Avian eggs from the Eocene Willwood and Chadron formations of Wyoming and Nebraska

ABSTRACT

Partial eggs from the Eocene Willwood Formation of Wyoming contain unidentifiable embryonic remains; the eggs are referable to a theropod on the basis of their structural layering of calcite and to an avian theropod because of their Eocene age. We assign the specimens to the oofamily Medioolithidae as Microolithus wilsoni, oogen. et oosp. nov., on the basis of the following unique combination of characters: 600-μm-thick eggshell composed of three structural layers; abrupt and undulating contact between the mammillary and continuous layers; smooth and glossy outer egg surface; faint or obscure prisms of the continuous layer; non-branching pores; and mammillary-to-total shell thickness ratio of 1:4. The eggshell microstructure resembles that of some extant neognath bird eggs. We refer a single egg from Chadron Formation of eastern Nebraska to incertae sedis as Metoolithus nebraskensis, oogen. et oosp. nov. A combination of characters distinguishes the egg from other fossil and modern avian eggs, namely, thicker eggshell, prominent ornamentation, flared upper portion of the prisms, variable mammillary thickness, and irregular squamatic texture in the continuous layer. These characteristics more closely resemble Mesozoic non-avian theropod eggs and likely reflect mosaic evolution in the non-avian to avian-theropod transition. Finally, an external layer occurs in a wide range of Paleogene and Cretaceous eggs of variable size and taxonomic affinities, indicating that this feature may not represent an apomorphic character for the avian crown group with respect to non-avian theropod eggshell. Therefore, an external layer cannot be used to identify neognath birds in the Mesozoic.

ACKNOWLEDGMENTS

This research was supported in part by National Science Foundation grant 0847777 (EAR) to D. Varricchio. We thank K. Chin, L. Wilson, and T. Culver, University of Colorado Museum, Boulder, Colorado, for all their help; R. Avci for use of the Imaging and Chemical Analysis Laboratory (ICAL) at Montana State University; and J. Horner, Paleontology Department of the Museum of the Rockies, for use of the Gabriel Laboratory for Cellular and Molecular Paleontology. We thank P. Gingerich, S. McCloud, and M. Walsh for their assistance in reconstructing the history of the Willwood specimens, and B. Beasley for help with the permit process and access to Nebraska National Forest. B. Jackson provided calculations related to egg mass and gas conductance and helpful discussions of the results. Finally, we thank N. Corsini for discovering the Chadron egg and T. Worthy, F. Bibi, and an anonymous reviewer for their helpful comments that improved the manuscript.

Handling editor: Trevor Worthy