The earliest bird-line archosaurs and the assembly of the dinosaur body plan

The archosaur species Teleocrater rhadinus , part of the new clade Aphanosauria, is an example of the earliest divergence of the avian stem lineage (Avemetatarsalia), the lineage that contains dinosaurs (including birds). Earliest avian archosaur The early history of the bird-line archosaurs, a group including dinosaurs, birds and pterosaurs, but excluding crocodilians, is not well defined. This is due in part to a fragmentary fossil record, but the distinctive morphology of pterosaurs has also obscured their ancestry. Sterling Nesbitt and colleagues describe a new species, Teleocraterrhadinus , from the Middle Triassic of Tanzania, that represents the most primitive known member of the bird-line archosaurs. Teleocrater provides the best guide so far to the ancestral bird-line condition. It was a lightly built, quadrupedal carnivore, so more like a crocodile than the small bipeds often depicted at this point in archosaur evolution. These are long-awaited findings on Teleocrater , which was undergoing study by the late Alan Charig of the Natural History Museum in London, and remained unpublished on his death in 1997. The relationship between dinosaurs and other reptiles is well established 1 , 2 , 3 , 4 , but the sequence of acquisition of dinosaurian features has been obscured by the scarcity of fossils with transitional morphologies. The closest extinct relatives of dinosaurs either have highly derived morphologies 5 , 6 , 7 or are known from poorly preserved 8 , 9 or incomplete material 10 , 11 . Here we describe one of the stratigraphically lowest and phylogenetically earliest members of the avian stem lineage (Avemetatarsalia), Teleocrater rhadinus gen. et sp. nov., from the Middle Triassic epoch. The anatomy of T. rhadinus provides key information that unites several enigmatic taxa from across Pangaea into a previously unrecognized clade, Aphanosauria. This clade is the sister taxon of Ornithodira (pterosaurs and birds) and shortens the ghost lineage inferred at the base of Avemetatarsalia. We demonstrate that several anatomical features long thought to characterize Dinosauria and dinosauriforms evolved much earlier, soon after the bird–crocodylian split, and that the earliest avemetatarsalians retained the crocodylian-like ankle morphology and hindlimb proportions of stem archosaurs and early pseudosuchians. Early avemetatarsalians were substantially more species-rich, widely geographically distributed and morphologically diverse than previously reco