The developing bird pelvis passes through ancestral dinosaurian conditions

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All baby birds have a moment prior to hatching when their hip bone is a tiny replica of a dinosaur's pelvis.

That's one of the findings in a new, Yale-led study in the journal Nature that explores the evolutionary underpinnings of the avian hip bone. It is also a modern-day nod to the dramatic transformation that led from dinosaurs to birds over tens of millions of years.

"Every single bird, in its early life, possesses this dinosaurian form," said Bhart-Anjan S. Bhullar, assistant professor of Earth & planetary science at Yale and senior and corresponding author of the new study. "Then, at the last minute, it's like it remembers it's a bird and needs a bird's pelvis."

(...) "It was unexpected to find these initial stages of bird development look so much like the hips of an early dinosaur," Griffin said. "During just two days, the developing embryo changes in a way that reflects how they changed in evolution, transitioning from looking like an early dinosaur to looking like a modern bird."

The hip bone is the core of a bird's body. It runs the length of the avian frame, engulfing the torso, while also enabling a bird to stand, move, and carry the weight of its entire body.

"The bird body is incredibly modified in virtually every way to create an optimized flying machine," Bhullar explained. "Its body structures are tightly constrained by the necessities of aeronautic design."