New remains of the Eocene Prophaethon and the early evolution of tropicbirds (Phaethontiformes)

A partial skeleton of the early Eocene tropicbird Prophaethon shrubsolei (Prophaethontidae) from the London Clay of Walton‐on‐the‐Naze (UK) is described. Most of the bones preserved in this fossil have not been described previously for Prophaethon or prophaethontids in general. The pelvis and leg bones of Prophaethon, including the previously unknown tarsometatarsus, are very different from modern tropicbirds and show a strong similarity to procellariiform birds, especially albatrosses. Rather than employing plunge‐diving like their extant relatives, prophaethontids therefore probably used foraging strategies similar to those of modern albatrosses, which seize food on the sea surface. Prophaethontids also appear to have been less pelagic than extant tropicbirds, and these different life modes, as well as the disappearance of Phaethontiformes from northern latitudes, attest to major changes in Northern Hemispheric marine avifaunas during the Cenozoic, which also affected other pelagic birds. The reasons for the profound changes in the historical biogeography and way of living of tropicbirds are unknown, but ecological competition and predation at breeding sites are likely to have played a role.