Before a bird hatches from the egg, its hip looks like a dinosaur’s pelvis, changing into a bird’s just days before it hatches, according to research by Yale University scientists.
“It’s amazing that we have anatomies in living animals that until now we thought were locked away in the fossil record,” said Christopher Griffin, a post-doctoral researcher and lead author of a paper published in the journal Nature. “But it turns out they’re present — temporarily present, but present — in birds.”
That means the anatomy can be “studied directly,” Griffin said. “You can actually look at the genetic changes and the genetic architecture that underlies this anatomy. You can’t do that in a dinosaur that’s 250 million years old.” There’s no DNA in a fossil, he said.
Griffin and Bhart-Anjan Bhullar, assistant professor of Earth and planetary science at Yale and senior author, along with others, examined how the pelvis of a bird looks much like that of a Tyrannosaurus rex or other dinosaur until late in its development, just before it turns from cartilage to bone, a phase known as “terminal addition.”
While all vertebrates develop similarly, the dramatic change inside the egg is a new insight, enabled by technology that allows the scientists to look at the embryo’s bones, muscles and nerves even on a cellular level, Griffin said.
“That’s why this hasn’t been noticed before,” Griffin said. “People have been studying bird embryos since Aristotle.”
“It’s still the age of dinosaurs,” Bhullar said. “I think vertebrates are inherently interesting. … They’re certainly the most complex life form that has ever appeared on Earth.”
And birds have unique features that make them even more fascinating, Bhullar said. “Among the vertebrates, birds are extraordinarily successful, and there are numerous superlatives that apply only to them.”
In order to be able to fly, “among the most demanding forms of travel but also among the most powerful [and] transcendent,” birds’ bodies are “the most exquisitely machined bodies of any vertebrate,” Bhullar said.
But their abilities “in a way also constrain the body form of birds,” he said. “There are characteristic shapes of all the body parts of birds and they all have to work together. You can instantly recognize a bird hip or a bird arm. That is something that they inherited from their dinosaurian ancestors.”
As dinosaurs evolved into birds, the changes in their anatomy evolved as well, Bhullar said. “The beginnings of becoming a bird involved a set of modifications that permitted early dinosaurs to walk on two legs, and these modifications were very substantially concentrated in the hip or the pelvis,” he said.
The changes were “basically an elongation of everything,” so that a bird’s pelvis extends most of the length of its stiff, compact body. Along with that came the expansion of the muscles needed to support the bird’s weight.
“In birds, it’s expanded beyond other dinosaurs,” Bhullar said.
That contrasts with lizard legs, for example, which are “not unmuscular, but they’re kind of cylindrical and skinny.” Chicken thighs, on the other hand, have meat on the bone.
The change from dinosaur to bird hip so late in its development also is unusual. “My expectation at least was that most of these alterations would be present early on,” Bhullar said. “The earlier you mess up a process, the more messed up it will be in the end.”
As vertebrates develop from a single cell, the genes tell the cells how to change and what to become: skin, bone or brain. “You’ve got to change instructions during assembly,” Bhullar said.
He expected changes that reach so far back in evolutionary history to occur early in the embryo’s development. “We had found previously that was the case … for the bird beak, but it turns out not to be the case for the pelvis,” he said. “In the lifetime of every single bird in the little microcosm that is the egg, there is a passage through stages that resemble those of their dinosaurian ancestors.”
Since this is true, that means that every bird genome still has the instructions to make “dinosaur-like hips,” Bhullar said. Usually, genetic instructions that are no longer used “get scrambled because there’s nothing operating to stabilize them,” he said. “They get thrown in an attic somewhere and left to degrade.”
Other structures in the bird embryo also resemble those of dinosaurs, but they “appear and then disappear, so they’re transient,” Bhullar said. One is the pubic boot, which is at the end of the pubic bone, and which scientists thought had disappeared 65 million years ago.
“The assumption was that there was nothing on Earth that has that particular structure at any point during its lifetime,” Bhullar said.
The technology the research team used is called CLARITY, Griffin said, and consists of three steps: making the embryo transparent, then staining it so the nerves, muscles and bones glow under a laser, and finally viewing it through a confocal microscope.
Bhullar said he considers himself a historian as well as a scientist.
“What my lab does fundamentally is, we look around frankly at the modern world and we look at extant biodiversity and we try to identify those vertebrates that have had a disproportionate impact on the modern world and the tapestry of life on the face of the planet,” he said.
“Indisputably, flourishing of birds is one of the great events in the history of life on Earth.”
Ed Stannard can be reached at estannard@courant.com, 203-993-8109.
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