The ankle joint that unites pigeons, pterosaurs and Tyrannosaurus in one family
Every bird at your feeder belongs to a lineage that also produced the giant dinosaurs and the leathery-winged pterosaurs. Palaeontologists call it Avemetatarsalia, meaning bird metatarsals, and it includes everything closer to birds than to crocodiles. What set the group apart early on was a new kind of ankle.
Archosaurs split into two great branches in the Triassic: one leading to crocodilians, the other to birds. The bird-line branch produced the group's two big successes. Dinosaurs were the largest land animals for much of the Mesozoic, and a single group of small feathered ones, the birds, never died out. Pterosaurs became the first vertebrates to fly under their own power and lasted until the mass extinction at the end of the Cretaceous.
The defining feature is the so-called advanced mesotarsal ankle, with a large astragalus bone and a small calcaneum working on one simple hinge. That mobility probably let the group's common ancestor stand upright on two legs held vertically beneath the body, much as mammals do. Bird-line archosaurs were also lighter than their crocodile-line cousins, with smaller heads and usually no bony armour plates in the skin. Filaments turn up widely too, from the fuzzy pycnofibres of pterosaurs to quill-like bristles on the plant-eating Psittacosaurus and true feathers on theropods. Flight evolved twice: pterosaurs stretched a skin membrane to an enormously long fourth finger, while birds much later used feathered arms.
The earliest members were not especially birdlike. Aphanosaurs, rare four-legged carnivores recognised as a group only in 2017, sit at the base of the tree, and one of them, Teleocrater, shows that many signature traits appeared later. Body fossils such as Asilisaurus date to about 245 million years ago, and footprints from the Holy Cross Mountains of Poland may push the origin back to around 249 million years ago, soon after the Permian extinction, though some researchers doubt who made the tracks.
The name itself has a tangled history. Jacques Gauthier proposed related clades in 1986, and Michael Benton coined Avemetatarsalia in 1999 after deciding the small reptile Scleromochlus fell outside Gauthier's grouping. Gauthier later offered Panaves, meaning all birds, for essentially the same branch.
Source: Avemetatarsalia
