This paper reviews the microstructure, stratigraphic setting, and paleoenvironment of abundant avian and crocodillian eggshell from 10 sites located on the margins of three large Eocene lakes: Lake Gosiute, Fossil Lake, and Lake Uinta. The avian eggshell is known from all three Eocene lakes and the crocodillian eggshell is known from Lake Gosiute, and Lake Uinta. Often the eggshell occurs in multiple stratigraphic units at each of the 10 sites.

The eggshell microstructure was studied in radial cross-section by several techniques: SEM (scanning electron microscopy), PLM (polarized light microscopy), and by LM (light microscopy). The basic shell unit of the crocodillian eggshell has tabular ultrastructure arranged in large irregularly shaped wedges. At the apex of the wedges (at the base of the shell unit) are bulbous basal plate groups with a loose ultrastructure. The avian eggshell fragments show the following macrostructure zones (from internal to external): 1) wedges of the mamillary layer (diverging outward from the central core), grading into 2) long vertical columns of the prismatic layer. An external zone of vertical calcite crystals is sometimes preserved in the avian eggshell.

The eggshell is associated with carbonate mudflats, deltas, beach sands, floodplain channels and ponds, stromatolites, tufa encrusted branches, carbonate caddisfly mounds, oolites, ostracodes, gastropods, wave ripples, and strandlines of avian, crocodilian, turtle and fish bones. The avian eggshell is always associated with abundant bones of Presbyornis (Aves: Anseriformes). The dense concentration of eggshell fragments at most of the sites argues for little transport and therefore supports the nesting site hypothesis.