Abstract
We report the first leg bones of the anseriform bird Nettapterornis (“ Anatalavis ”) oxfordi from the type locality of the species, the early Eocene London Clay of Walton-on-the-Naze (Essex, UK). In order to exclude the possibility that the specimens are from bony-toothed birds (Pelagornithidae), we also survey the pelagornithid material from Walton-on-the-Naze, which belongs to a large species identified as Dasornis cf. emuinus . The tarsometatarsus of N. oxfordi closely resembles that of the late Oligocene/early Miocene taxon Paranyroca and corresponds to the tarsometatarsus of the Anatidae in that it lacks a fossa metatarsi I. By contrast, the hypotarsus exhibits a plesiomorphic morphology more similar to that of the Anhimidae. Phylogenetic analyses yielded conflicting results concerning the position of Nettapterornis , which was either recovered outside crown group Anseriformes or—in a taxonomically pruned data set—as the sister taxon of the Anseranatidae. We consider the underlying character matrix to be inadequate for a robust placement of the taxon and hypothesise that Nettapterornis is a stem group representative of the Anatidae. If so, it would be the earliest and phylogenetically most basal representative of the clade. Based on its tarsometatarsus morphology, N. oxfordi probably had a predominantly aquatic ecology and may have been feeding on molluscs, crustaceans, and other marine invertebrates. The fact that both tarsometatarsi are from juvenile or subadult individuals shows that Nettapterornis was breeding in the palaeoenvironment of Walton-on-the-Naze.