Wednesday, August 24, 2011

Latoplatecarpus willistoni, a new Mosasaur from the Western Interior Seaway

Fig 1.- Latoplatecarpus willistoni had a streamline body.

Mosasaurs are a group of highly successful marine lizards from the Late Cretaceous period that are related to the modern day land-living Komodo dragons and varans. Their diversity ranges from the small 3 meters long mollusk eating Carinodens to the giant 18 meters long predator Tylosaurus. To date, around 30 genera with a total of some 40 species have been described. Their geographical distribution is worldwide but fossils are particularly abundant in the ancient Western Interior seaway that used to divide the continent of North America during the Late Cretaceous period. The number of specimens found there is amounting to more than 3000.

The Plioplatecarpines form a particular group of mosasaurs consisting of shorted headed and long bodied critters, known from some 500 specimens from North America. In a general revision of the Plioplatecarpines published in the Journal of Vertebrate Paleontology in 2011, Takuya Konishi and Michael W. Caldwell name two new genera, Latoplatecarpus and Plesioplatecarpus and one new species, Latoplatecarpus willistoni, while two other taxa were referred to the above mentioned genera, Latoplatecarpus nichollsae (formerly Plioplatecarpus nichollsae) and Plesioplatecarpus planifrons (formerly Clidastes planifrons and then Platecarpus planifrons). In total, Konishi and Caldwell recognize 7 genera and 11 species to the Plioplatecarpines.

L. willistoni is known from 4 partial skeletons from the lowermost middle Campanian period of the Late Cretaceous, one (the holotype), from the Lower Pierre Shale Formation of Southern Manitoba, Canada, two from the Pierre Shale Formation of Wyoming and one from the Ozan Formation of Texas. The contemporary L. nichollsae is known from remains from Manitoba, Wyoming, South Dakota and Alabama, while P. planifrons, dating from the more ancient Upper Middle Coniacian-Middle Santonian period of the Late cretaceous, is known from Kansas and Alabama. 

Fig 2.- Old representation of Platecarpus with an eel like body.

The fish eating plioplatecarpines, like the other mosasaurs, were traditionally depicted as eel-like creature that propelled by lateral motion of the body. But the description of an exceptionally preserved complete specimen of Platecarpus in 2010 indicates that the general body plan of the most derived mosasaur Plotosaurus was also present in plioplatecarpines. The specimen is complete with skin impressions and possible internal organ impressions, and shows a sharp downward turn of the tail indicating the presence of a tail fluke. By an effect of convergent evolution, mosasaurs, it turned out, had a streamlined body shape similar to other marine creatures such as ichthyosaurs, metriorhynchids and sharks.

References:

Lindgren J, Caldwell MW, Konishi T, Chiappe LM. 2010. Convergent Evolution in Aquatic Tetrapods: Insights from an Exceptional Fossil Mosasaur. PLoS ONE 5(8): e11998.

Konishi, T. and Caldwell, M.W. 2011. Two new plioplatecarpine (Squamata, Mosasauridae) genera from the Upper Cretaceous of North America, and a global phylogenetic analysis of plioplatecarpines. Journal of Vertebrate Paleontology 31 (4): 754-783.


Original artworks on Paleoexhibit are copyrighted to Nobu Tamura. Do not use without permission.

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