Mosasaur

Mosasaurs Fossil range: Late Cretaceous, 85–65 Ma PreЄ Є O S D C P T J K Pg N
Tylosaurus specimen nicknamed "Bruce"
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Superorder:	Lepidosauria
Order:	Squamata
Suborder:	Scleroglossa
Infraorder:	Anguimorpha
Superfamily:	†Mosasauroidea
Family:	†Mosasauridae Gervais, 1853
Subfamilies
†Halisaurinae †Mosasaurinae †Plioplatecarpinae †Tylosaurinae

Mosasaurs (from Latin Mosa meaning the 'Meuse river', and Greek sauros meaning 'lizard') are large extinct marine lizards. The first fossil remains were discovered in a limestone quarry at Maastricht on the Meuse in 1764. Mosasaurs are now considered to be the closest relatives of snakes, due to cladistic analyses that have taken into account similarities in jaw and skull anatomies.^[1] Mosasaurs were varanoids closely related to terrestrial monitor lizards. They probably evolved from semi-aquatic squamates^[2] 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 pliosaurs, mosasaurs became the dominant marine predators.

Description

Plioplatecarpus primaevus skull Oxford University Museum of Natural History.

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 to 3.5 m long and probably lived in shallow waters near shore, cracking mollusks and sea urchins with its bulbous teeth. Larger mosasaurs were more typical: mosasaurs ranged in size up to 17 m. Tylosaurus holds the record for longest mosasaur, at 17.5 m.

Reconstruction of Platecarpus showing crescent-shaped fluke.

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 tales 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.^[3] The animal may have lurked and pounced rapidly and powerfully on passing prey, rather than hunting for it.

Skeletal drawings of three types of mosasaur

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.

Based on features such as the double row of pterygoid ("flanged") teeth on the palate, the double-hinged jaw, modified/reduced limbs and probable methods of locomotion, many researchers believe that snakes and mosasaurs may have had a common ancestor. This theory was first suggested in 1869, by Edward Drinker Cope, who coined the term "Pythonomorpha" to include them. The idea lay dormant for more than a century, before being revived in the 1990s.^[4][5]

A 43 ft. (13 m) complete fossil specimen of Mosasaur species Tylosaurus proriger (with a human for scale) on display at the Academy of Natural Sciences in Philadelphia.

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 perservation, 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^[6] 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.^[7] 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,^[6] 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,^[8] it is suggested mosasaurs benefited greatly from the non-reflective keeled scales.^[6]

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.^[3]

Environment

Mosasaurus hoffmannii skeleton, Natural History Museum of Maastricht, The Netherlands.

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, Denmark, Sweden, Angola, Morocco), Portugal, Australia, New Zealand, and on Vega Island off the coast of Antarctica. Mosasaurs have been found in Canada in Manitoba^[9] 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,^[10] Colorado, Nebraska, South Dakota, Montana, and the Pierre Shale/Fox Hills formations of North Dakota.^[11] Lastly, mosasaur bones and teeth are also known from California, Mexico, and Peru.

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

Discovery

The Mosasaurus discovered in a Maastricht limestone quarry, 1770 (contemporary engraving)

The first publicized discovery of a partial fossil mosasaur skull in 1764 by quarry-workers in a subterranean gallery of a limestone quarry 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.

The Mosasaur skull found in Maastricht in 1764

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 nineteenth 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 that they have given their name to the final six-million-year epoch of the Cretaceous, the Maastrichtian.

Taxonomy

Classification

Taniwhasaurus.

Clidastes.

Prognathodon.

• Family Mosasauridae
  • Subfamily Tylosaurinae
    • Hainosaurus
    • Tylosaurus
    • Taniwhasaurus
  • Subfamily Plioplatecarpinae
    • Platecarpus
    • Angolasaurus
    • Ectenosaurus
    • Selmasaurus
    • Igdamanosaurus
    • Yaguarasaurus
    • Plioplatecarpus
  • Subfamily Mosasaurinae
    • Dallasaurus
    • Clidastes
    • Mosasaurus
    • Moanasaurus
    • Amphekepubis
    • Plotosaurus
    • Globidens
    • Prognathodon
    • Plesiotylosaurus
    • Carinodens
  • Subfamily Halisaurinae
    • Eonatator
    • Halisaurus

Incertae sedis

• Liodon
• Goronyosaurus
• Pluridens
• Kourisodon
• Russellosaurus
• Tethysaurus

Phylogeny

Cladogram of mosasaurs and related taxa modified from Bell and Polcyn, 2005:^[12]

Evolutionary antecedents

Based on features such as the loosely-hinged jaw, modified/reduced limbs and probable 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.^[4] Recently, the discovery of Najash rionegrina, a fossorial snake from South America cast doubt on the marine origin theory.

On 2005-11-16, research reported in Netherlands Journal of Geosciences, confirmed that the recently uncovered Dallasaurus turneri is an early link between land-based monitor lizards (such as the Komodo dragon) and the aquatic mosasaurs.^[13]

Distribution

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

In popular culture

• Mosasaurs appear in the BBC television series Sea Monsters.
• Mosasaurs also feature heavily in the ITV television series Primeval. In this show, the mosasaurs are depicted incorrectly as having skin more like a crocodile's.
• A highly evolved mosasaur appeared in an episode of Godzilla: The Series. It was inhabiting Loch Ness as the legendary Loch Ness Monster.
• The IMAX 3D film "Sea Monsters" features mosasaurs (including Tylosaurus) with detailed animated recreations of their movements and activities.

References

External links

• 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