Mosasaur

Mosasaurs (from Latin Mosa meaning the 'Meuse river' in the Netherlands, and Greek sauros meaning 'lizard') were serpentine marine reptiles. The first fossil remains were discovered in a limestone quarry at Maastricht on the Meuse in 1778. These ferocious marine predators are now considered to be the closest relatives of snakes, due to cladistic analysis of symptomatic similarities in jaw and skull anatomies. Mosasaurs were not dinosaurs but lepidosaurs, reptiles with overlapping scales. These predators evolved from semi-aquatic squamates known as the aigialosaurs, close relatives of modern-day monitor lizards, in the Early Cretaceous Period. 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.

Genera
Known genera include:


 * Clidastes
 * Mosasaurus
 * Prognathodon
 * Globidens
 * Plotosaurus
 * Plesiotylosaurus
 * Carinodens
 * Dallasaurus
 * Igdamanosaurus
 * Halisaurus
 * Tylosaurus
 * Platecarpus
 * Selmasaurus


 * Plioplatecarpus
 * Amphekepubis
 * Goronyosaurus
 * Liodon
 * Moanasaurus
 * Pluridens
 * Yaguarasaurus
 * Eonatator
 * Hainosaurus
 * Tethysaurus
 * Angolasaurus
 * Kourisodon
 * Russellosaurus

Description
Mosasaurs breathed air and were powerful swimmers that 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. Hainosaurus holds the record for longest mosasaur, at 17.5 m.

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. The animal may have lurked and pounced rapidly and powerfully on passing prey, rather than hunting for it.

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 that has 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.

Fossils
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, in Sweden, in Africa, in Australia, in New Zealand and on Vega Island, off the coast of Antarctica. In Canada and the United States, complete or partial specimens have been found in Alabama, Mississippi, Tennessee, and Georgia and in almost all the states covered by the seaway: Texas, southwest Arkansas, New Mexico, Kansas, Colorado, Nebraska, the Dakotas, Montana, Manitoba, and the Pierre Shale and Fox Hills formations of North Dakota. Mosasaurs are also known from California, Mexico, Peru and Denmark.

Many of the 'dinosaur' remains found on New Zealand are actually mosasaurs and plesiosaurs, another group of Mesozoic predatory marine reptiles.

Taxonomy
Incertae sedis
 * 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
 * Liodon
 * Goronyosaurus
 * Pluridens
 * Kourisodon
 * Russellosaurus
 * Tethysaurus