New species from the early Eocene London Clay suggest an undetected early Eocene diversity of the Leptosomiformes, an avian clade that includes a living fossil from Madagascar

We report the first records of the leptosomiform taxon from the early Eocene London Clay of Walton-on-the-Naze (Essex, UK). Two species could be distinguished, one of which is described as , sp. nov. (the other is classified as sp.). We furthermore describe two small leptosomiform-like birds from Walton-on-the-Naze, , gen. et sp. nov. and , gen. et sp. nov., which are represented by partial skeletons of multiple individuals. In plesiomorphic characters of the coracoid and furcula, the new taxon is distinguished from and the extant ; the skull of furthermore exhibits basipterygoid processes, which are absent in . Another new species, , gen. et sp. nov., is based on a partial skeleton from Walton-on-the-Naze and closely resembles in the overall shape of the tarsometatarsus. However, is clearly distinguished from leptosomiform birds in the morphology of the hypotarsus, the more massive coracoid, and other skeletal features. Although we consider close affinities to and, hence, the Leptosomiformes to be possible, we could not identify derived characters that enable an unambiguous phylogenetic placement of . In any case, the new species document an unexpected diversity of -like fossil in the London Clay and may open a new perspective for a reconstruction of the ancestral morphology of the avian clade Afroaves, which includes the Leptosomiformes and other arboreal landbirds.