What dinosaurs evolved into birds?
Birds evolved from dinosaurs that lived in trees and learned to fly, like Archaeopteryx – right? Wrong, according to two Australian researchers.
By measuring the angles of modern birds’ claws and comparing them with claws from fossils of Archaeopteryx and other feathered or “fuzzy” dinosaurs, the researchers have concluded that ancestral “dino-birds” were small, agile ground dwellers.
Ph.D. student Chris Glen and Associate Professor Mike Bennett from the University of Queensland’s School of Biomedical Sciences, studied the claws of thousands of birds and dinosaurs to come to conclude that the dinosaur ancestors of modern birds were mostly ground foragers.
“This is a really interesting and important piece of the puzzle in terms of the evolution of birds,” Chris Glen said.
He explained to me that the shape of a bird’s claw can clearly indicate whether they are tree dwellers or ground foragers.
“We found a clear pattern that shows species that spend more time foraging in trees will have claws that are more,” he said.
“You can tell whether a bird species spend more time in trees or on the ground by seeing if they have more hooked claws (tree dwellers) or straight claws (ground foragers).”
Comparing Bird Claws with Dinosaur Claws
Having found the key to the bird’s foraging behavior from the shape of their claws, Bennett and Glen then compared bird claws with fossil dinosaur feet.
Archaeopteryx has long been considered the ancestor of birds, as it appears to be half dinosaur-half bird.
But Archaeopteryx isn’t the only ‘missing link’ between dinosaurs and birds.
There are also the more recently discovered Deinonychus and the velociraptors – “small, nimble predatory animals with very good eyesight and balancing tails” according to Chris. They also had feathers.
Since the discovery of these dinosaurs in the late 1960s, and particularly with more recent finds in China of hundreds of “fuzzy dinosaurs” (some of them almost like modern birds), paleontologists have argued whether birds evolved from dinosaurs living in trees like Archaeopteryx or ground-dwelling ones like Deinonychus.
Flying: Tree Down Or Ground Up?
It was a dispute between the ‘tree-down’ hypothesis – a progression from gliding from branches of trees just as small mammals like squirrel gliders do today, to flying, or the ‘ground-up’ hypothesis.
Glen and Bennett measured the feet of a range of dinosaurs from nonflying species with proto-feather fuzz, through the famous feathered Archaeopteryx, to the early birds with well developed flying abilities.
The ‘ground-up’ hypothesis – where somehow the ancestral birds went from jumping to flying – seems to have been validated by their claw measurements.
“For a large part of the 20th century the favored argument in bird evolution was that their ancestors went into trees to carve out a niche looking for food,” Dr. Bennett said.
Proto-Birds Lived on the Ground
However, their findings contradicted those arguments. The measurements showed the “fuzzy dinosaurs” and proto-birds had straight claws indicating they lived on the ground.
“We were very surprised by our findings as we expected to see some may have been tree dwellers, as there is some reasonable logic to the idea that flight would have evolved in a tree dweller, but what we saw was such a clear signal to the contrary,” Chris Glen said.
Dr. Bennett said their work adds another part of the puzzle that, along with other lines of evidence, helps build the modern view of how birds evolved from their early feathered dinosaur beginnings to the many species today.
Their research was published in the November 2007 issue of Current Biology.