FOSSIL EGGSHELL HIGHLIGHTS HIDDEN DIVERSITY IN THE UPPER CRETACEOUS (CAMPANIAN) JUDITH RIVER FORMATION, NORTH-CENTRAL MONTANA

The Upper Cretaceous Judith River Formation (JRF) of north-central Montana has a rich history of paleontological discovery. The unit is particularly noteworthy for its exceptional preservation of vertebrate microfossil bonebeds (VMBs), which are taxonomically diverse concentrations of small bones, teeth, scales and eggshells. VMBs play an important role in reconstructing terrestrial vertebrate paleofaunas in Mesozoic ecosystems. In the Late Cretaceous, sediment accumulated in lacustrine and fluvial settings adjacent to the Western Interior Sea, forming the JRF and preserving abundant VMBs. Here, we analyze millimeter to sub-millimeter scale eggshell fragments to broaden our perspectives on diversity in a Late Cretaceous paleocommunity.

We sieved bulk samples from four VMB localities through 2 mm and 500 μm sieves, and sorted the recovered sample using a dissecting microscope to extract over 1,000 pieces of eggshell. We recognize fourteen distinctive eggshell morphs on the basis of thickness, surface sculpture, porosity, and color. We also utilized X-Ray diffraction to differentiate between calcitic and aragonitic eggshells, which aids in identifying turtle morphs without destructive sampling.

The most abundant morph (n=619) is attributed to crocodiles on the basis of its calcitic composition, undulating vitreous outer surface, and nodular, white, crystalline precipitates on the inner surface. The next two most abundant morphs (n=244 and n=76) are attributed to two distinct species of turtles based on their aragonitic composition and smooth, highly porous surfaces. The abundance of crocodilian and turtle eggshells is consistent with the depositional environments of JRF VMBs. Dinosaur eggshells are also present, but rare. Several unusual morphs are still under investigation, and may indicate the presence of other hard-shelled egg layers (e.g., champsosaurs, lizards). Radial thin sections will provide detail of eggshell microstructure that will allow identification of still unknown morphs. Taphonomic data indicate that eggshells are more rounded, abraded, and polished in channel-hosted VMBs. Scanning Electron Microscopy also reveals evidence of microbial degradation of eggshells in JRF VMBs.