Crucial Avian Wrist Bone Found in Theropods Millions of Years Earlier Than Anticipated

A groundbreaking new study published in the journal Nature reveals that a key wrist bone essential for avian flight, the pisiform, was present in theropod dinosaurs millions of years earlier than previously understood. This discovery, led by James Napoli of Stony Brook University, significantly reshapes the timeline of how birds evolved the anatomical features necessary for flight. The research challenges the long-held belief that this specific bone was lost in early theropod evolution and only reappeared with the advent of birds.

In modern birds, the pisiform bone plays a critical role in stabilizing the wing during flight and facilitating the automatic folding of the wing when not in use. Its unique shape, including a V-shaped notch, allows it to clasp the hand bones, preventing dislocation. Prior to this study, paleontologists largely believed that the pisiform, a sesamoid bone that functions similarly to a kneecap, had disappeared from the theropod lineage and was re-acquired by birds as a specialized adaptation for flight.

The team identified the pisiform in two exceptionally preserved Late Cretaceous theropod fossils from the Gobi Desert in Mongolia: an oviraptorid (specifically, a Citipati) and an unnamed troodontid. The use of high-resolution micro-computed tomography (µCT) allowed researchers to create detailed three-dimensional visualizations, revealing the tiny, bead-like carpals in their correct anatomical positions. Alex Ruebenstahl of Yale University noted the surprise of finding a bone "that wasn’t supposed to be there."

This finding suggests that the "reorganization of the theropod wrist preceded the origin of avian flight," as stated in the paper's title. The presence of the pisiform in these non-avian theropods, part of the Pennaraptora group (which includes dromaeosaurids, troodontids, and oviraptorosaurs), indicates that this vital component of the avian flight apparatus was in place before the full development of powered flight. It implies that the evolutionary path towards flight was a more gradual, stepwise process than previously envisioned.

The study contributes to a growing body of evidence indicating that many traits traditionally considered unique to birds, such as thin-walled bones, enlarged brains, and feathers, actually originated much earlier within their theropod dinosaur ancestors. The discovery of the ancient pisiform underscores that the complex architecture of the avian wrist, crucial for the extraordinary maneuverability of modern birds, was assembled over a prolonged evolutionary period, hinting that avian flight may indeed have been "all in the wrist" from a much earlier stage.