Exceptionally well-preserved early Eocene fossil reveals cranial and vertebral features of a stem group roller (Aves: Coraciiformes)

Abstract

Three-dimensionally preserved skulls of small Paleogene land birds are very rare. Here, we describe a cranium and associated partial postcranial remains of an early Eocene stem group roller (Aves: Coraciiformes) from the London Clay of the Isle of Sheppey (England). The fossil shows features of the skull and vertebral column in great detail. It is distinguished from extant Coraciidae and Brachypteraciidae in several presumably plesiomorphic characteristics, which are likely to reflect differences in diet and/or foraging strategy between Eocene and extant rollers. Preserved stomach contents in other early Eocene fossils indicate that fruits were a regular part of the diet of stem group rollers. The extant Coraciidae and Brachypteraciidae, by contrast, almost exclusively feed on larger-sized invertebrates and small vertebrates, which are usually dispatched by beating before being swallowed. Stronger biting forces as well as the characteristic prey manipulation behavior of extant rollers may account for some of the observed differences in the cranial and vertebral morphology of the fossil and extant taxa, but the exact functional correlations remain elusive. We furthermore identify a previously undescribed cranial feature of rollers: a very large foramen for the ramus occipitalis of the arteria ophthalmica externa, which is of unknown functional significance and constitutes a potentially promising research target for future studies.