An overview of the oldest and most comprehensive avifauna of the early Eocene British London Clay

Abstract Early Eocene birds from the British London Clay have been known for two centuries, but a comprehensive understanding of the avifauna of the London Clay Formation is still in its infancy. In the present study, the bird assemblage of the Walton Member is reviewed, which represents one of the oldest subdivisions of the formation. With more than 80 currently recognized species, it is one of the most comprehensive early Cenozoic avifaunas known to date. Some taxa are unknown from middle Eocene sites, while others have undergone few morphological changes over several million years, which may constitute evidence for niche conservatism in balanced ecosystems. The fossils show a high taxonomic concordance with coeval bird assemblages in North America, but the extent to which early Eocene avifaunas were similar on a global scale is unknown owing to the limited fossil record from the southern continents. The avian diversity of Walton‐on‐the‐Naze is not evenly distributed across neornithine phylogeny, and taxa of the Telluraves predominate. Other clades that produced arboreal birds are conspicuously absent. The occurrence of stem group representatives of multiple telluravian taxa suggests an initial diversification of the clade on the northern continents, which conflicts with biogeographic scenarios derived from the extant taxa. A notable feature of the avifauna of Walton‐on‐the‐Naze is the very small size of many species of the Telluraves and Strisores, which is attributed to the fact that these birds were at the beginning of an adaptive radiation.