Primobucco mcgrewi (Aves: Coracii) from the Eocene Green River Formation: new anatomical data from the earliest constrained record of stem rollers

Abstract

ABSTRACT The Eocene Green River Formation provides one of the richest records of fossil birds worldwide. As part of a reevaluation of this avifauna, we describe 12 new specimens of the stem roller Primobucco mcgrewi from the well-dated (51.66 ± 0.09 Ma) Fossil Butte Member (FBM) of the Green River Formation of Wyoming. FBM specimens represent most of avian diversity in the Green River Formation and include the oldest well-constrained record of the roller lineage (Coracii). These fossils provide new anatomical data, including the first observations on the palate, and broaden our understanding of the distribution, abundance, and taphonomy of P. mcgrewi. Using museum records and lithological comparisons, 14 of the 15 known P. mcgrewi specimens can now be assigned to specific quarries within Fossil Lake. The species is now known from five distinct localities representing both nearshore and mid-lake environments and accounts for >10% of the 148 FBM avian specimens reviewed in this study. Pectoral elements are disproportionately represented in the FBM P. mcgrewi specimens, and more than half of the sample exhibits broken elements. These new fossils and other key specimens from the Eocene of North America and Europe clarify our understanding of the evolution of the clade Coracii. Extant parts of this lineage (i.e., Coraciidae and Brachypteraciidae) have specialized ecologies and restricted Old World distributions, whereas stem representatives appear more generalized and were a major component of some North American avifaunas.