Look out your window, and you may just spot a living dinosaur. Instead of slipping into total annihilation 66 million years ago, the avian line of dinosaurs managed to not only survive but thrive in the aftermath of a mass extinction, giving rise to modern birds.
Fossils can help us trace the ways the fearsome reptiles transformed into the feathered flocks we see today. But it’s also possible to work backwards, using the rich evolutionary record written in birds’ soft tissues and genes.
Yale paleontologist Bhart-Anjan Bhullar has been picking away at this intersection where the modern meets the prehistoric. Pairing ancient bones and tracks with an understanding of the way modern species develop, Bhullar has been using “time-tested evolutionary biological techniques” to explore the dinosaurs’ transformation.
At purely superficial levels, Bhullar says, the differences between ancestral dinosaurs and today’s birds can seem overwhelming: “Birds are a lot smaller. The bird skull is a lot smaller,” but it holds an enlarged brain, he says. “They have no teeth. They actually have a very short face. They have a very different architecture.”
The secret is that some of those major changes spring from relatively minor tweaks early on in an animal’s development. “At first this seems like a million changes,” Bhullar says, “but it turns out that many of these changes are potentially explainable if you look at birds as potentially juvenilized versions of the ancestral dinosaur.”
That’s what Bhullar and colleagues concluded in a 2012 paper surveying changes in dinosaur skull shape. The large brain, big eyes and short faces of birds are all traits shared with infant dinosaurs, meaning that today’s birds are baby-faces.
But Bhullar hasn’t stopped there. Part of the mystery of the change from bird to dinosaur has been the evolution of the beak. The upper beak of birds is built upon a single, large bone called the premaxilla. This bone is small, often has teeth and makes up just the tip of the snout in most dinosaurs. But during the evolution of birds, the paired premaxillae bones expanded and became the main skeletal anchor for a toothless beak. How did this happen?
“It’s something to do with more specific patterning genes,” Bhullar says. These are the genetic instructions that tell early embryos how cells should move around to sculpt an adult animal.
As detailed in a paper published earlier this year, Bhullar and his colleagues found that making small changes to these patterning genes in chickens allowed them to re-create a face more similar to that of their non-avian dinosaur ancestors. The experiment resulted in chicken embryos “growing to have skeletons that were in many ways more ancestral,” Bhullar says. That included small, rounded jaw bones that “were strikingly more like the ancestral form.”
In his latest work, announced at the annual Society of Vertebrate Paleontology meeting last month, Bhullar described how a different jaw bone changed with the evolution of birds. This bone, the maxilla, is huge and holds most of the upper teeth in dinosaurs but is reduced to a tiny strut in birds. Paired with the previous research, the new science lays out how the tip of the dinosaur snout grew to become the major part of the bird beak, while the toothiest part of the dinosaur jaw shrunk to almost nothing.
Element by element, Bhullar and his colleagues are starting to understand the underpinnings of one of the most transcendent transitions in evolutionary history. Not that Bhullar is interested in creating a “chickenosaurus”—such proposals make headlines but would only mask the real wonder of what time and evolution have gifted us with.
“We’ve known that animals, organisms, carry in their morphology the legacy of their histories,” Bhullar says. “In the tiny parts of their morphology, in the nucleic acid that forms the genome itself, there are far more features—burdens of history, relics that are left, molecular fossils—and these perhaps represent a treasure trove of potentialities that we can use to explain the history of life, and maybe even its future.”
source: smithsonian.com By Brian Switek