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Artist's rendering of Mosasaurus maximus - The Onion Creek MosasaurMosasaurus maximus
The Onion Creek Mosasaur

TMM 313-1
Navarro Group, Cretaceous
Travis County, Texas

Mosasaurus maximus was a giant extinct marine reptile.  It lived in shallow seas that covered much of Texas about 70 million years ago, during the Cretaceous Period.  It is a member of the mosasaur lineage, which included many other species and achieved a worldwide distribution before becoming extinct about 65 million years ago.  Their fossilized bones are fairly common across the state, and they are especially abundant in Central Texas.  But skeletons as complete as this one are very rare.  Some mosasaur species were small (under 6 feet long) but others evolved to huge size.  This specimen is one of the larger mosasaurs, being nearly 30 feet long.  Its head alone is nearly 5 feet long and its open jaws had a gape of 3 feet. 

Mosasaurs lived only during the Mesozoic and are sometimes confused with dinosaurs.  But lizards, snakes, and mosasaurs form their own distinctive branch of the reptilian family tree, and they are only distantly related to dinosaurs.  Today the closest living relatives of the extinct mosasaurs are the members of a lizard lineage that includes the Komodo dragon and the Gila monster. 

Mosasaurs were marine animals that spent virtually their entire lives in the oceans and seaways of the Cretaceous world.  Unlike modern sea turtles, which come out on dry land to lay eggs, mosasaurs gave birth in the water to live young.  For reasons that are not fully understood, the ancestors of the mosasaur lineage left the dry land and adapted to life in the seas. 

Mosasaurs quickly evolved to tremendous size in the environment of the Cretaceous seas.  With long snake-like tails and paddles for limbs, they were probably excellent swimmers, and they reached all of the oceans and seas of the Cretaceous world.  Their large pointed teeth leave little doubt that they were predatory, hunting other marine animals.  Several known specimens preserve possible stomach contents, which indicate that mosasaurs ate other vertebrates (sharks, bony fish, turtles, other marine reptiles, etc.).  The shells of extinct molluscs known as ammonites have also been found with holes interpreted as bite marks made by mosasaurs. 

The specimen buried at the Dino Pit was cast from a beautiful skeleton found in 1935 in Travis County, along the banks of Onion Creek.  W. Clyde Ikins and John Peter Smith, geology students at The University of Texas at Austin, discovered the skeleton.  They alerted paleontologists at the Texas Memorial Museum, who excavated the skeleton.  It was first put on public display at the Texas Centennial in 1936.  The mounted and reconstructed skeleton is currently on display at the Texas Memorial Museum, the exhibit hall of the Texas Memorial Museum.


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