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Mosasaurs (from Latin ''Mosa'' meaning the ' Meuse', and Greek ' meaning 'lizard') comprise a group of extinct, large marine reptiles from the Late Cretaceous. Their first fossil remains were discovered in a limestone quarry at Maastricht on the Meuse in 1764. They belong to the order Squamata, which includes
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 and snakes. Mosasaurs probably evolved from an extinct group of aquatic
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 known as
aigialosaur Aigialosauridae (from Greek, ''aigialos'' = "seashore" and ''sauros''= lizard) is a family of Late Cretaceous semiaquatic pythonomorph lizards closely related to the mosasaurs. Regarded by some paleontologists as a distinct monophyletic group an ...
s in the Earliest Late Cretaceous with 42 described genera. During the last 20 million years of the Cretaceous period ( TuronianMaastrichtian ages), with the extinction of the ichthyosaurs and pliosaurs, mosasaurs became the dominant marine predators. They themselves became extinct as a result of the K-Pg event at the end of the Cretaceous period, about 66 million years ago.


Description

Mosasaurs breathed air, were powerful swimmers, and were well-adapted to living in the warm, shallow
inland seas An inland sea (also known as an epeiric sea or an epicontinental sea) is a continental body of water which is very large and is either completely surrounded by dry land or connected to an ocean by a river, strait, or "arm of the sea". An inland se ...
prevalent during the Late Cretaceous period. Mosasaurs were so well adapted to this environment that they most likely gave birth to live young, rather than returning to the shore to lay eggs as sea turtles do. The smallest-known mosasaur was '' Dallasaurus turneri'', which was less than long. Larger mosasaurs were more typical, with many species growing longer than . '' Mosasaurus hoffmannii'', the largest known species reached up to , but it has been considered to be probably overestimated by Cleary ''et al.'' (2018). Currently, the largest publicly exhibited mosasaur skeleton in the world is on display at the
Canadian Fossil Discovery Centre The Canadian Fossil Discovery Centre, formerly known as the Morden and District Museum, is located in Morden, Manitoba in the lower level of the Access Events Centre. The museum currently houses the largest collection of marine reptile fossils i ...
in Morden, Manitoba. The specimen, nicknamed "Bruce", is just over long, but this might be an overestimate as the skeleton was assembled for display prior to a 2010 reassessment of the species that found its original number of vertebrae to be exaggerated, implying that the actual size of the animal was likely smaller. Mosasaurs had a body shape similar to that of modern-day
monitor lizard Monitor lizards are lizards in the genus ''Varanus,'' the only extant genus in the family Varanidae. They are native to Africa, Asia, and Oceania, and one species is also found in the Americas as an invasive species. About 80 species are recogn ...
s (varanids), but were more elongated and streamlined for swimming. Their limb bones were reduced in length and their paddles were formed by webbing between their long finger and toe bones. Their tails were broad, and supplied their locomotive power. Until recently, mosasaurs were assumed to have swum in a method similar to the one used today by
conger eel ''Conger'' ( ) is a genus of marine congrid eels. It includes some of the largest types of eels, ranging up to 2 m (6 ft) or more in length, in the case of the European conger. Large congers have often been observed by divers during ...
s and sea snakes, undulating their entire bodies from side to side. However, new evidence suggests that many advanced mosasaurs had large, crescent-shaped flukes on the ends of their tails, similar to those of sharks and some ichthyosaurs. Rather than use snake-like undulations, their bodies probably remained stiff to reduce drag through the water, while their tails provided strong propulsion. These animals may have lurked and pounced rapidly and powerfully on passing prey, rather than chasing after it. Early reconstructions showed mosasaurs with dorsal crests running the length of their bodies, which were based on misidentified remains of tracheal cartilage. By the time this error was discovered, depicting mosasaurs with such crests in artwork had already become a trend.


Paleobiology

Mosasaurs had double-hinged jaws and flexible skulls (much like those of snakes), which enabled them to gulp down their prey almost whole. A skeleton of ''
Tylosaurus ''Tylosaurus'' (from the ancient Greek (') 'protuberance, knob' + Greek (') 'lizard') is a genus of mosasaur, a large, predatory marine reptile closely related to modern monitor lizards and to snakes, from the Late Cretaceous. Description A ...
proriger'' from South Dakota included remains of the diving
seabird Seabirds (also known as marine birds) are birds that are adapted to life within the marine environment. While seabirds vary greatly in lifestyle, behaviour and physiology, they often exhibit striking convergent evolution, as the same envir ...
''
Hesperornis ''Hesperornis'' (meaning "western bird") is a genus of cormorant-like bird that spanned the first half of the Campanian age of the Late Cretaceous period (83.5–78 mya). One of the lesser-known discoveries of the paleontologist O. C. Marsh i ...
'', a marine
bony fish Osteichthyes (), popularly referred to as the bony fish, is a diverse superclass of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primarily composed of cartilag ...
, a possible shark, and another, smaller mosasaur (''Clidastes''). Mosasaur bones have also been found with shark teeth embedded in them. One of the food items of mosasaurs were
ammonites Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttlefis ...
, molluscs with shells similar to those of '' Nautilus'', which were abundant in the Cretaceous seas. Holes have been found in fossil shells of some ammonites, mainly ''
Pachydiscus ''Pachydiscus'' is an extinct genus of ammonite from the Late Cretaceous and Early Paleocene with a worldwide distribution, and type for the desmoceratacean family Pachydiscidae. The genus' type species is ''P. neubergicus''. Altogether some 28 s ...
'' and ''
Placenticeras ''Placenticeras'' is a genus of ammonites from the Late Cretaceous. Its fossils have been found in Asia, Europe, North and South America. Taxonomy ''Placenticeras'', named by Fielding Bradford Meek, 1870, is the type genus for the Placenticera ...
''. These were once interpreted as a result of limpets attaching themselves to the ammonites, but the triangular shape of the holes, their size, and their presence on both sides of the shells, corresponding to upper and lower jaws, is evidence of the bite of medium-sized mosasaurs. Whether this behaviour was common across all size classes of mosasaurs is not clear. Virtually all forms were active predators of fish and ammonites; a few, such as ''
Globidens ''Globidens'' ("Globe teeth") is an extinct genus of mosasaur lizard classified as part of the Globidensini tribe in the Mosasaurinae subfamily. ''Globidens alabamaensis'' was the first species of ''Globidens'' described, in a publication by Ch ...
'', had blunt, spherical teeth, specialized for crushing mollusk shells. The smaller genera, such as ''
Platecarpus ''Platecarpus'' ("flat wrist") is an extinct genus of aquatic lizards belonging to the mosasaur family, living around 84–81 million years ago during the middle Santonian to early Campanian, of the Late Cretaceous period. Fossils have been found ...
'' and ''
Dallasaurus ''Dallasaurus'' ("Dallas lizard") is a basal mosasauroid from the Upper Cretaceous of North America. Along with '' Russellosaurus'', ''Dallasaurus'' is one of the two oldest mosasauroid taxa currently known from North America. It is also the smal ...
'', which were about long, probably fed on fish and other small prey. The smaller mosasaurs may have spent some time in fresh water, hunting for food. The largest mosasaur ''Mosasaurus hoffmannii'' was the apex predator of the Late Cretaceous oceans, reaching more than in length and weighing up to in body mass.


Soft tissue

Despite the many mosasaur remains collected worldwide, knowledge of the nature of their skin coverings remains in its early stages. Few mosasaurid specimens collected from around the world retain fossilized scale imprints. This lack may be due to the delicate nature of the scales, which nearly eliminates the possibility of preservation, in addition to the preservation sediment types and the marine conditions under which the preservation occurred. Until the discovery of several mosasaur specimens with remarkably well-preserved scale imprints from late Maastrichtian deposits of the Muwaqqar Chalk Marl Formation of Harrana in Jordan, knowledge of the nature of mosasaur integument was mainly based on very few accounts describing early mosasaur fossils dating back to the upper
Santonian The Santonian is an age in the geologic timescale or a chronostratigraphic stage. It is a subdivision of the Late Cretaceous Epoch or Upper Cretaceous Series. It spans the time between 86.3 ± 0.7 mya (million years ago) and 83.6 ± 0.7 mya. T ...
–lower
Campanian The Campanian is the fifth of six ages of the Late Cretaceous Epoch on the geologic timescale of the International Commission on Stratigraphy (ICS). In chronostratigraphy, it is the fifth of six stages in the Upper Cretaceous Series. Campanian ...
, such as the famous ''
Tylosaurus ''Tylosaurus'' (from the ancient Greek (') 'protuberance, knob' + Greek (') 'lizard') is a genus of mosasaur, a large, predatory marine reptile closely related to modern monitor lizards and to snakes, from the Late Cretaceous. Description A ...
'' specimen (KUVP-1075) from Gove County, Kansas. Material from Jordan has shown that the bodies of mosasaurs, as well as the membranes between their fingers and toes, were covered with small, overlapping, diamond-shaped scales resembling those of snakes. Much like those of modern reptiles, mosasaur scales varied across the body in type and size. In Harrana specimens, two types of scales were observed on a single specimen: keeled scales covering the upper regions of the body and smooth scales covering the lower. As ambush predators, lurking and quickly capturing prey using stealth tactics, they may have benefited from the nonreflective, keeled scales. Additionally, mosasaurs had large pectoral girdles, and such genera as ''Plotosaurus'' may have used their front flippers in a breaststroke motion to gain added bursts of speed during an attack on prey. More recently, a fossil of ''
Platecarpus tympaniticus ''Platecarpus'' ("flat wrist") is an extinct genus of aquatic lizards belonging to the mosasaur family, living around 84–81 million years ago during the middle Santonian to early Campanian, of the Late Cretaceous period. Fossils have been found ...
'' has been found that preserved not only skin impressions, but also internal organs. Several reddish areas in the fossil may represent the heart, lungs, and kidneys. The trachea is also preserved, along with part of what may be the retina in the eye. The placement of the kidneys is farther forward in the abdomen than it is in monitor lizards, and is more similar to those of
cetacean 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 ...
s. As in cetaceans, the
bronchi A bronchus is a passage or airway in the lower respiratory tract that conducts air into the lungs. The first or primary bronchi pronounced (BRAN-KAI) to branch from the trachea at the carina are the right main bronchus and the left main bronchus ...
leading to the lungs run parallel to each other instead of splitting apart from one another as in monitors and other terrestrial reptiles. In mosasaurs, these features may be internal adaptations to fully marine lifestyles. In 2011, collagen protein was recovered from a ''
Prognathodon ''Prognathodon'' is an extinct genus of marine lizard belonging to the mosasaur family. It is classified as part of the Mosasaurinae subfamily, alongside genera like ''Mosasaurus'' and '' Clidastes''. ''Prognathodon'' has been recovered from dep ...
'' humerus dated to the Cretaceous. In 2005, a
case study A case study is an in-depth, detailed examination of a particular case (or cases) within a real-world context. For example, case studies in medicine may focus on an individual patient or ailment; case studies in business might cover a particular fi ...
by A.S. Schulp, E.W.A Mulder, and K. Schwenk outlined the fact that mosasaurs had paired
fenestra A fenestra (fenestration; plural fenestrae or fenestrations) is any small opening or pore, commonly used as a term in the biological sciences. It is the Latin word for "window", and is used in various fields to describe a pore in an anatomical st ...
e in their palates. In monitor lizards and snakes, paired fenestrae are associated with a
forked tongue A forked tongue is a tongue split into two distinct tines at the tip; this is a feature common to many species of reptiles. Reptiles smell using the tip of their tongue, and a forked tongue allows them to sense from which direction a smell is co ...
, which is flicked in and out to detect chemical traces and provide a directional sense of
smell Smell may refer to; * Odor, airborne molecules perceived as a scent or aroma * Sense of smell, the scent also known scientifically as olfaction * "Smells" (''Bottom''), an episode of ''Bottom'' * The Smell, a music venue in Los Angeles, Californ ...
. They therefore proposed that mosasaurs probably also had a sensitive forked tongue.


Metabolism

A study published in 2016 by T. Lyn Harrell, Alberto Pérez-Huerta and Celina Suarez showed that mosasaurs were endothermic. The study contradicted findings published in 2010 indicating mosasaurs were
ectotherm An ectotherm (from the Greek () "outside" and () "heat") is an organism in which internal physiological sources of heat are of relatively small or of quite negligible importance in controlling body temperature.Davenport, John. Animal Life a ...
ic. The 2010 study did not use warm-blooded animals for comparison but analogous groups of common marine animals. Based on comparisons with modern warm-blooded animals and fossils of known cold-blooded animals from the same time period, the 2016 study found mosasaurs likely had body temperatures similar to those of contemporary seabirds and were able to internally regulate their temperatures to remain warmer than the surrounding water.


Coloration

The coloration of mosasaurs was unknown until 2014, when the findings of Johan Lindgren of
Lund University , motto = Ad utrumque , mottoeng = Prepared for both , established = , type = Public research university , budget = SEK 9 billion 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 ...
in the fossilized scales of a mosasaur. Mosasaurs were likely
countershaded 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, an ...
, with dark backs and light underbellies, much like a great white shark or leatherback sea turtle, the latter of which had fossilized ancestors for which color was also determined. The findings were described in ''
Nature Nature, in the broadest sense, is the physical world or universe. "Nature" can refer to the phenomena of the physical world, and also to life in general. The study of nature is a large, if not the only, part of science. Although humans are p ...
''.


Teeth

Mosasaurs possessed a thecodont dentiton, meaning that the roots were cemented deeply into the jaw bone. Mosasaurs did not use permanent teeth but instead constantly shed them. Replacement teeth developed within a pit inside the roots of the original tooth called the resorption pit. This is done through a distinctively unique eight-stage process. The first stage was characterized by the mineralization of a small tooth crown developed elsewhere that descended into the resorption pit by the second stage. In the third stage, the developing crown firmly cemented itself within the resorption pit and grew in size; by the fourth stage, it would be of the same size as the crown in the original tooth. Stages five and six were characterized by the development of the replacement tooth's root: in stage five the root developed vertically, and in stage six the root expanded in all directions to the point that the replacement tooth became exposed and actively pushed on the original tooth. In the seventh stage, the original tooth was shed and the now-independent replacement tooth began to anchor itself into the vacancy. In the eighth and final stage, the replacement tooth has grown to firmly anchor itself.


Ontogeny and growth

Mosasaur growth is not well understood, as specimens of juveniles are rare, and many were mistaken for hesperornithine birds when discovered 100 years ago. However, the discovery of several specimens of juvenile and neonate-sized mosasaurs unearthed more than a century ago indicate that mosasaurs gave birth to live young, and that they spent their early years of life out in the open ocean, not in sheltered nurseries or areas such as shallow water as previously believed. Whether mosasaurs provided parental care, like other marine reptiles such as plesiosaurs, is currently unknown. The discovery of young mosasaurs was published in the journal ''Palaeontology''. In late 2014 Guinness World Records awarded the museum with a record for Largest Publicly Displayed Mosasaur – Bruce. The record was added to the 2016 print edition of ''Guinness World Records''.


Possible eggs

A 2020 study published in ''Nature'' described a large fossilized hatched egg from
Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest contine ...
from the very end of the Cretaceous, about 68 million years ago. The egg is considered one of the largest
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 ...
eggs ever known, rivalling that of the
elephant bird Elephant birds are members of the extinct ratite family Aepyornithidae, made up of flightless birds that once lived on the island of Madagascar. They are thought to have become extinct around 1000-1200 CE, probably as a result of human activity ...
, and due to its soft, thin, folded texture, it likely belonged to a marine animal. While the organism that produced it remains unknown, the egg's pore structure is very similar to that of extant lepidosaurs such as lizards and snakes, and presence of mosasaur fossils nearby indicates that it may have been a mosasaur egg. It is unknown whether the egg was laid on land or in the water. The egg was assigned to the newly described
oospecies Egg fossils are the fossilized remains of eggs laid by ancient animals. As evidence of the physiological processes of an animal, egg fossils are considered a type of trace fossil. Under rare circumstances a fossil egg may preserve the remains of t ...
''
Antarcticoolithus bradyi ''Antarcticoolithus'' is an oogenus of large fossil eggs from the Maastrichtian part of the Lopez de Bertodano Formation of Seymour Island, Antarctica. The genus contains the type species ''A. bradyi'', described by Legendre et al. in 2020. The ...
''.


Environment

Paleontologists compared the taxonomic diversity and patterns of morphological disparity in mosasaurs with sea level, sea surface temperature, and stable carbon isotope curves for the Upper Cretaceous to explore factors that may have influenced their evolution. No single factor unambiguously accounts for all radiations, diversification, and extinctions; however, the broader patterns of taxonomic diversification and morphological disparity point to niche differentiation in a "fishing up" scenario under the influence of "bottom-up" selective pressures. The most likely driving force in mosasaur evolution was high productivity in the Late Cretaceous, driven by tectonically controlled sea levels and climatically controlled ocean stratification and nutrient delivery. When productivity collapsed at the end of the Cretaceous, coincident with bolide impact, mosasaurs became extinct. Sea levels were high during the Cretaceous period, causing marine transgressions in many parts of the world, and a great inland seaway in what is now North America. Mosasaur fossils have been found in the Netherlands, Belgium, Denmark, Portugal, Sweden, South Africa, Spain, France, Germany, Poland, the Czech Republic, Italy
Bulgaria Bulgaria (; bg, България, Bǎlgariya), officially the Republic of Bulgaria,, ) is a country in Southeast Europe. It is situated on the eastern flank of the Balkans, and is bordered by Romania to the north, Serbia and North Maced ...
, the United Kingdom,
Russia Russia (, , ), or the Russian Federation, is a transcontinental country spanning Eastern Europe and Northern Asia. It is the largest country in the world, with its internationally recognised territory covering , and encompassing one-eight ...
, Ukraine, Kazakhstan,
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, Japan, Egypt, Israel, Jordan, Syria, Turkey, Niger,
Angola , national_anthem = " Angola Avante"() , image_map = , map_caption = , capital = Luanda , religion = , religion_year = 2020 , religion_ref = , coordina ...
, Morocco, Australia, New Zealand, and on
Vega Island Vega Island is a small island to the northwest of James Ross Island, on the Antarctic Peninsula. It is separated from James Ross Island by Herbert Sound. The island was named by Otto Nordenskjold, leader of the Swedish Antarctic Expedition (1 ...
off the coast of
Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest contine ...
. Tooth taxon ''Globidens timorensis'' is known from the island of Timor; however, the phylogenetic placement of this species is uncertain and it might not even be a mosasaur. Mosasaurs have been found in Canada in Manitoba and Saskatchewan and in much of the contiguous United States. Complete or partial specimens have been found in
Alabama (We dare defend our rights) , anthem = " Alabama" , image_map = Alabama in United States.svg , seat = Montgomery , LargestCity = Huntsville , LargestCounty = Baldwin County , LargestMetro = Greater Birmingham , area_total_km2 = 135,7 ...
, Mississippi, New Jersey, Tennessee, and
Georgia Georgia most commonly refers to: * Georgia (country), a country in the Caucasus region of Eurasia * Georgia (U.S. state), a state in the Southeast United States Georgia may also refer to: Places Historical states and entities * Related to the ...
, as well as in states covered by the Cretaceous seaway: Texas, southwest
Arkansas Arkansas ( ) is a landlocked state in the South Central United States. It is bordered by Missouri to the north, Tennessee and Mississippi to the east, Louisiana to the south, and Texas and Oklahoma to the west. Its name is from the Osage la ...
, New Mexico, Kansas,
Colorado Colorado (, other variants) is a state in the Mountain West subregion of the Western United States. It encompasses most of the Southern Rocky Mountains, as well as the northeastern portion of the Colorado Plateau and the western edge of the ...
, Nebraska, South Dakota, Montana, Wyoming, and the
Pierre Shale The Pierre Shale is a geologic formation or series in the Upper Cretaceous which occurs east of the Rocky Mountains in the Great Plains, from Pembina Valley in Canada to New Mexico. The Pierre Shale was described by Meek and Hayden in 1862 in t ...
/ Fox Hills formations of North Dakota. Lastly, mosasaur bones and teeth are also known from California, Mexico, Colombia, Brazil, Peru, and
Chile Chile, officially the Republic of Chile, is a country in the western part of South America. It is the southernmost country in the world, and the closest to Antarctica, occupying a long and narrow strip of land between the Andes to the east a ...
. Many of the so-called 'dinosaur' remains found on New Zealand are actually mosasaurs and plesiosaurs, both being Mesozoic predatory marine reptiles.


Discovery

The first publicized discovery of a partial fossil mosasaur skull in 1764 by quarry workers in a subterranean gallery of a limestone quarry in
Mount Saint Peter Mount Saint Peter ( French: ''Montagne Saint-Pierre''; Dutch: ''Sint-Pietersberg''), also referred to as Caestert Plateau, is the northern part of a plateau running north to south between the valleys of the river Geer to the west, and the Meuse ...
, near the Dutch city of Maastricht, preceded any major dinosaur fossil discoveries, but remained little known. However, a second find of a partial skull drew the
Age of Enlightenment The Age of Enlightenment or the Enlightenment; german: Aufklärung, "Enlightenment"; it, L'Illuminismo, "Enlightenment"; pl, Oświecenie, "Enlightenment"; pt, Iluminismo, "Enlightenment"; es, La Ilustración, "Enlightenment" was an intel ...
's attention to the existence of fossilized animals that were different from any known living creatures. When the specimen was discovered between 1770 and 1774,
Johann Leonard Hoffmann Johann Leonard Hoffmann (1710–1782) was a Maastricht army surgeon and amateur geologist who collected fossils from the local Mount Saint Peter. He is known for fossils named after him, and was one of the three people contacted on the discovery o ...
, a surgeon and fossil collector, corresponded about it with the most influential scientists of his day, making the fossil famous. The original owner, though, was Godding, a canon of Maastricht cathedral. When the French revolutionary forces occupied Maastricht in 1794, the carefully hidden fossil was uncovered, after a reward, it is said, of 600 bottles of wine, and transported to Paris. After it had been earlier interpreted as a fish, a crocodile, and a sperm whale, the first to understand its lizard affinities was the Dutch scientist
Adriaan Gilles Camper Adriaan Gilles Camper (March 31, 1759 – February 5, 1820) was a 19th-century Dutch mathematics and physics professor at the University of Franeker who took to politics and became a statesman in his later years. He was the son of Petrus Camper is ...
in 1799. In 1808, Georges Cuvier confirmed this conclusion, although ''le Grand Animal fossile de Maëstricht'' was not actually named ''Mosasaurus'' (' Meuse reptile') until 1822 and not given its full species name, ''Mosasaurus hoffmannii,'' until 1829. Several sets of mosasaur remains, which had been discovered earlier at Maastricht but were not identified as mosasaurs until the 19th century, have been on display in the Teylers Museum, Haarlem, procured from 1790. The Maastricht limestone beds were rendered so famous by the mosasaur discovery, they have given their name to the final six-million-year epoch of the Cretaceous, the Maastrichtian.


Classification


Relation with snakes or monitor lizards

The specific placement of mosasaurs within the Squamata has been controversial since its inception. Cuvier was the first scientist to deeply analyze their possible taxonomic placement through ''Mosasaurus''. While his original 1808 hypothesis that the genus was a lizard with affinities to monitor lizards remained the most popular, Cuvier was uncertain, even at the time, about the accuracy of this placement. He simultaneously proposed a number of alternative hypotheses, with one such alternative suggesting that ''Mosasaurus'' instead had closer affinities with iguanas due to their shared presence of pterygoid teeth. With the absence of sufficient fossil evidence, researchers during the early and mid-19th century had little to work with. Instead, they primarily relied on stratigraphic associations and Cuvier's 1808 research on the holotype skull. Thus, in-depth research on the placement of ''Mosasaurus'' was not undertaken until the discovery of more complete mosasaur fossils during the late 19th century, which reignited research on the placement of mosasaurs among squamates. In a span of about 30 to 40 years during the late 19th to early 20th centuries, paleontologists fiercely debated the issue, which created two major schools of thought: one that supported a monitor lizard relationship and one that supported a closer relationship with snakes. The proposition of a snake relationship was spearheaded by Cope, who first published such a hypothesis in 1869 by proposing that mosasaurs, which he classified under a group called the Pythonomorpha, was the sister group of snakes. Some scientists went as far as to interpret mosasaurs as direct ancestors of snakes. Opponents supporting a relationship with monitor lizards argued that mosasaurs should be placed within the infraorder
Anguimorpha The Anguimorpha is a suborder of squamates. The group was named by Fürbringer in 1900 to include all autarchoglossans closer to '' Varanus'' and ''Anguis'' than '' Scincus''. These lizards, along with iguanians and snakes, constitute the propo ...
, with lower classifications varying from placing mosasaurs within Varanoidea or its sister taxa, or as true monitor lizards within
Varanidae The Varanidae are a family of lizards in the superfamily Varanoidea within the Anguimorpha group. The family, a group of carnivorous and frugivorous lizards, includes the living genus '' Varanus'' and a number of extinct genera more closely re ...
. These debates spawned higher taxonomic groups that were erected in attempts to classify the placement of mosasaurs (although not all are compatible). One of these was the Mosasauria, a loosely-defined group erected by Marsh in 1880 but is still used by some researchers today that includes all descendants of the last common ancestor of ''Mosasaurus'' and some of its ancestral relatives, including the marine squamates dolichosaurs '' Dolichosaurus'' and '' Coniasaurus'' and '' Adriosaurus''. In 1923,
Charles Lewis Camp Charles Lewis Camp (March 12, 1893 Jamestown, North Dakota – August 14, 1975 San Jose, California) was a palaeontologist and zoologist, working from the University of California, Berkeley. He took part in excavations at the 'Placerias Quarry', in ...
published ''Classification of the Lizards'', in which he proposed through the review and rebuttal of previous arguments using his own anatomical observations that all taxa more closely related to ''Mosasaurus'' than ''Dolichosaurus'' should be classified into a superfamily called the Mosasauroidea, which would be a sister superfamily to the Varanoidea. Camp's take on the subject virtually ended the snake-monitor lizard debate for approximately 70 years, with nearly all subsequent studies supporting a relationship with monitor lizards. However, many studies continued to support going further than Camp in the monitor lizard relationship, placing mosasaurs within the Varanoidea. The debate was reignited with the publication of a 1997 cladistical study by Michael S. Y. Lee, which recovered the Mosasauroidea as a sister taxon to the snake suborder Serpentes and resurrected the argument for a snake relationship. Prior, there had never been a modern phylogenetic study specifically testing the relationships between mosasaurs or snakes. Lee also resurrected the defunct Pythonomorpha and redefined it to unify the Mosasauroidea and Serpentes under one clade. Multiple subsequent studies conducted by scientists such as Lee, Caldwell, and Alessandro Palci refined this hypothesis. In some studies, the Mosasauria clade was used to represent mosasaurs. However, there still remained little consensus. For example, a large-scale 2008 phylogenetic study by paleontologist Jack Conrad of the American Museum of Natural History recovered the Mosasauria clade in a polytomy, or a clade with unresolved relationships, with monitor lizards and beaded lizards; and a 2012 study by Gauthier ''et al.'' recovered Mosasauria as a clade basal to both monitor lizards and snakes. With the advent of
molecular genetics Molecular genetics is a sub-field of biology that addresses how differences in the structures or expression of DNA molecules manifests as variation among organisms. Molecular genetics often applies an "investigative approach" to determine the ...
during the 2010s, some scientists argued for the combining of molecular and morphological data to examine relationships between mosasaurs and living squamates. An early study was Lee (2009), which based on
nuclear Nuclear may refer to: Physics Relating to the nucleus of the atom: *Nuclear engineering *Nuclear physics *Nuclear power *Nuclear reactor *Nuclear weapon *Nuclear medicine *Radiation therapy *Nuclear warfare Mathematics *Nuclear space *Nuclear ...
and mitochondrial DNA in living squamates and morphological data recovered mosasaurs as a
stem group In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor. ...
to snakes, which some later authors interpreted as placement of snakes within the Mosasauria clade itself. However, a 2010 study by Weins ''et al.'' attempted to replicate Lee (2009) using a larger dataset but instead yielded results that recovered the Mosasauria as a sister clade to the monitor lizards. The frequent discrepancies are due to the high prevalence of convergent evolution in squamates, which creates much room for interpreting molecular and morphological data; many of these studies had results that often contradicted each other, for example in completely different phylogenetic results by simply adding more datasets or varying which taxa are represented, which resulted in various conflicts creating even more uncertainty. Due to this, some scientists argued that a molecular perspective should be abandoned entirely. Nevertheless, other scientists have attempted to resolve these problems. One approach was utilized by a 2015 study by Reeder ''et al.'': it closely integrated morphological, molecular, and paleontological data in a large dataset to overcome previous conflicts, which revealed new morphological support for molecular results that recovered Mosasauria as a sister clade to Serpentes. Another approach was developed by R. Alexander Pyron in a 2016 study, which also recovered Mosasauria as a sister clade to the Serpentes.


Lower classifications

The traditional view of mosasaur evolution held that all paddle-limbed (hydropedal) mosasaurs originated from a single common ancestor with functional legs (plesiopedal). However, this was shaken with the discovery of ''
Dallasaurus ''Dallasaurus'' ("Dallas lizard") is a basal mosasauroid from the Upper Cretaceous of North America. Along with '' Russellosaurus'', ''Dallasaurus'' is one of the two oldest mosasauroid taxa currently known from North America. It is also the smal ...
'', a plesiopedal mosasauroid that was found by multiple phylogenetic analyses to be an ancestor of the Mosasaurinae but unrelated to other mosasaurs. Bell and Polycn (2005) grouped these outside mosasaurs into two clades: the Russellosaurina, whose basal members include plesiopedal genera (Tethysaurinae) of their own and derived members consisting of the Plioplatecarpinae and Tylosaurinae; and the Halisauromorpha, containing the Halisaurinae. The placement of ''Dallasaurus'' suggested that the Russellosaurina and Halisauromorpha may have evolved a hydropedal form independently, the former through the tethysaurines, meaning that their placement within the Mosasauridae creates an unnatural polyphyly and thus potentially invalid. Caldwell informally proposed in a 2012 publication that the definition of a mosasaur must thus be redefined into one that does not consider russellosaurines and halisauromorphs as true mosasaurs, but as an independent group of marine lizards. However, phylogenetic studies of mosasaurs can be fickle, especially when wild card taxa like ''Dallasaurus'' remain poorly understood. For example, some studies such as a 2009 analysis by Dutchak and Caldwell instead found that ''Dallasaurus'' was ancestral to both russellosaurines and mosasaurines, although results were inconsistent in later studies. A 2017 study by Simoes ''et al.'' noted that utilization of different methods of phylogenetic analyses can yield different findings and ultimately found a consistent indication that tethysaurines were a case of hydropedal mosasaurs reversing back to a plesiopedal condition rather than an independent ancestral feature.


Phylogeny

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 ...
of the Mosasauroidea from Madzia and Cau (2017):


Distribution

Though no individual genus or subfamily is found worldwide, the Mosasauridae as a whole achieved global distribution during the Late Cretaceous with many locations typically having complex mosasaur faunas with multiple different genera and species in different ecological niches. Two African countries are particularly rich in mosasaurs: Morocco and Angola.


In contemporary culture

Mosasaurs, along with other extinct aquatic Mesozoic reptiles, are often erroneously depicted as 'swimming dinosaurs' in popular culture. The belief that mosasaurs are dinosaurs is a
common misconception Each entry on this list of common misconceptions is worded as a correction; the misconceptions themselves are implied rather than stated. These entries are concise summaries of the main subject articles, which can be consulted for more detail. ...
.


References


External links


Palaeos: Vertebrates: Mosasaurs

BBC Science and Nature: Mosasaurs


New Zealand fossil fauna

life in the Sea of Kansas, illus. by Carl Buell

until 1829.
''Mosasaurus maximus'' mounted skeleton at University of Texas Memorial Museum

Canadian Fossil Discovery Centre

"The Mosasaur of Maastricht"
by Hennie Reuvers i
Crossroads web magazine

"Mosasaurs terrorized Cretaceous rivers" Planet Earth online

Georgia Southern University Museum Mosasaur Exhibit

Kansas Geological Survey Vol IV (1899)
containing the famous summary of American mosasaurs by Samuel Williston. * William R. Wahl
MOSASAUR BITE MARKS ON AN AMMONITE. PRESERVATION OF AN ABORTED ATTACK?


{{Taxonbar, from=Q729456 01 Cretaceous lizards Turonian first appearances Maastrichtian extinctions Taxa named by Paul Gervais