Analysis of a newly described fossil, reported online in Nature this week, suggests that certain features that have been presumed to be characteristic of dinosaurs, actually evolved much earlier, soon after the bird-crocodylian split. This finding forces a rethink of early dinosaur evolution.
Birds and crocodylians, both modern-day archosaurs, diverged from their common ancestor during the Triassic period. This major transition in terrestrial vertebrate evolution involved changes in limb proportions and body size, but these innovations are poorly documented in the fossil record. Sterling Nesbitt and colleagues describe the fossil remains of one of the earliest members of the avian stem lineage. Teleocrater rhadinus, from the Middle Triassic epoch of Tanzania, was a lightly built, quadrupedal carnivore - more like a crocodile than the small bipeds often depicted for animals at this point in archosaur evolution.
Teleocrater is assigned to an entirely new clade of reptiles, dubbed Aphanosauria, which sits at the base of the bird-line archosaurs before the split between pterosaurs and dinosaurs. The new clade has transitional morphologies, combining features present in the last common ancestor of birds and crocodylians, such as a crocodilian-like ankle joint, with some classic dinosaur characteristics. Overall, the study hints that these early stem avians were substantially more species-rich, widely geographically distributed and morphologically diverse than previously thought.