"This one lineage of dinosaurs that became ever-more-birdy over time happened to hit on something successful."
Image: Jason Brougham
Bird evolution is like a Legend of Zelda puzzle: the pieces may take a while to come together, but once they do, it's next-level.
Paleontologists have created what they claim is the most detailed family tree of meat-eating dinosaurs ever, revealing fresh insights into the origin of birds. By analyzing 853 body features across 150 specimens of coelurosaurs (a group including tyrannosaurs, raptors, and birds), researchers discovered that the transition from dinosaur to bird was gradual—that is, until classic bird morphology clicked into place.
At that point, some 150 millions years ago, the team observed a sudden explosion of diversity within the clade.
"We hypothesize that the classic avian body plan was assembled gradually over many millions of years, but when it came together fully, then bam, something changed," lead author Steve Brusatte told me. "Something was unlocked, allowing birds to evolve much, much faster than other dinosaurs."
The study, published today in the journal Current Biology, has broader implications for evolutionary biology as a whole. The sudden explosion of bird success, characterized by species like Archaeopteryx, substantiates a controversial theory first proposed by George Gaylord Simpson in the 1940s. Simpson argued that novel anatomical arrangements periodically result in a surge of evolutionary success.
"Simpson's ideas have been debated for many decades," said Brusatte. "There is currently a big debate about whether there are 'bursts' of evolution during the origin and early history of major groups like birds, mammals, etc. Some data and methods say yes, others say no. Our data and methods quite clearly indicate an early burst in the history of birds."
Brusatte and his colleagues mapped out this expansive family tree at the Theropod Working Group based at the American Museum of Natural History. The specimens they studied represented several epochs of dinosaurian evolution, tracing the origin of birds back to its murkiest beginnings in the Middle Jurassic. The oldest coelurosaur they studied lived 170 million years ago, while the youngest coelurosaurs were the extant birds of today
"The Theropod Working Group is an amazing thing," he said. "There are a few datasets out there that are bigger than ours, [with] more species and characters, but these are hodgepodge datasets that people make by combining stuff from the literature, without seeing the specimens themselves."
The study raises a flurry of new questions, including identifying why the avian body plan turned out to be such an evolutionary dynamo. Was it simply the ability to fly? That's certainly a game-changing adaptation, though it wasn't unique even in the Mesozoic. Could flight alone have propelled birds to evolve at such a "supercharged rate" respective to their fellow dinosaurs? Brusatte suspects it was more complicated.
"I think that over tens of millions of years of evolution, this one lineage of dinosaurs that became ever-more-birdy over time happened to hit on something successful," he said. "A new body plan, a new repertoire of behaviors, including flight, small size and perhaps faster metabolism, that opened up new niches and evolutionary possibilities."
Along those lines, birds may not be the only group that can back up Simpson's long-debated theory about evolutionary surges.
"Some recent evidence also suggests that dinosaurs as a whole experienced an early burst of change in the Triassic," said Brusatte. "Similar analyses haven't been conducted on other dinosaur groups, so we await new studies!"