Temporal range: Late Cretaceous
|Mosasaurus hoffmannii skeleton, Natural History Museum of Maastricht, The Netherlands|
Mosasaurs (from Latin Mosa meaning the 'Meuse river', and Greek σαύρος sauros meaning 'lizard') are large extinct marine reptiles. The first fossil remains were discovered in a limestone quarry at Maastricht on the Meuse in 1764. Mosasaurs probably evolved from semi-aquatic squamates known as aigialosaurs, which were more similar in appearance to modern-day monitor lizards, in the Early Cretaceous. During the last 20 million years of the Cretaceous Period (Turonian-Maastrichtian), with the extinction of the ichthyosaurs and decline of plesiosaurs, mosasaurs became the dominant marine predators.
Mosasaurs breathed air, were powerful swimmers, and were well-adapted to living in the warm, shallow epicontinental seas prevalent during the Late Cretaceous Period. Mosasaurs were so well adapted to this environment that they gave birth to live young, rather than return to the shore to lay eggs, as sea turtles do.
The smallest-known mosasaur was Carinodens belgicus, which was about 3.0 metres (9.8 ft) to 3.5 metres (11 ft) long and probably lived in shallow waters near shore, cracking mollusks and sea urchins with its bulbous teeth. Larger mosasaurs were more typical: Hainosaurus holds the record for longest mosasaur, at 17.5 metres (57 ft).
Mosasaurs had a body shape similar to that of modern-day monitor lizards (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 elongated digit-bones. Their tails were broad, and supplied the locomotive power. This method of locomotion may have been similar to that used by the conger eel or sea snakes today. However, more recent evidence suggests that many advanced mosasaurs had large crescent-shaped flukes on the ends of their tails similar to those of sharks and ichthyosaurs. Rather than snake-like undulatory movement, the body probably remained stiff in these mosasaurs to improve hydrodynamic efficiency through the water while the end of the tail provided strong propulsion. The animal may have lurked and pounced rapidly and powerfully on passing prey, rather than hunting for it.
Early reconstructions showed mosasaurs with dorsal crests running the length of the body, which were based on misidentified tracheal cartilage. When the error was discovered, depicting mosasaurs with such crests was already a trend.
Mosasaurs had a double-hinged jaw and flexible skull (much like that of a snake), which enabled them to gulp down their prey almost whole, a snakelike habit which helped identify the unmasticated gut contents fossilized within mosasaur skeletons. A skeleton of Tylosaurus proriger from South Dakota included remains of the diving seabird Hesperornis, a marine bony fish, 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, molluscs with a shell similar to that of Nautilus, which were very abundant in the Cretaceous seas. On fossil shells of some ammonites (mainly Pachydiscus and Placenticeras) were found round holes, once interpreted as a result of limpets attaching themselves to the ammonites. The triangular formation of the holes, their size and shape and their presence on both sides of the shells, corresponding to the upper and lower jaws, is evidence of the bite of a medium-sized mosasaur. It is not clear if this behaviour was common across all size classes of mosasaurs.
Virtually all forms were active predators of fish and ammonites, a few, like Globidens had blunt spherical shaped teeth, specialized to crush mollusk shells. The smaller genera, such as Platecarpus and Dallasaurus, which were about 1–6 m (10–20 ft) long, probably preyed on fish and other small prey. The smaller mosasaurs may have spent some time in freshwater, hunting for food. The larger mosasaurs such as Tylosaurus, and Mosasaurus, reached sizes of 10–15 m (33–49 ft) long and were the apex predators of the Late Cretaceous oceans, attacking other marine reptiles, in addition to preying on large fish and ammonites.
Soft tissue 
Despite the relatively high number of mosasaur remains collected worldwide, knowledge of the nature of their skin coverings remains in its early stages. An incredibly small amount of mosasaurid specimens collected from around the world retain fossilized scale imprints; this lack of knowledge is possibly due to the delicate nature of the scales, which nearly eliminates possibility of preservation, in addition to the preservation sediments types and the marine conditions under which the preservation occurred. Until the discovery of several mosasaur specimens along with their 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-lower Campanian such as the famous Tylosaurus specimen (KUVP-1075) from Cove County, Kansas. Material from Jordan has shown that the body of mosasaurs, as well as the membrane between the fingers and toes, was covered with small overlapping diamond-shaped scales resembling those of snakes. Much like modern reptiles, there existed regional variations in the type and size of the scales that covered the mosasaurs. In Harrana specimens, two types of scales were observed on a single specimen, keeled scales covering the upper regions of the body as well as smooth scales covering the lower regions. As ambush predators, lurking and quickly capturing prey using stealth tactics, it is suggested mosasaurs benefited greatly from the non-reflective keeled scales.
More recently, a well preserved fossil of Platecarpus tympaniticus has been found that preserves not only skin impressions, but also internal organs. There are several reddish areas in the fossil that 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 that of cetaceans. As in cetaceans, the bronchi 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 a fully marine lifestyle.
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, Spain, France, Germany, Poland, Bulgaria, the United Kingdom, Russia, Ukraine, Kazakhstan, Azerbaijan, Japan, Egypt, Israel, Jordan, Syria, Turkey, Niger, Angola, Morocco, Australia, New Zealand, and on Vega Island off the coast of Antarctica. 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, Mississippi, Tennessee, and Georgia—as well as in states covered by the Cretaceous seaway: Texas, southwest Arkansas, New Mexico, Kansas, Colorado, Nebraska, South Dakota, Montana, and the Pierre Shale/Fox Hills[disambiguation needed] formations of North Dakota. Lastly, mosasaur bones and teeth are also known from California, Mexico, Colombia, Brazil, Peru and Chile.
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, 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'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, 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 six hundred 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 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, that 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.
- Family Mosasauridae
- Subfamily Tylosaurinae
- Subfamily Plioplatecarpinae
- Subfamily Mosasaurinae
- Subfamily Halisaurinae
- Subfamily Yaguarasaurinae
- Subfamily Tethysaurinae
Evolutionary antecedents 
Based on features such as the double row of pterygoid ("flanged") teeth on the palate, the loosely hinged jaw, modified/reduced limbs and probable methods of locomotion, many researchers believe that snakes share a common marine ancestry with mosasaurs, a suggestion advanced in 1869, by Edward Drinker Cope, who coined the term "Pythonomorpha" to unite them. The idea lay dormant for more than a century, to be revived in the 1990s. Recently, the discovery of Najash rionegrina, a fossorial snake from South America cast doubt on the marine origin hypothesis.
The skeleton of Dallasaurus turneri, described by Bell and Polcyn (2005), has a mixture of features present in the skeletons of derived mosasaurs and in the skeletons of mosasaurid ancestors such as aigialosaurids. Dallasaurus retains facultatively terrestrial limbs similar in their structure to the limbs of aigialosaurids and terrestrial squamates (plesiopedal limb condition), unlike derived mosasaurids which evolved paddle-like limbs (hydropedal limb condition). However, the skeleton of Dallasaurus simultaneously had several characters that linked it with derived members of the subfamily Mosasaurinae; the authors of its description listed "invasion of the parietal by medial tongues from the frontal, teeth with smooth medial enamel surface, high coronoid buttress on surangular, interdigitate anterior scapulo-coracoid suture, humeral postglenoid process, elongate atlas synapophysis, sharp anterodorsal ridge on synapophyses, vertically oriented vertebral condyles, elongate posterior thoracic vertebrae, and fused haemal arches" as the characters uniting Dallasaurus with Mosasaurinae. The phylogenetic analysis conducted by Bell and Polcyn indicated that hydropedal mosasaurids did not form a clade that wouldn't also include plesiopedal taxa such as Dallasaurus, Yaguarasaurus, Russellosaurus, Tethysaurus, Haasiasaurus and Komensaurus (in 2005 only informally known as "Trieste aigialosaur"); the analysis indicated that hydropedal limb condition evolved independently in three different groups of mosasaurs (Halisaurinae, Mosasaurinae and the group containing the subfamilies Tylosaurinae and Plioplatecarpinae). The result of this phylogenetic study was subsequently mostly confirmed by the analyses conducted by Caldwell and Palci (2007) and Leblanc, Caldwell and Bardet (2012); the analysis conducted by Makádi, Caldwell and Ősi (2012) indicated that hydropedal limb condition evolved independently in two group of mosasaurs (in Mosasaurinae and in the clade containing Halisaurinae, Tylosaurinae and Plioplatecarpinae). Conrad et al. (2011), on the other hand, recovered hydropedal mosasaurs forming a clade that excluded their plesiopedal relatives. If the hypothesis of Bell and Polcyn (2005) is correct, then mosasaurs in the traditional sense of the word, i.e. "lizards that evolved paddle-like limbs and radiated into aquatic environments in the late Mesozoic, going extinct at the end of that era", are actually polyphyletic; Bell and Polcyn (2005) maintained monophyletic Mosasauridae by including Dallasaurus and other aforementioned plesiopedal taxa in the family as well, while Caldwell (2012) suggested (though explicitly stated that it was not "a formal proposal of new nomenclature") to restrict Mosasauridae only to the genus Mosasaurus and its closest hydropedal relatives.
The exact phylogenetic position of the clade containing mosasaurids and their closest relatives (aigialosaurids and dolichosaurs) within Squamata remains uncertain. Some cladistic analyses recovered them as the closest relatives of snakes, taking into account similarities in jaw and skull anatomies; however, this has been disputed and the morphological analysis conducted by Conrad (2008) recovered them as varanoids closely related to terrestrial monitor lizards instead. Subsequent analysis of anguimorph relationships conducted by Conrad et al. (2011) based on morphology alone recovered mosasaurids, aigialosaurids and dolichosaurs as anguimorphs lying outside the least inclusive clade containing monitor lizards and helodermatids; the analysis based on combined datasets of morphological and molecular data, on the other hand, found them more closely related to monitor lizards and the earless monitor lizard than helodermatids and the Chinese crocodile lizard were. The large morphological analysis conducted by Gauthier et al. (2012) recovered mosasaurids, aigialosaurids and dolichosaurids in an unexpected position as basal members of the clade Scincogekkonomorpha (containing all taxa sharing a more recent common ancestor with Gekko gecko and Scincus scincus than with Iguana iguana) that didn't belong to the clade Scleroglossa. The phylogenetic position of these taxa turned out to be highly dependent on which taxa were included in or excluded from the analysis. When mosasaurids were excluded from the analysis, dolichosaurs and aigialosaurids were recovered within Scleroglossa, forming a sister group to the clade containing snakes, amphisbaenians, dibamids and the American legless lizard. When mosasaurids were included in the analysis, and various taxa with reduced or absent limbs other than snakes (such as dibamids or amphisbaenians) were excluded, mosasaurids, aigialosaurids and dolichosaurs were recovered inside Scleroglossa forming the sister group to snakes. Longrich, Bhullar and Gauthier (2012) conducted a morphological analysis of squamate relationships using a modified version of the matrix from the analysis of Gauthier et al. (2012); they found the phylogenetic position of the clade containing mosasaurs and their closest relatives within Squamata to be highly unstable, with the clade "variously being recovered outside Scleroglossa (as in Gauthier et al., 2012) or alongside the limbless forms".
The following is a list of geologic formations that have produced mosasaur fossils.
|Fox Hills Formation||Cretaceous||North Dakota||Estuarine deposits|
|Pierre Shale Formation||Cretaceous||North Dakota, Manitoba||Oceanic deposits|
|Mooreville Chalk Formation||Cretaceous||Alabama, Mississippi||Oceanic deposits|
|Demopolis Chalk Formation||Cretaceous||Alabama||Oceanic deposits|
|Eutaw Formation||Cretaceous||Georgia, Alabama, Mississippi||Oceanic deposits|
|Prairie Bluff Chalk Formation||Cretaceous||Alabama||Oceanic deposits|
|Ripley Formation||Cretaceous||Alabama||Oceanic deposits|
- Squamates include the living varanoid lizards, snakes and their fossil relatives the mosasaurs.
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- Kaddumi, H.F. (2009). "On the latest scale coverings of mosasaurs (Squamata: Mosasauridae) from the Harrana Fauna in addition to the description of s new species of Mosasaurus". Fossils of the Harrana Fauna and the Adjacent Areas. Amman: Eternal River Museum of Natural History. pp. 80–94.
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- Massare, J. A. (1987). "Tooth morphology and prey preference of Mesozoic marine reptiles". Journal of Vertebrate Paleontology 7 (2): 121–137. doi:10.1080/02724634.1987.10011647.
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- Glenn W. Storrs, Maxim S. Arkhangelskii and Vladimir M. Efimov (2000). "Mesozoic marine reptiles of Russia and other former Soviet republics". In Benton, M.J.; Shishkin, M.A.; and Unwin, D.M. Cambridge: Cambridge University Press. pp. 187–210. ISBN 0521554764.
- Takuya Konishi, Masahiro Tanimoto, Satoshi Utsunomiya, Masahiro Sato and Katsunori Watanabe (2012). "A Large Mosasaurine (Squamata: Mosasauridae) from the Latest Cretaceous of Osaka Prefecture (Sw Japan)". Paleontological Research 16 (2): 79–87. doi:10.2517/1342-8144-16.2.079.
- N. Bardet, X. Pereda Suberbiola, M. Iarochène, M. Amalik and B. Bouya (2005). "Durophagous Mosasauridae (Squamata) from the Upper Cretaceous phosphates of Morocco, with description of a new species of Globidens". Netherlands Journal of Geosciences 84 (3): 167–175.
- Nathalie Bardet and Cemal Tunoğlu (2002). "The first mosasaur (Squamata) from the Late Cretaceous of Turkey". Journal of Vertebrate Paleontology 22 (3): 712–715.
- Theagarten Lingham-Soliar (1991). "Mosasaurs from the upper Cretaceous of Niger". Palaeontology 34 (3): 653–670.
- Theagarten Lingham-Soliar (1998). "A new mosasaur Pluridens walkeri from the Upper Cretaceous, Maastrichtian of the Iullemmeden Basin, southwest Niger". Journal of Vertebrate Paleontology 18 (4): 709–717.
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- Michael J. Everhart (2005). "Chapter 9: Enter the Mosasaurs". Oceans of Kansas: a natural history of the western interior sea. Bloomington: Indiana University Press. ISBN 0-253-34547-2.
- Getman, Myron RC (1994). Occurrences of Mosasaur and other reptilian fossil remains from the Fox Hills Formation (Maastrichtian: late Cretaceous) of North Dakota. St. Lawrence University Dept. of Geology theses.
- Páramo-Fonseca, M. 2012. Mosasauroids from Colombia. Bulletin de la Societe Geologique de France, v. 183, p. 83
- Rodrigo A. Otero, James F. Parham, Sergio Soto-Acuña, Paulina Jimenez-Huidobro and David Rubilar-Rogers (2012). "Marine reptiles from Late Cretaceous (early Maastrichtian) deposits in Algarrobo, central Chile". Cretaceous Research 35: 124–132. doi:10.1016/j.cretres.2011.12.003.
- Aaron R. H. Leblanc, Michael W. Caldwell and Nathalie Bardet (2012). "A new mosasaurine from the Maastrichtian (Upper Cretaceous) phosphates of Morocco and its implications for mosasaurine systematics". Journal of Vertebrate Paleontology 32 (1): 82–104. doi:10.1080/02724634.2012.624145.
- Palaeos Vertebrates 260.100 Pythonomorpha: Pythonomorpha
- Mosasaurs: Last of the Great Marine Reptiles
- G.L. Bell Jr. and M.J. Polcyn (2005). "Dallasaurus turneri, a new primitive mosasauroid from the Middle Turonian of Texas and comments on the phylogeny of Mosasauridae (Squamata)". Netherlands Journal of Geosciences 84 (3): 177–194.
- Michael W. Caldwell (2012). "A challenge to categories: "What, if anything, is a mosasaur?"". Bulletin de la Société Géologique de France 183 (1): 7–34.
- Michael W. Caldwell and Alessandro Palci (2007). "A new basal mosasauroid from the Cenomanian (U. Cretaceous) of Slovenia with a review of mosasauroid phylogeny and evolution". Journal of Vertebrate Paleontology 27 (4): 863–880.
- Makádi, L. S.; Caldwell, M. W.; Ősi, A. (2012). "The First Freshwater Mosasauroid (Upper Cretaceous, Hungary) and a New Clade of Basal Mosasauroids". In Butler, Richard J. PLoS ONE 7 (12): e51781. doi:10.1371/journal.pone.0051781.
- Jack L. Conrad, Jennifer C. Ast, Shaena Montanari, Mark A. Norel (2011). "A combined evidence phylogenetic analysis of Anguimorpha (Reptilia: Squamata)". Cladistics 27 (3): 230–277. doi:10.1111/j.1096-0031.2010.00330.x.
- Lee MSY (1997-01-29). "The phylogeny of varanoid lizards and the affinities of snakes". Philos Trans R Soc Lond B Biol Sci. 352 (1349): 53–91. doi:10.1098/rstb.1997.0005. PMC 1691912.
- Michael S.Y Lee (2005). "Molecular evidence and marine snake origins". Biology Letters 1 (2): 227–230. doi:10.1098/rsbl.2004.0282.
- Conrad J (2008). "Phylogeny and systematics of Squamata (Reptilia) based on morphology". Bulletin of the American Museum of Natural History. 310: 1–182. doi:10.1206/310.1.
- Vidal N, Hedges SB (2004). "Molecular evidence for a terrestrial origin of snakes". Philos Trans R Soc Lond B Biol Sci. 271: S226–S229. doi:10.1098/rsbl.2003.0151.
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- Jacques A. Gauthier, Maureen Kearney, Jessica Anderson Maisano, Olivier Rieppel, Adam D.B. Behlke (2012). "Assembling the Squamate Tree of Life: Perspectives from the Phenotype and the Fossil Record". Bulletin of the Peabody Museum of Natural History 53 (1): 3–308. doi:10.3374/014.053.0101.
- Nicholas R. Longrich, Bhart-Anjan S. Bhullar and Jacques A. Gauthier (2012). "Mass extinction of lizards and snakes at the Cretaceous–Paleogene boundary". Proceedings of the National Academy of Sciences of the United States of America 109 (52): 21396–21401. doi:10.1073/pnas.1211526110.
|Wikimedia Commons has media related to: Mosasauridae|
- Palaeos: Vertebrates: Mosasaurs
- BBC Science and Nature: Mosasaurs
- Mike Everhart and David Lewis, "Mesozoic marine monsters of the Mangahouanga": New Zealand fossil fauna
- Mike Everhart, "A day in the life of a Mosasaur": life in the Sea of Kansas, illus. by Carl Buell
- Mike Everhart, "Mosasaurus hoffmani" until 1829.
- Mosasaurus maximus mounted skeleton at University of Texas Memorial Museum
- Canadian Fossil Discovery Centre
- "The Mosasaur of Maastricht" by Hennie Reuvers in 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?
- Mosasaur diet