Mosasaurus - The Ultimate Mosasaur Copyright © 2000-2009 by Mike Everhart Last updated 07/18/2009

Mosasaurus was a genus of Late Cretaceous mosasaurs that lived up until the end of the Age of Dinosaurs (65 mya). Several species grew to sizes that can only be described as HUGE (greater than 50' in length). Mosasaurus does not occur in the Smoky Hill Chalk of Kansas but is found commonly in the Pierre Shale of South Dakota. We have had two opportunities to work on Mosasaurus remains (both M. conodon), the first of which occurred during the summer of 1996 while working with the Field School sponsored by the South Dakota School of Mines. Later (August, 1998), I found the remains of another Mosasaurus and was able to participate in that dig.

In 1995, one of the SDSMT Field School students had discovered the remains of a medium sized mosasaur eroding from a shale hillside above the Missouri River northwest of Chamberlain, South Dakota (see picture above). The remains of "Zanc's Mosasaur", as it came to be called, were scattered across what had been a shallow, near-shore sea bottom. From the way that the bones were randomly scattered, it appears that they had been moved around by tides or other currents before finally being covered. Click here to see other specimens of Mosasaurus. LEFT: A nearly complete Mosasaurus conodon on exhibit in the Museum of Geology at the South Dakota School of Mines and Technology. Click on the photo to see additional pictures of this specimen.

The 'lucky' quadrate that identified "Zanc's Mosasaur". Scale is 6 inches. When we arrived at the site, some of the bones had already been removed (1995) and Diane, one of the Field School students, was trying to locate others and complete the mapping of the remains. At that point, there had been no really diagnostic material found, so the mosasaur was still unidentified. About noon on our first day, Diane lifted a piece of shale and was surprised to find a bowl shaped bone underneath. The bone turned out to be a quadrate and was identified by Gordon Bell as coming from a Mosasaurus conodon. I got to be the lucky one that applied the plaster jacket to the quadrate. A portion of the 'Big Bend' of the Missouri River is in the background. This area is part of the DeGrey Formation of the Pierre Shale in Hyde County, South Dakota. The shale was deposited about 75 million years ago in a shallow, near shore portion of the Western Interior Sea. Once the plaster jacket had dried and quadrate had been 'mapped in', it was removed for safe keeping. Unfortunately, very little else was uncovered before the site was determined to be finished and closed. A couple hundred yards away, however, the remains of the first 'mother mosasaur' ever to be found were being excavated. This Plioplatecarpus specimen contained the remains of at least three mosasaur embryos.

Take a look at Samuel W. Williston's drawings of a Mosasaurus horridus skull from Volume IV of the University Geological Survey of Kansas. as published in 1898. This specimen was later re-identified as Mosasaurus missouriensis and is in the collection of the University of Kansas at Lawrence.

The first Mosasaurus to be described from South Dakota was found in the 1830's. It was actually first described as an Ichthyosaur. There is an interesting story behind the specimen of Mosasaurus Maximiliana and the first mosasaur to be described from the American West.