DISTRIBUTION OF DINOSAUR EGGSHELL FRAGMENTS IN AN OVERBANK DEPOSIT, TWO MEDICINE FORMATION, CHOTEAU, MT: A preliminary report

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Elaine Kennedy, Geoscience Research Institute, located on the campus of Loma Linda University, Loma Linda, CA 92350 email: ekennedy@ccmail.llu.edu

Dinosaur eggs and eggshell fragments were located at the 'Nest in Camp' site in the Upper Two Medicine Formation (Upper Cretaceous) near Choteau, MT. The eggs and fragments occurred in an overbank deposit of very fine silt to clay-size, laminated to thinly-bedded muds. Distribution and orientation of the eggshell fragments found in this deposit were plotted in an effort to determine the nature of the deposit, i.e. 'in situ' nesting vs. transported shell material. Research results were compared with recently reported (Hayward, et al. SVP Abstracts 1997), modern studies of 'in situ' and transported shell fragments from gull colonies, and with previous work (Kennedy & Spencer GSA Abstracts 1995) on dinosaur eggshell orientation within Upper Cretaceous mudflows in Patagonia, Argentina. 'In situ' gull shell material had nearly 70% of the fragments oriented concave up. Values for concave up/down orientation of transported gull shell material were about 50%; with no significant difference in orientation. Orientation of the dinosaur shell material from mudflows in Patagonia was 52% with 7% margin of error concave down. The orientation data in this research, 53% with 6% margin of error concave down, suggest that the eggs and fragments associated with the 'Nest in Camp' were transported into the crevasse splay. The distribution data within the muds suggest the eggs and shell material settled from the water column concurrently with the muds to form the overbank deposit. There is no shell material in the underlying bed and the undisturbed nature of the muds supports the conclusion that the muds, eggs and shell material were deposited synchronously.

Published in: Geological Society of America, Abstracts with Programs, 1997; vol. 29, no. 6, A-272