Preliminary Results of the First Avian Eggs and Possible Clutches in the Bridger Formation, Wyoming

FERGUSON, Ashley L.; Idaho State University, Pocatello, ID. MCGREGOR, Douglas; Idaho State University, Pocatello, ID.


Avian eggs and nests in the fossil record are rare, particularly when compared to their non-avian ancestors. Currently, the record of avian nesting in the Paleogene is limited, with only three documented eggs and a possible nesting locality worldwide. Locating and describing eggs and nests from the Paleogene is crucial to understanding when the diversity of nesting behaviors in birds arose. Here, we describe preliminary results of the first partial avian eggs discovered in the Eocene Bridger Formation. The eggs were discovered in 2017 in the upper Blacks Fork Member in a gray siltstone. Steinkerns of unionid bivalves occur just below the locality in a gray lithic arenite. Two concentrations of eggs and eggshell were located approximately 25 cm apart. The first concentration contained four partial eggs with additional eggs and the second concentration contains several eggs as well. Both need to be prepared. A radial thin section of the eggshell confirms an ornithoid basic type. Counts of eggshell fragments produce a ratio of 55:45 concave up to concave down eggshell consistent with in situ preservation of the eggs. Future work will include scanning electron microscopy, computed tomography, further thin section analysis and preparation of the specimens.