THE EARLY EVOLUTION OF GREGARIOUS SOCIAL BEHAVIOR IN CROWN BIRDS: EVIDENCE FROM A UNIQUE EARLY EOCENE BONEBED OF ARBOREAL MOUSEBIRDS (AVES: COLIIFORMES) FROM WYOMING, USA

A unique monodominant multitaxic vertebrate microfossil assemblage from the early Eocene (~53 Ma) Willwood Formation in northwestern Wyoming, USA is the largest single locality collection of fossil mousebirds known, comprising >100 skeletal fragments derived from at least 12 individuals of stem mousebirds (Coliiformes) collected over an area of <2 m². This unexpected collection provides valuable new paleobiological data on stem members of the lineage including particular support for the early evolution of gregarious social behavior in this clade of arboreal herbivorous birds, a prominent aspect of the biology of the living species (distributed exclusively in sub-Saharan Africa today). The fossil locality in a thin paleosol horizon contains parts of all major skeletal elements (from the skull to the tail, and all limbs) derived from a minimum of 10 adults referable to the sandcoleid Anneavis anneae, one juvenile of A. anneae (confirmed histologically), and one adult of an as yet undescribed smaller sized stem mousebird species. Consistent with previous taphonomically derived methodologies for identifying gregarious behavior in fossil vertebrate assemblages, the arboreal mousebird dominated fossil assemblage was not concentrated by water, lacks indicators of attritional or carnivore accumulation, contains a large number of juvenile and adult individuals (beyond those of reproductive requirements), and the bones were on the surface less than one year prior to burial. Along with the early acquisition of their typical arboreal herbivory among stem mousebird taxa, it appears that their gregarious habits also evolved quite early during their evolution, and it has been maintained to the present day. This hypothesis of early aggregational behavior in a crown bird group appears to be the oldest paleontological evidence of socially gregarious behavior within crown Aves and examination of the phylogenetic distribution of (non-reproductive) gregarious behaviors in close clade members suggests that social behaviors may be primitive for an even larger clade beyond coliiforms. We should consider that perhaps avian sociality broadly helped to fuel the evolutionary radiation and success among land birds (Telluraves) and its component clades.