists of a partial
rostrum Rostrum may refer to: * Any kind of a platform for a speaker: **dais **pulpit ** podium * Rostrum (anatomy), a beak, or anatomical structure resembling a beak, as in the mouthparts of many sucking insects * Rostrum (ship), a form of bow on naval ...
(snout) and mandible, fragmentary ribs, and a complete right
humerus The humerus (; : humeri) 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 (bone), radius and ulna, and consists of three sections. The humeral upper extrem ...
(upper-arm bone). Palaeontologist Jeff Liston recognised the significance of the Speeton Clay ichthyosaur while working at the Hunterian Museum, and Appleby's widow Valerie asked him to help finish Appleby's unpublished monograph. Liston approached ichthyosaur specialist Valentin Fischer about writing a description of the animal. Fischer examined the specimen in 2011 and realised it represented the same ichthyosaur as a specimen from
Cremlingen Cremlingen is a municipality in the district of Wolfenbüttel, Lower Saxony, Germany. It borders Lehre, Königslutter, Sickte, and Braunschweig. Parts of it are adjacent to the Elm hills. History From May 8, 1944 until April 12, 1945 a concent ...
in northern Germany about which he had recently written a draft paper with several colleagues. This German specimen was discovered in
2005 2005 was designated as the International Year for Sport and Physical Education and the International Year of Microcredit. The beginning of 2005 also marked the end of the International Decade of the World's Indigenous Peoples, Internationa ...
when private fossil collector Hans-Dieter Macht found some vertebrae in a construction area. Macht notified the director of the State Natural History Museum of Braunschweig (SNHM), whereafter excavation began; the specimen was collected within three days because construction work had to continue. It was
prepared The Scout Motto of the Scout movement is, in English, "Be Prepared", with most international branches of the group using a close translation of that phrase. These mottoes have been used by millions of Scouts around the world since 1907. Most of t ...
and mounted in 2005 at the museum, where it is catalogued as SNHM1284-R. It consists of a partial skeleton of a subadult and includes a fragmentary skull roof, a complete mandible, a partial axial skeleton, and a partial scapular girdle. It was assigned to the genus ''Platypterygius'' in a 2008 article. Historically, the genus ''Platypterygius'' has been treated as a catch-all
wastebasket taxon Wastebasket taxon (also called a wastebin taxon, dustbin taxon or catch-all taxon) is a term used by some taxonomists to refer to a taxon that has the purpose of classifying organisms that do not fit anywhere else. They are typically defined by e ...
for Cretaceous ichthyosaurs which contained multiple distantly-related species. Liston and Fischer recognised the specimens were distinct from other species referred to ''Platypterygius'' and belonged to a new species and genus. After determining the Speeton Clay specimen is much larger than the Cremlingen specimen and thus likely more mature, Liston and Fischer decided to make it the
holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...
of the new species because juvenile specimens often have characteristics absent in adults. The Cremlingen specimen and the other Speeton Clay specimen became
paratype In zoology and botany, a paratype is a specimen of an organism that helps define what the scientific name of a species and other taxon actually represents, but it is not the holotype (and in botany is also neither an isotype (biology), isotype ...
s (additional specimens in the series of type specimens). In 2012, a team of palaeontologists led by Fischer formally named the new genus and species ''Acamptonectes densus''. The generic name is derived from the Greek words ''akamptos'' and ''nektes'', which together mean "rigid swimmer"; the specific name means "compact" or "tightly packed". In full, the
scientific name In Taxonomy (biology), taxonomy, binomial nomenclature ("two-term naming system"), also called binary nomenclature, is a formal system of naming species of living things by giving each a name composed of two parts, both of which use Latin gramm ...
refers to the robust, tightly fitting bones of the
occiput The occipital bone () is a cranial dermal bone and the main bone of the occiput (back and lower part of the skull). It is trapezoidal in shape and curved on itself like a shallow dish. The occipital bone lies over the occipital lobes of the ...
(back and lower part of the skull) and the tightly interlocking centra ("bodies") of the cervical (neck) and
dorsal Dorsal (from Latin ''dorsum'' ‘back’) may refer to: * Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism * Dorsal, positioned on top of an aircraft's fuselage The fus ...
(back) vertebrae. The holotype, GLAHM 132855, was listed under the incorrect specimen number GLAHM 132588 in the original version of the description. Multiple basioccipitals (a bone at the lower part of the occiput),
stapes The ''stapes'' or stirrup is a bone in the middle ear of humans and other tetrapods which is involved in the conduction of sound vibrations to the inner ear. This bone is connected to the oval window by its annular ligament, which allows the f ...
(one of the ear bones), and a basisphenoid (a bone within the lower part of the
braincase In human anatomy, the neurocranium, also known as the braincase, brainpan, brain-pan, or brainbox, is the upper and back part of the skull, which forms a protective case around the brain. In the human skull, the neurocranium includes the calv ...
) from the Cambridge Greensand Formation of
Cambridge Cambridge ( ) is a List of cities in the United Kingdom, city and non-metropolitan district in the county of Cambridgeshire, England. It is the county town of Cambridgeshire and is located on the River Cam, north of London. As of the 2021 Unit ...
, England, were also assigned to ''Acamptonectes'' sp. (i.e., an uncertain species within the genus ''Acamptonectes'') by Fischer and colleagues in 2012. This assumed that ''Acamptonectes'' was the only Cretaceous ichthyosaur of the
subfamily In biological classification, a subfamily (Latin: ', plural ') is an auxiliary (intermediate) taxonomic rank, next below family but more inclusive than genus. Standard nomenclature rules end botanical subfamily names with "-oideae", and zo ...
Ophthalmosaurinae Ophthalmosaurinae is an extinct subfamily of ophthalmosaurid thunnosaur ichthyosaurs from the Middle Jurassic to the late Early Cretaceous (Bajocian - Albian) of Europe, North America and South America. Currently, the oldest and the basalmost, kn ...
from
Eurasia Eurasia ( , ) is a continental area on Earth, comprising all of Europe and Asia. According to some geographers, Physical geography, physiographically, Eurasia is a single supercontinent. The concept of Europe and Asia as distinct continents d ...
. Some of the specimens are housed at the
Sedgwick Museum of Earth Sciences The Sedgwick Museum of Earth Sciences, is the geology museum of the University of Cambridge. It is part of the Department of Earth Sciences and is located on the university's Downing Site in Downing Street, central Cambridge, England. The Sedg ...
,
University of Cambridge The University of Cambridge is a Public university, public collegiate university, collegiate research university in Cambridge, England. Founded in 1209, the University of Cambridge is the List of oldest universities in continuous operation, wo ...
(CAMSM), and several others are housed at the Hunterian Museum and the Natural History Museum. Several of the bones are essentially identical to those of ''A. densus'', while others differ in some details. The bones are generally small so their differences from ''A. densus'' were considered to be related to either the ages of the animals or evolutionary changes. In 2014, Fischer and colleagues identified a basioccipital and humerus belonging to ''
Ophthalmosaurus ''Ophthalmosaurus'' (Greek ὀφθάλμος ''ophthalmos'' 'eye' and σαῦρος ''sauros'' 'lizard') is a genus of ichthyosaur known from the Middle-Late Jurassic. Possible remains from the earliest Cretaceous, around 145 million years ago, a ...
'' (or a closely related ichthyosaur) from
Berriasian In the geological timescale, the Berriasian is an age/ stage of the Early/Lower Cretaceous. It is the oldest subdivision in the entire Cretaceous. It has been taken to span the time between 143.1 ±0.6 Ma and 137.05 ± 0.2 (million years ago) ...
-aged rocks (dating to between 145 and 139.8 million years old) near
Nettleton, Lincolnshire Nettleton is a village and civil parish in the West Lindsey district of Lincolnshire, England. It is situated less than south-west from the town of Caistor. At the 2001 census, the village had a population of 579. Nettleton contains a village ...
. Therefore, since their prior assumption no longer held, Fischer and colleagues re-listed the Cambridge Greensand specimens as belonging to indeterminate ophthalmosaurines that are not identifiable below the subfamily level. Additional ''Acamptonectes'' remains known may also include an ichthyosaur specimen from
Hannover Hanover ( ; ; ) is the capital and largest city of the States of Germany, German state of Lower Saxony. Its population of 535,932 (2021) makes it the List of cities in Germany by population, 13th-largest city in Germany as well as the fourth-l ...
, Germany. In 1909, German palaeontologist Ferdinand Broili named it as a new species of the genus ''
Ichthyosaurus ''Ichthyosaurus'' (derived from Greek () meaning 'fish' and () meaning 'lizard') is a genus of ichthyosaurs from the Early Jurassic (Hettangian - Pliensbachian) of Europe (Belgium, England, Germany and Portugal). Some specimens of the ichthy ...
'', ''Ichthyosaurus brunsvicensis'', but considered its assignment to the genus tentative. Broili's specimen consisted of an incomplete
basicranium The base of skull, also known as the cranial base or the cranial floor, is the most inferior area of the skull. It is composed of the endocranium and the lower parts of the calvaria. Structure Structures found at the base of the skull are for ...
(skull base) and an incomplete
interclavicle An interclavicle is a bone which, in most tetrapods, is located between the clavicles. Therian mammals ( marsupials and placentals) are the only tetrapods which never have an interclavicle, although some members of other groups also lack one. In ...
(a bone between the
clavicles The clavicle, collarbone, or keybone is a slender, S-shaped long bone approximately long that serves as a strut between the shoulder blade and the sternum (breastbone). There are two clavicles, one on each side of the body. The clavicle is the ...
, or collar bones); however, the specimen was destroyed during
World War II World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...
. Palaeontologist Christopher McGowan regarded it as a member of ''Platypterygius'' in 1972 and 2003 but Fischer and colleagues assigned it to
cf. The abbreviation cf. (short for either Latin or , both meaning 'compare') is generally used in writing to refer the reader to other material to make a comparison with the topic being discussed. However some sources offer differing or even contr ...
''Acamptonectes'' (i.e., possibly ''Acamptonectes'' or a related animal). They found it similar in several features to ''Acamptonectes'' but also different in others; they suggested the specimen was a juvenile because of the size and shape of its basicranium. Due to its fragmentary and inaccessible nature, they considered ''I. brunsvicensis'' a ''
nomen dubium In binomial nomenclature, a ''nomen dubium'' (Latin for "doubtful name", plural ''nomina dubia'') is a scientific name that is of unknown or doubtful application. Zoology In case of a ''nomen dubium,'' it may be impossible to determine whether a ...
'' (dubious name).


Description

''Acamptonectes'' was a small ichthyosaur, with an estimated body length of . Like other ichthyosaurs, it had a long, thin snout, large eye sockets, and a
tail fluke A fin is a thin component or appendage attached to a larger body or structure. Fins typically function as foil (fluid mechanics), foils that produce lift (force), lift or thrust, or provide the ability to steer or stabilize motion while travelin ...
that was supported by vertebrae in the lower half. Ichthyosaurs were superficially similar to
dolphin A dolphin is an aquatic mammal in the cetacean clade Odontoceti (toothed whale). Dolphins belong to the families Delphinidae (the oceanic dolphins), Platanistidae (the Indian river dolphins), Iniidae (the New World river dolphins), Pontopori ...
s and had flippers rather than legs, and most (except for early species) had
dorsal fin A dorsal fin is a fin on the back of most marine and freshwater vertebrates. Dorsal fins have evolved independently several times through convergent evolution adapting to marine environments, so the fins are not all homologous. They are found ...
s. Although the colour of ''Acamptonectes'' is unknown, at least some ichthyosaurs may have been uniformly dark-coloured in life, which is evidenced by the discovery of high concentrations of
eumelanin Melanin (; ) is a family of biomolecules organized as oligomers or polymers, which among other functions provide the pigments of many organisms. Melanin pigments are produced in a specialized group of cells known as melanocytes. There are ...
pigments in the preserved skin of an early ichthyosaur fossil. ''Acamptonectes'' was similar in
morphology Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines *Morphology (archaeology), study of the shapes or forms of artifacts *Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies, ...
to the related but earlier ophthalmosaurines ''Ophthalmosaurus'' and ''
Mollesaurus ''Mollesaurus'' is an extinct genus of large ophthalmosaurine ichthyosaur known from northwestern Patagonia of Argentina. Etymology ''Mollesaurus'' was named by Marta S. Fernández in 1999 and the type species is ''Mollesaurus periallus''. T ...
''. Features of the humerus in specimen SNHM1284-R are indicative of its immaturity; however, the humerus lacks the sandpaper-like texture of the humeral shafts (shafts of the upper arm bones) of juvenile ichthyosaurs and is thus thought to represent a subadult. The holotype and specimen NHMUK R11185 are large compared to other members of the wider
family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...
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 ...
; the holotype is thought to have been an adult because of the extensive fusion of its bones, including within the occiput, and the smooth texture of the humerus.


Skull

The snout of ''Acamptonectes'' was elongated and extremely slender; in the holotype, it is only wide in front of the bony nostrils. The snout was also only 0.044 times as deep as it was long, one of the lowest ratios among ophthalmosaurids. Much of the snout was 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 mammals h ...
e, which formed the front portion of the upper jaw. The fossa praemaxillaris, a groove that ran parallel to the tooth row of the upper jaw, was deep and continuous, and ended in a series of aligned
foramina In anatomy and osteology, a foramen (; : foramina, or foramens ; ) is an opening or enclosed gap within the dense connective tissue (bones and deep fasciae) of extant and extinct amniote animals, typically to allow passage of nerves, arter ...
(depressions). Behind and above the premaxillae were the
nasal Nasal is an adjective referring to the nose, part of human or animal anatomy. It may also be shorthand for the following uses in combination: * With reference to the human nose: ** Nasal administration, a method of pharmaceutical drug delivery * ...
s, which the holotype preserves in three dimensions, documenting the shape of the upper side of the snout. The back part of the nasal had a downward-extending bulge that was similar to that of related genera such as ''Ophthalmosaurus''. This bulge gave rise to a short, robust, wing-like extension that formed an overhang over the rear of the bony nostril; this feature was also present in ''Ophthalmosaurus'' and ''Platypterygius australis''. The edge of this overhang was roughened, indicating this was probably the attachment site for a
soft tissue Soft tissue connective tissue, connects and surrounds or supports internal organs and bones, and includes muscle, tendons, ligaments, Adipose tissue, fat, fibrous tissue, Lymphatic vessel, lymph and blood vessels, fasciae, and synovial membranes. ...
structure. The back part of the skull roof is incompletely known from the hind part of the
lacrimal bone The lacrimal bones are two small and fragile bones of the facial skeleton; they are roughly the size of the little fingernail and situated at the front part of the medial wall of the orbit. They each have two surfaces and four borders. Several bon ...
(in front of the eye opening), the
postfrontal The postfrontal is a paired cranial bone found in many tetrapods. It occupies an area of the skull roof between and behind the orbits (eye sockets), lateral to the frontal and parietal bones, and anterior to the postorbital bone. The postfrontal ...
(above and behind the eye opening), the parietal (at the rear of the skull roof), and parts of a
supratemporal The supratemporal bone is a paired Skull, cranial bone present in many Tetrapod, tetrapods and Tetrapodomorpha, tetrapodomorph fish. It is part of the temporal region (the portion of the skull roof behind the eyes), usually lying medial (inwards) re ...
that formed the rear corners of the skull roof. A forward-directed extension of the supratemporal formed the internal rear edge of the
supratemporal fenestra Temporal fenestrae are openings in the temporal region of the skull of some amniotes, behind the orbit (eye socket). These openings have historically been used to track the evolution and affinities of reptiles. Temporal fenestrae are commonly (al ...
, an opening in the skull roof situated behind the eyes. The parietal, which would have formed the inner margin of the supratemporal fenestra, had a convex front margin that would have interdigitated (interlocked) with either the frontal or postfrontal bones, which are not preserved in the known specimens. When viewed from the side, the
quadrate bone The quadrate bone is a skull bone in most tetrapods, including amphibians, sauropsids ( reptiles, birds), and early synapsids. In most tetrapods, the quadrate bone connects to the quadratojugal and squamosal bones in the skull, and forms up ...
, which connected to the lower jaw to form the jaw joint, was C-shaped. Two probable
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 verte ...
s (tongue bones) are preserved in specimen SNHM1284-R; these bones were rod-like with one spatula-shaped end. The stapes had a shaft that was more slender than in any other ichthyosaur, and its head was large and square; these features are regarded as an
autapomorphy In phylogenetics, an autapomorphy is a distinctive feature, known as a Synapomorphy, derived trait, that is unique to a given taxon. That is, it is found only in one taxon, but not found in any others or Outgroup (cladistics), outgroup taxa, not ...
—a characteristic that distinguishes the genus from related genera. The basisphenoid had a well-developed crest on its upper surface; this is considered another autapomorphy because this surface was a wide, flat plateau in other ichthyosaur species. At its front end, the basisphenoid was fused to the
parasphenoid The parasphenoid is a bone which can be found in the cranium of many vertebrates. It is an unpaired dermal bone which lies at the midline of the roof of the mouth. In many reptiles (including birds), it fuses to the endochondral (cartilage-derived ...
(another bone within the lower part of the braincase) and no suture (border between the two bones) can be seen. The
supraoccipital The occipital bone () is a cranial dermal bone and the main bone of the occiput (back and lower part of the skull). It is trapezoidal in shape and curved on itself like a shallow dish. The occipital bone lies over the occipital lobes of the cere ...
at the upper rear of the braincase (part of the skull which encloses the brain) was only weakly arched; it thus differed from those of ''Platypterygius'' and ''
Baptanodon ''Baptanodon'' is an ichthyosaur of the Late Jurassic period (160-156 million years ago), named for its supposed lack of teeth (although teeth of this genus have since been discovered). It had a graceful long dolphin-shaped body, and its jaws we ...
'', which were U-shaped. Below the supraoccipital were the two exoccipitals, which formed the sides of the
foramen magnum The foramen magnum () is a large, oval-shaped opening in the occipital bone of the skull. It is one of the several oval or circular openings (foramina) in the base of the skull. The spinal cord, an extension of the medulla oblongata, passes thro ...
(the canal for the
spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue that extends from the medulla oblongata in the lower brainstem to the lumbar region of the vertebral column (backbone) of vertebrate animals. The center of the spinal c ...
). Located further below was the basioccipital, which formed the floor of the foramen magnum. The midline canal that formed this floor was bordered by ridges, giving a bilobed appearance when seen from above; this is also regarded as an autapomorphy of the genus. Below the foramen magnum, the basioccipital formed 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 (anatomy), atlas vertebra. The condyles are oval or reniform (kidney-shaped) in shape ...
, which connected with the first vertebra of the neck to form the head joint. The occipital condyle was well-demarcated from the remainder of the bone by a constricted band, unlike most other ophthalmosaurids. The condyle was rounded and had visible growth rings, as in related genera. The opisthotics, which are on either side of the basioccipital, possessed extensions called the paroccipital processes which pointed backwards and upwards. These processes were elongated and slender in ''Acamptonectes'' and ''Ophthalmosaurus'', but short and stout in other ophthalmosaurids. The dentary (the tooth-bearing bone at the front of the lower jaw) was elongated, straight, and had a blunt front tip; this contrasts with the down-turned and beak-like tips of some ichthyosaurs in the subfamily Platypterygiinae. The splenial bones expanded in depth at their rear, forming the lower margin of the mandible and much of its midline surface. A groove similar in morphology to the fossa premaxilliaris of the upper jaw, termed the fossa dentalis, ran parallel to the dentary. Two ''Acamptonectes'' specimens lack the "3"-shaped upper surface of the
angular bone The angular is a large bone in the lower jaw (mandible) of amphibians and reptiles (birds included), which is connected to all other lower jaw bones: the dentary (which is the entire lower jaw in mammals), the splenial, the suprangular, and the ...
s that are otherwise typical of ophthalmosaurids; instead, the surface in these specimens consists of a simple, flat groove bordered by two walls. Because the "3" shape is present in the holotype specimen, however, this feature may have varied between individuals or growth stages. The
articular bone The articular bone is part of the lower jaw of most vertebrates, including most jawed fish, amphibians, birds and various kinds of reptiles, as well as ancestral mammals. Anatomy In most vertebrates, the articular bone is connected to two othe ...
in one specimen was stouter than those in other ophthalmosaurids, which were nearly as thick as it was long. The teeth of ''Acamptonectes'' had striated bases and quadrangular roots, as in numerous ophthalmosaurids, but they were not square like those of ''Platypterygius''. Some of the roots of SNHM1284-R had resorption pits, indicating its teeth were still growing. The only-known complete
tooth crown In dentistry, the crown is the visible part of the tooth above the gingival margin and is an essential component of dental anatomy. Covered by Tooth enamel, enamel, the crown plays a crucial role in cutting, tearing, and grinding food. Its shap ...
was small compared to those of other ophthalmosaurids; it was also slender and sharply pointed, and similar to the teeth from the rear of the jaw in ''Baptanodon''. The bottom two-thirds of the crown had subtle, longitudinal ridges and was covered in a coarse texture that was nevertheless finer than those in ''
Aegirosaurus ''Aegirosaurus'' is an extinct genus of platypterygiine ophthalmosaurid ichthyosaurs known from the late Jurassic and early Cretaceous of Europe. It was originally named as a species of '' Ichthyosaurus''. Discovery and species Originally des ...
'' and some ''Platypterygius'' specimens. The base of the crown was slightly bulbous and almost smooth, unlike those in other ophthalmosaurids.


Postcranial skeleton

As was typical for ichthyosaurs, the vertebral centra of ''Acamptonectes'' were disc-shaped and deeply concave on both ends. The processes (bony projections that serve as muscle and rib attachments) projecting from the centra were greatly reduced as an adaptation for its fully aquatic lifestyle. In ''Acamptonectes'', the front-most cervical (neck) centra were high and short, and the following cervical and dorsal (trunk) centra become progressively longer. In the rear dorsal vertebral column, the centra became shorter and higher; this trend peaked at the first caudal (tail) vertebra, which was 3.12 times as high as it was long. The remaining caudals became longer and lower again; the caudals, which comprised the fin, were as long as they were high, a feature that was previously identified only in ''P. platydactylus''. The first two cervicals—the
atlas An atlas is a collection of maps; it is typically a bundle of world map, maps of Earth or of a continent or region of Earth. Advances in astronomy have also resulted in atlases of the celestial sphere or of other planets. Atlases have traditio ...
and
axis An axis (: axes) may refer to: Mathematics *A specific line (often a directed line) that plays an important role in some contexts. In particular: ** Coordinate axis of a coordinate system *** ''x''-axis, ''y''-axis, ''z''-axis, common names ...
—were fused into a single complex that was wide when viewed from the rear. The front dorsal vertebrae have diapophyses (sideways-protruding processes to which ribs attach) fused to the centra; this feature was shared with several other ophthalmosaurids. The centra of the dorsal vertebrae were autapomorphic, being tightly interlocking and having extensive posterolateral lamellae (ridges lining the rear surfaces of the centra). This interlocking stiffened the front section of the vertebral column in conjunction with the strong occiput of the skull. Such stiffening can be observed in other ichthyosaurs belonging to the wider group
Thunnosauria Thunnosauria (Ancient Greek, 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) o ...
, though not to the degree as seen in ''Acamptonectes''. The neural arches of the vertebrae had narrow pre- and postzygapophyses (
articular processes The articular process or zygapophysis ( + apophysis) of a vertebra is a projection of the vertebra that serves the purpose of fitting with an adjacent vertebra. The actual region of contact is called the ''articular facet''.Moore, Keith L. et al. ...
projecting forward and backward from the centra) that were unpaired (fused into a single element) in all vertebrae. In contrast, in ''P. hercynicus'' and ''
Sveltonectes ''Sveltonectes'' (meaning "agile swimmer" in Greek) is an extinct genus of platypterygiine ophthalmosaurid ichthyosaurs known from Ul’yanovsk region, western Russia. History of discovery ''Sveltonectes'' is known from the holotype Royal Belgi ...
'', these processes were paired in the front of the vertebral column. The neural spines (large upward-projecting processes) were of variable height within each specimen; they were markedly longer in some dorsals than others, reaching 1.25 times the height of the largest centrum. These long spines may be bony extensions that are analogous to the extraneural processes, a row of bones located above the tops of the neural spines that are preserved in two juvenile ''
Stenopterygius ''Stenopterygius'' is an extinct genus of thunnosaur ichthyosaur known from Europe (England, France, Germany, Luxembourg and Switzerland). History ''Stenopterygius'' was originally named by Quenstedt in 1856 as a species of ''Ichthyosaurus'', '' ...
'' specimens. The top surfaces of the neural spines were often pitted, indicating they had a
cartilage Cartilage is a resilient and smooth type of connective tissue. Semi-transparent and non-porous, it is usually covered by a tough and fibrous membrane called perichondrium. In tetrapods, it covers and protects the ends of long bones at the joints ...
covering. The ribs were distinct in being robust with a round cross-section; this contrasted with the "8"-shaped cross-section that is seen in other thunnosaurian ichthyosaurs. The
coracoid A coracoid 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 present as part of the scapula, but this is n ...
(a paired bone in the scapular girdle) was roughly hexagonal, contrasting with the rounded shape in ''Platypterygius'', and had outer and midline edges that were straight and parallel. The upper and lower surfaces were slightly paddle-shaped, and the mid-line surface was eye-shaped as in ''Ophthalmosaurus'', although it was not as thick as those in ''Sveltonectes'' and ''P. australis''. The mid-line surface was unfinished and had deep pits, indicating the presence of a thick layer of cartilage. At the front, the mid-line margin was strongly deflected outward, forming the rugose (roughened and wrinkled) edge of a wide, sheet-like process similar to that in ''Ophthalmosaurus''. The process was separated from the scapular facet (articulation with the
scapula The scapula (: 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 side ...
) by a deep, wide notch, as in many ''Ophthalmosaurus'' specimens. The scapular facet of the coracoid was small, deeply pitted, and triangular while the
glenoid facet The glenoid fossa of the scapula or the glenoid cavity is a bone part of the shoulder. The word ''glenoid'' is pronounced or (both are common) and is from , "socket", reflecting the shoulder joint's ball-and-socket form. It is a shallow, pyrifo ...
(articulation with the humerus) was large and eye-shaped. These facets were not markedly separated, unlike those in ''Sveltonectes'', where they were set at an angle of 100°. The hind margin of the coracoid was sheet-like and lacked a notch. As with its coracoid, the scapula (shoulder blade) of ''Acamptonectes'' was similar to that of ''Ophthalmosaurus''. It was strongly compressed from side to side, unlike that in ''P. hercynicus'', in which the shaft was thick and rod-like. The lower part of the scapula was expanded from front to back, forming a wide, rugose, articular, tear-drop-shaped surface that articulated with the coracoid and glenoid facets. It had a large, flat, fan-like
acromial In human anatomy, the acromion (from Greek: ''akros'', "highest", ''ōmos'', "shoulder", : acromia) or summit of the shoulder is a bony process on the scapula (shoulder blade). Together with the coracoid process, it extends laterally over the shou ...
process at the front (which connected with the clavicle), like those in ''Ophthalmosaurus'' and ''P. americanus''. The coracoid facet of the scapula was triangular and continuous with the larger glenoid facet, as in ''Ophthalmosaurus'' but unlike ''P. australis''. The side and mid-line surfaces of the acromial process were slightly concave. The deltopectoral crest (to where the
deltoid muscle The deltoid muscle is the muscle forming the rounded contour of the shoulder, human shoulder. It is also known as the 'common shoulder muscle', particularly in other animals such as the domestic cat. Anatomically, the deltoid muscle is made up o ...
attached) on the upper-front part of the humerus was more prominent in ''Acamptonectes'' than in ''Ophthalmosaurus'' and ''
Arthropterygius ''Arthropterygius'' is a widespread genus of ophthalmosaurid ichthyosaur which existed in Canada, Norway, Russia, and Argentina from the late Jurassic period and possibly to the earliest Cretaceous. Description ''Arthropterygius'' appears to h ...
'', but less so than in ''Sveltonectes'' and ''Platypterygius''. On the opposite side of the upper humerus, the
trochanter A trochanter is a tubercle of the femur near its joint with the hip bone. In humans and most mammals, the trochanters serve as important muscle attachment sites. Humans have two, sometimes three, trochanters. Etymology The anatomical term ' ...
dorsalis (a
tubercle In anatomy, a tubercle (literally 'small tuber', Latin for 'lump') is any round nodule, small eminence, or warty outgrowth found on external or internal organs of a plant or an animal. In plants A tubercle is generally a wart-like projectio ...
or protrusion where muscles attached) was tall and narrow, as in ''Sveltonectes'' and many species of ''Platypterygius''. The humerus had three facets on its lower side, including a facet for a bone at the front and a backward-deflected facet for the
ulna The ulna or ulnar bone (: ulnae or ulnas) is a long bone in the forearm stretching from the elbow to the wrist. It is on the same side of the forearm as the little finger, running parallel to the Radius (bone), radius, the forearm's other long ...
, which was also similar to that of ''Ophthalmosaurus'', and had five articular processes. The expanded upper surface that articulated with the humerus was slightly concave and pitted, unlike that of ''Arthropterygius'', in which the ulna's humerus facet formed a pyramidal-shaped process. The facet for the
radius In classical geometry, a radius (: radii or radiuses) of a circle or sphere is any of the line segments from its Centre (geometry), center to its perimeter, and in more modern usage, it is also their length. The radius of a regular polygon is th ...
was straight and trapezoidal, and merged with facets for two wrist bones, the intermedium and the ulnare. The facet for the pisiform, another wrist bone, was small and triangular, and was located at the back of the ulna's lower side. The ulna had a concave and edge-like hind margin. The
phalanx bones The phalanges (: 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. Struct ...
(finger bones within the flipper) were oval as in ''Ophthalmosaurus'', ''Arthropterygius'', and some species of ''Brachypterygius''; they tapered away from the body, and the edges of the peripheral phalanx bones were irregular and slightly concave.


Classification

In 2012, a
phylogenetic analysis In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical data ...
conducted by Fischer and colleagues found ''Acamptonectes'' to be a member of the family Ophthalmosauridae based on several characteristics. These include: the reduced extracondylar area (a band of bone surrounding the occipital condyle), the plate-like dorsal trochanter of the humerus, the presence of a facet at the front of the humerus' bottom end for a paddle bone, and the lack of notching in the paddle bones that was considered to be homoplastic (independently acquired). It was also found to be more closely related to other ophthalmosaurids than ''Arthropterygius'' based on the large processes of the basipterygoids (bones at the base of the braincase), the lack of a peg on the basioccipital, and the large trochanters of the femur. Relationships within Ophthalmosauridae have historically been unstable in analyses due to the fragmentary nature of many ophthalmosaurid specimens; furthermore, many ophthalmosaurid genera are known from a single specimen. Removal of these fragmentary genera, however, has degraded the resolution of analyses even further. The phylogenetic analysis conducted by Fischer and colleagues in 2012 recovered two novel
clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...
s (groups) within Ophthalmosauridae; the Ophthalmosaurinae and Platypterygiinae, the existence of which had long been suspected by ichthyosaur researchers—Maxim Arkhangelsky had named the clades as subfamilies as early as 2001—but had not yet been supported robustly by the results of phylogenetic analyses. Fischer and colleagues placed ''Acamptonectes'' was placed in the former clade, although its placement there represented a secondary reversal of the group's only uniting characteristic; a notch on the bottom of the basioccipital. Within the Ophthalmosaurinae, various positions have been recovered for ''Acamptonectes'' due to the same issues. In 2012, Fischer and colleagues found that it grouped closest with ''"Ophthalmosaurus" natans'', with ''Ophthalmosaurus icenicus'' and ''Mollesaurus'' being successively less-closely related. The relationship with ''"O." natans'' was formed on account of the reduced presence of striations on the teeth, although Fischer and colleagues indicated this characteristic was homoplastic so they did not consider it sufficient to resurrect the previously used genus name ''Baptanodon'' for ''"O." natans''. In 2013, they recovered the same arrangement in a derivative analysis for the description of ''
Malawania ''Malawania'' is an extinct genus of basal thunnosaur ichthyosaur known from the middle Early Cretaceous (Hauterivian or Barremian stage) of Iraq. The type species is ''Malawania anachronus'' and it was named by Valentin Fischer, Robert M. Appl ...
'', as did Nikolay Zverkov and colleagues in a 2015 analysis focusing on '' Grendelius''—albeit with a clade consisting of ''
Cryopterygius ''Undorosaurus'' is an extinct genus of ophthalmosaurid ichthyosaur known from western Russia, Svalbard, and Poland. It was a large ichthyosaur, with the type species measuring long. Discovery and naming ''Undorosaurus'' was named by Vladimir ...
'', ''
Undorosaurus ''Undorosaurus'' is an extinct genus of ophthalmosaurid ichthyosaur known from western Russia, Svalbard, and Poland. It was a large ichthyosaur, with the type species measuring long. Discovery and naming ''Undorosaurus'' was named by Vladimir ...
'', and '' Paraophthalmosaurus'' being closer to ''Acamptonectes'' than ''Mollesaurus''. Arkhangelsky and Zverkov previously recovered all of these species with the exception of ''Mollesaurus'' in a
polytomy An internal node of a phylogenetic tree is described as a polytomy or multifurcation if (i) it is in a rooted tree and is linked to three or more child subtrees or (ii) it is in an unrooted tree and is attached to four or more branches. A tree ...
(unresolved clade) in 2014. A 2019 analysis by Zverkov and Vladimir Efimov found an otherwise identical arrangement, in which the positions of ''Mollesaurus'' and ''Acamptonectes'' were exchanged, which was also found in another 2019 analysis by Zverkov and Natalya Prilepskaya, and in the 2020 description of a new specimen of ''
Muiscasaurus ''Muiscasaurus'' is an extinct genus of ophthalmosaurid ichthyosaur that lived in modern Colombia during the Early Cretaceous. The only known species is the type ''Muiscasaurus catheti''. Discovery The fossils of ''Muiscasaurus'' were found in ...
'' by María Páramo-Fonseca and colleagues, in which ''Muiscasaurus'' was the next-closest relative of these species. In their description of '' Acuetzpalin'', a 2020 analysis by Jair Barrientos-Lara and Jesús Alvarado-Ortega found ''"O." natans'' and ''O. icenicus'' to form a clade with the exclusion of ''Mollesaurus'' and then ''Acamptonectes'', which was also recovered by Megan Jacobs and
David Martill {{Short pages monitor