A DIVERSE ASSEMBLAGE OF FOSSIL EGGSHELL FROM THE UPPER CRETACEOUS JUDITH RIVER FORMATION OF NORTH-CENTRAL MONTANA

We describe a unique collection of fragmentary fossil eggshell from the type area of the Upper Cretaceous Judith River Formation in north-central Montana. The sample under investigation was recovered from four vertebrate microfossil bonebeds, which are localized concentrations of fossils characterized by predominantly small, disarticulated and dissociated vertebrate hard parts. Sites that yielded eggshell specimens represent both lacustrine and fluvial paleoenvironments. Sediment recovered from the four bonebeds was processed in an automated sieve system that sorted fossils down to 500 microns. A total of 855 eggshell fragments were recovered from the processed matrix, and 13 distinct morphologies identified. Eggshell morphologies were identified at higher taxonomic levels when possible through assessment of surface features and microstructure in radial thin section. Diagnostic criteria included total thickness, the ratio between the mammillary layer and the continuous layer, and the structure of the mammillae as revealed by polarized light microscopy. The most abundant morphology in our sample (n=464) exhibits horizontal laminations and irregular wedges that radiate from the former basal plate group, a morphology indicative of the fossil and modern crocodilian eggshell. Another morphology (n=165) reveals distinct pore canals and straight, narrow shell units whose height to width ratio exceeds 5:1, and correlates well with previously described fossil turtle eggshell. Two theropod ootaxa identified in the sample include 18 specimens of Porituberoolithus and 7 of Elongatoolithus; these were identified, respectively, by their distinct teardrop shaped nodes on the outer surface and the abrupt boundary between the continuous mammillary layer boundary, along with other features. Continuing work focuses on identification of the remaining eggshell morphtypes. These preliminary results document the presence of multiple nesting species in the coastal wetlands of the Upper Cretaceous Judith River Formation. The abundance of crocodilian and turtle eggshell correlates well with the known paleoecology of these sites based on the recovered fossil bone sample.