Fossil eggs and perinatal remains from the upper Cretaceous Two Medicine Formation of Montana : description and implications

Abstract

Egg Mountain is a dinosaur nesting site located in the Upper Cretaceous, Two Medicine Formation of western Montana. The site was located on a coastal plain with seasonal variations in rainfall and was utilized as a nesting location by several taxa. Egg Mountain hosts a remarkable diversity of fossil eggshell and is a window into the reproductive behavior of multiple extinct taxa. A recent 4x6 m excavation revealed two clusters of unidentified eggs, 185 Spheroolithus eggshell fragments, and perinatal osteological remains within homogenous siliciclastic mudstone. Insect burrows (Celliforma) and cocoons (Rebuffoichnus) were excavated from the micritic limestone bed capping the excavation. The objectives of this thesis are to 1) describe the eggs and eggshell fragments, 2) determine nesting environment, 3) assign the osteological remains to taxon, and 4) investigate the taphonomic history of the site. Analytical methods include scanning electron microscopy, cathodoluminescence, petrographic microscopy, and ImageJ photo analysis. The lithologically compressed, unidentified 12 cm diameter eggs occur in two clusters containing 7-22 eggs. Diagenetic alteration obscures eggshell microstructure, inhibiting ootaxonomic assignment of the 0.5 mm thick eggshell. The 0.8-1.3 mm thick fragmentary eggshell is assigned to Spheroolithus albertensis based on microstructure, sagenotuberculate ornamentation, and prolatocanaliculate/ rimocanaliculate pores. To assess taphonomic history of the Spheroolithus fragments, chicken (Gallus gallus domesticus) and ostrich (Struthio camelus) eggshells were placed in a tumbler with water and quartz sand to simulate transport. The resulting wear on these fragments was compared to unabraded eggshell. In addition, modern eggshell was compared to fossil eggshells from a fossil nesting site, crevasse splay and channel deposits, and Egg Mountain. Spheroolithus eggshell from Egg Mountain lack edge rounding and resemble fossil eggshell from a nesting site and unabraded modern eggshell, suggesting a parautochthonous assemblage. Spheroolithus and unidentified eggs from cluster 1 respectively have gas conductance values 16-32x and 4-13x higher than avian eggs of the same mass, suggesting enclosed nest environments. The morphology of the humerus and skull elements of the perinatal osteological remains is consistent with the Hadrosauridae, though the juvenile status and incomplete nature of the specimen inhibits further taxonomic assignment.