Mosasaur

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Mosasaurs
Temporal range: Late Cretaceous
Mosasaurus hoffmannii - skeleton.jpg
Mosasaurus hoffmannii skeleton, Natural History Museum of Maastricht, The Netherlands
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Squamata
Superfamily: Mosasauroidea
Family: Mosasauridae
Gervais, 1853
Subfamilies

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 semiaquatic squamates[1] 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.

Description[edit]

Restoration of Prognathodon saturator with asymmetrically bilobed tail fluke
Tylosaurus proriger erroneously depicted with a dorsal crest and no tail fluke, by Charles R. Knight, 1899

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[citation needed].

The smallest-known mosasaur was Dallasaurus turneri, which was less than a 1 metre (3.3 ft) long. Larger mosasaurs were more typical with a number of species longer than 4 metres (13 ft). Mosasaurus hoffmannii may have been as much as 18 metres (59 ft) in length.

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. Until recently, mosasaurs were assumed to swim in a similar method used today by the conger eel or sea snakes. However, new evidence suggests 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, their bodies probably remained stiff in these mosasaurs to improve hydrodynamic efficiency through the water, while the end of their tails provided strong propulsion.[2] The animal may have lurked and pounced rapidly and powerfully on passing prey, rather than hunting for it. A juvenile Prognathodon found in Jordan's Harrana Site in 2008 and described in 2013 supports this, as the outline of its tail fluke was also preserved with the skeleton.[3]

Early reconstructions showed mosasaurs with dorsal crests running the length of their bodies, which were based on misidentified tracheal cartilage. When the error was discovered, depicting mosasaurs with such crests was already a trend.[4][5]

Currently the largest publicly exhibited mosasaur in the world is housed at the Canadian Fossil Discovery Centre in Morden, Manitoba, Canada. 'Bruce' is just over 13 metres or 43 feet in length.[6]

Paleobiology[edit]

Fossil shell of ammonite Placenticeras whitfieldi showing punctures caused by the bite of a mosasaur, Peabody Museum of Natural History, Yale

Mosasaurs had double-hinged jaws and flexible skulls (much like those of snakes), which enabled them to gulp down their prey almost whole, a snake-like 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 shells similar to that of Nautilus, which were very abundant in the Cretaceous seas. On fossil shells of some ammonites (mainly Pachydiscus and Placenticeras), round holes were found, 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 medium-sized mosasaurs. 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, such as Globidens, had blunt spherical 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 fresh water, 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[edit]

Scales of Tylosaurus proriger (KUVP-1075)

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[7] 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 Gove County, Kansas.[8] 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 modern reptiles, regional variations existed 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,[7] 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,[9] they are suggested to have benefited greatly from the nonreflective, keeled scales.[7]

Soft tissues in the head and neck of Platecarpus tympaniticus specimen LACM 128319: Tracheal rings are shown in the bottom three photographs.

More recently, a well-preserved fossil of Platecarpus tympaniticus 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 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 fully marine lifestyles.[2]

Fibrous tissues and microstructures recovered from Prognathodon specimen IRSNB 1624

In 2011, collagen protein was recovered from a Prognathodon humerus dated to the Cretaceous.[10]

Coloration[edit]

Color has been unknown in mosasaurs until 2014, when the findings of Johan Lindgren of Lund University et al. revealed the pigment melanin in the fossilized scales of a mosasaur. The studies of the scales revealed that mosasaurs were likely countershaded; with black backs and white 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 the journal Nature.[11]

Environment[edit]

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, Czech Republic, Bulgaria, the United Kingdom,[12][13] Russia, Ukraine, Kazakhstan, Azerbaijan,[14] Japan,[15] Egypt, Israel, Jordan, Syria,[16] Turkey,[17] Niger,[18][19] 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.[20] Mosasaurs have been found in Canada in Manitoba and Saskatchewan[21] 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,[22] Colorado, Nebraska, South Dakota, Montana, and the Pierre Shale/Fox Hills formations of North Dakota.[23] Lastly, mosasaur bones and teeth are also known from California, Mexico, Colombia,[24] Brazil,[16] Peru, and Chile.[25]

Many of the so-called 'dinosaur' remains found on New Zealand are actually mosasaurs and plesiosaurs, both being Mesozoic predatory marine reptiles.

Discovery[edit]

The Mosasaurus hoffmannii skull found in Maastricht between 1770 and 1774

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 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 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.

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.

Taxonomy[edit]

Classification[edit]

Drawing from Williston (1898) that shows the skeletons of three common species of mosasaurs from Kansas; Clidastes propython, Platecarpus tympaniticus and Tylosaurus proriger
Selmasaurus johnsoni mounted skull in the Rocky Mountain Dinosaur Resource Center in Woodland Park, Colorado

Incertae sedis

Phylogeny[edit]

Cladogram of mosasaurs and related taxa modified from Aaron R. H. Leblanc, Michael W. Caldwell and Nathalie Bardet, 2012:[26]

Mosasauroidea
Aigialosauridae

Aigialosaurus dalmaticus



Mosasauridae
Mosasaurinae

Dallasaurus turneri





Clidastes liodontus



Clidastes moorevillensis



Clidastes propython





"Prognathodon" kianda




Globidens alabamaensis



Globidens dakotensis



Mosasaurini

Eremiasaurus heterodontus




Plotosaurus bennisoni




Mosasaurus conodon



Mosasaurus hoffmanni



Mosasaurus missouriensis







"Prognathodon" rapax




Plesiotylosaurus crassidens




Prognathodon overtoni




Prognathodon saturator




Prognathodon waiparaensis




Prognathodon solvayi



Prognathodon currii















Carsosaurus marchesettii



Komensaurus carrolli




Haasiasaurus gittelmani


Russellosaurina

Halisaurinae

Halisaurus platyspondylus



Halisaurus sternbergi





Tethysaurus nopcsai




Yaguarasaurus columbianus



Russellosaurus coheni






Plioplatecarpinae

Ectenosaurus clidastoides




Plesioplatecarpus planifrons




Angolasaurus bocagei




Platecarpus tympaniticus



Plioplatecarpus






Tylosaurinae

Tylosaurus nepaeolicus



Tylosaurus proriger









Evolutionary antecedents[edit]

Restoration of Aigialosaurus bucchichi, a basal mosasaur

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.[27][28] 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.[29] 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).[29][30] 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);[26][31] 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).[32] Conrad et al. (2011), on the other hand, recovered hydropedal mosasaurs forming a clade that excluded their plesiopedal relatives.[33] 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",[30] are actually polyphyletic; Bell and Polcyn (2005) maintained monophyletic Mosasauridae by including Dallasaurus and other aforementioned plesiopedal taxa in the family as well,[29] 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.[30]

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,[34][35] taking into account similarities in jaw and skull anatomies;[34] however, this has been disputed[36][37][38] and the morphological analysis conducted by Conrad (2008) recovered them as varanoids closely related to terrestrial monitor lizards instead.[36] 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.[33] 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[36]) 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.[39] 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".[40]

Distribution[edit]

The following is a list of geologic formations that have produced mosasaur fossils.

Name Age Location Notes
Mount Saint Peter Cretaceous Maastricht Limestone deposits since 1764
Fox Hills Formation Cretaceous North Dakota Estuarine deposits
Moreno Formation
Niobrara Formation
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, Mississippi Oceanic deposits
Ripley Formation Cretaceous Alabama Oceanic deposits
Navarro Formation Cretaceous Texas
Maastrichtian Cretaceous Bentiaba River

References[edit]

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External links[edit]