While many dinosaurs, including Tyrannosaurus rex, had straightened tibial surfaces and stiffened drumsticks that only allowed for hinge-like knees, certain avian ancestors, including Rahonavis ostromi and Ichthyornis dispar, showed indications of curved tibial surfaces and a shortened, thinner fibula that was free to move on its own.

They suspect the jointed beak was present in even older birds, because the rest of the specimen indicates it was a relative of Ichthyornis, another ancient bird that lived about 20 million years earlier.

Several skulls of its older Ichthyornis relative have been described in recent years with bones that suggested the bird’s upper palate might have been jointed, but the evidence was still fuzzy.

Ichthyornis dispar lived in North America about 86 million years ago.

The bones of Ichthyornis were first found in the 1870s by the US palaeontologist Othniel C Marsh.