A new mousebird (Aves, Coliiformes) from the early Eocene London Clay of Walton-on-the-Naze (Essex, United Kingdom) constitutes a morphological link between sandcoleids and coliids

We describe a new species of the Coliiformes from the early Eocene London Clay of Walton-on-the-Naze (Essex, United Kingdom). Ypresicolius sandcoleiformis n. gen., n. sp. is represented by two partial skeletons and shows an overall resemblance to the Sandcoleidae in the preserved bones. However, the hypotarsus of the tarsometatarsus exhibits a characteristic derived morphology that is different from sandcoleids and closely resembles the hypotarsus of crown group Coliiformes (Coliidae). A phylogenetic analysis supported a sister group between Ypresicolius n. gen. and Pan-Coliidae, the total group including stem and crown group members of the Coliidae. We conclude that the similarities between Ypresicolius n. gen. and sandcoleids are plesiomorphic for the Coliiformes, and the low degree of morphological disparity between Ypresicolius n. gen. and sandcoleids indicates that the divergence of the sandcoleid and coliid lineages did not occur long before the earliest Eocene. The length proportions of the pedal phalanges of Ypresicolius n. gen. and sandcoleid Coliiformes are strikingly similar to those of Paleogene stem group representatives of the Strigiformes. The significance of this resemblance depends on the relationships of coliiform birds. Most current analyses do not recover owls and mousebirds as sister taxa, in which case the similar foot morphology of stem group Coliiformes and Strigiformes is either plesiomorphic for a more inclusive clade or evolved convergently in mousebirds and owls.