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CHAMPSOSAURUS GIGAS ERICKSON: A RESTORED AND MOUNTED SKELETON (ND94-225.1) IN THE NORTH DAKOTA STATE FOSSIL COLLECTION

CAMPBELL, Johnathan M.l* and HOGANSON, John W., North Dakota Geological Survey

   

Isolated postcranial bones of the large late Paleocene freshwater eosuchian reptile Champsosaurus gigas Erickson are commonly found throughout western North Dakota in the Bullion Creek and Sentinel Butte Formations. In the late summer of 1993 a site in the Sentinel Butte Formation was discovered containing the remains of champsosaurs, crocodilians, turtles, fish, and freshwater mollusks. The site located on U. S. Forest Service (USFS) land in Billings County is called the Tracy Mountain Site.

A nearly complete, although mostly disarticulated, Champsosaurus gigas skeleton (ND94-225.1) was excavated at the site in 1994 from a 20-cm-thick carbonaceous claystone. C. gigas is the largest of the Champsosaurus species and one of the last to live before extinction of the group (1). Because of the completeness of the skeleton (about 85%) and excellent preservation of the bones, it was decided to restore the skeleton for display in the forthcoming Corridor of Time exhibit at the North Dakota Heritage Center, Bismarck. Only about a half dozen mounted skeletons of Champsosaurus exist, and no other specimens of C. gigas have ever been restored in a three dimensional skeletal mount. Descriptions and illustrations of C. gigas remains by Erickson (1, 2) and examination of prepared specimens in the paleontology collection at the Science Museum of Minnesota were relied on for fabrication of missing and broken skeletal elements.

Vertebral centrums were present from cervical 1 to about the middle of the tail. About 6 caudal vertebra are estimated to be missing from the back part of the tail. If this estimate is correct, tails of Champsosaurus gigas would have been shorter than previously thought (1). The skull is complete although it was badly crushed and fused to the lower jaws as a result of compaction. Separation of the skull from the lower jaws and reinflated of the skull was accomplished by slowly prying apart and removing bone pieces and gluing them back together with either Super Glue or 5-minute epoxy. Most limb bones were present. Parts that had to be fabricated were broken or missing pieces of the pectoral and pelvic girdles, neural arches, ribs, chevrons, and foot bones. These were made from Plaster of Paris with internal wire supports. Critter Clay was tried, but was found to be structurally inferior and its use was limited to small areas of non-support.

Because it is believed that Champsosaurus gigas was an aggressive aquatic predator with large, powerful back legs, the specimen was posed as if it were springing off the bottom of a pond after a fish as depicted by Erickson (1).

This specimen was discovered by Chris Quinn (Dickinson, ND) and Mark Luther (North Dakota Geological Survey). Funding for preparation and mounting of the specimen by Johnathan Campbell was from the State Historical Society of North Dakota and the U. S. Forest Service though a grant to the North Dakota Geological Survey.

References Cited

1Erickson, B. R. (1972) Science Museum of Minnesota Monograph 1: Paleontology 91 p.

2Erickson, B. R. (1985) J of Vert Paleo 5(2), pp 111-127.

   
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Updated: 08.11.06