Heavier Dino Arms Led Evolution To Birds
As paleontologists increasingly unearth evidence of feathers in prehistoric fossils, our conceptions of what dinosaurs looked like when they roamed the earth has gradually evolved.
Instead of the reptilian appearance we all recognize from childhood toys and films like Jurassic Park, many dinosaurs in fact more closely resembled birds, kind of like this recently discovered little guy, Eosinopteryx brevipenna, a flightless theropod dinosaur that roamed China during the Middle/Late Jurassic period.
Archaeopteryx, also known as Urvogel, the German word for "original bird" or "first bird," was first discovered in 1860 and later fossils of this species presented some of the earliest evidence of flight in these prehistoric animals.
An intermediate creature that was not quite dinosaur but not exactly a bird either when it lived 150 million years ago, Archaeopteryx had teeth, a long tail, and wings capable of flight with claws at the end for grabbing prey.
A century after the discovery of Archaeopteryx, paleobiologists increasingly found anatomical connections between birds and dinosaurs. In the 1970s, artists began to portray dinosaurs with feathers based on accumulating evidence.
Megapnosaurus, shown here, was another species with whom researchers began early to identify with feathers. A lightweight animal that could reach up to 10 feet in length and roamed Jurassic Zimbabwe, Megapnosaurus, also known as Syntarsus, traveled in packs and preyed on small reptiles and fish.
Many of the fossils unearthed that provided evidence of feathers had deteriotated over the eons they remained buried. It wasn't until 2010 that researchers identified color pigments in feathers from dinosaurs and early birds.
Sinosauropteryx, illustrated here, was a theropod dinosaur that had "simple bristles -- precursors of feathers -- in alternate orange and white rings down its tail," according to a description of the study's findings.
Given that so many feathered dinosaurs were in fact flightless, the purpose of the feathers has been a subject of debate. Some dinosaurs may have evolved feathers for social signaling; others had plumage to provide insulation.
In the cases of some dinosaurs, such as the two oviraptors, herbivores related to T. rex that lived during the Cretaceous period, researchers believe the feathers were used for mating displays, similar to modern-day peacocks and turkeys.
Jason Brougham/University of Texas
You might be fooled into thinking the animals in this illustration are something between a murder of crows and a band of blue jays. In fact they are Microraptors that lives more than 130 million years ago.
These four-winged, plumed dinosaurs were no larger than modern-day pigeons and sported iridescent tail feathers.
Researchers believe the shimmering plumage was likely used in mating and other social interactions.
Like the diversity among birds today, not all feathered dinosaurs were lightweight, agile animals. A massive tyrannosaur that lived in China until about 65 million years ago, Yutyrannus huali, meaning "beautiful feathered tyrant," grew up to 30 feet long and could weigh more than 3,000 pounds.
This titanic tyrannosaur, as it was described, significantly increases the size range for feathered dinosaurs.
In a stunning find published in the journal Science in 2011, paleontologists uncovered dinosaur feather preserved in amber that dated back some 79 million years ago.
This discovery provided scientists a new window into the evolution of feathers in terms of structure in the evolutionary timeline from dinosaurs to birds. Even shades of color remained well preserved in the amber.
Many dinosaurs, like T. rex, had scrawny arms, but paleontologists have discovered that as dinosaurs gradually evolved bigger arms, they began to stand and move more like birds.
The change, documented in the journal Nature, passed on to the descendants of dinosaurs -- birds themselves.
"Our study shows how mass was allocated to the forelimbs, starting in non-flying dinosaurs, to turn them into longer, heavier, more muscular wings that became more and more effective for flapping during flight," co-author John Hutchinson of the Royal Veterinary College’s Structure and Motion Lab told Discovery News.
Hutchinson and his colleagues used digitizing technology to create 3D images of the skeletons of 17 archosaurs, a group that included living crocodiles and birds as well as extinct dinosaurs. The researchers then digitally added flesh around the skeletons to estimate the overall shape of the body as well as the individual body parts, such as the head, forelimbs and tail.
The scientists found that as the arms got bigger, eventually turning into wings in some species, the hind limb posture got progressively more crouched as the center of mass moved forward. Before then, dinosaurs that stood on two legs had a fairly straight posture, similar to that of humans. Maniraptoran dinosaurs, such as Velociraptor, which were in the lineage that evolved into birds, dramatically developed this crouching trait.
Posture varies among today’s birds, with larger species like ostriches tending to have straighter legs to more economically support their own weight. But even these birds still have fairly crouched limbs overall.
The discovery calls into question an earlier theory that held dinosaurs became more bird-like when their tails grew shorter and lighter. Hutchinson and his team acknowledge that happened, but only after the other changes took place.
He explained that "the shortening of the tail coincided with a reduction of a once-large tail muscle that connected to the thigh and moved the leg back and forth during locomotion. Birds only have a tiny remnant of that muscle."
Why the changes occurred is not yet clear.
Hutchinson suspects that "based on the anatomy of the dinosaurs that re-enlarged their forelimbs, they were using them for grasping -- various food, each other, whatever -- and maybe for climbing sometimes, and then much later they were used for flight. So the grasping/predatory/feeding/manipulatory functions of the forelimb probably drove that trend early on, then once flight evolved that added more impetus."
The crouched position would seem to be less sturdy, but that’s actually not the case.
"Biomechanical studies suggest that more crouched limbs can make animals more stable, more stealthy and more bouncy -- better for running, perhaps -- among other things," he said. "All birds maintain some sort of crouched posture."
Paul Barrett, a dinosaur researchers in the Department of Paleontology at the Natural History Museum, told Discovery News that the new paper is "a nice piece of work that has tested a previously widely-accepted idea about how birds acquired some of the features we associate with flight."
"The idea that enlargement of the arms and some other changes to the hips has more influence on bringing the center of mass forward than the reduction of the tail, the usual explanation, is interesting and new," Barrett added. "The linkage between changes to the forelimbs and hind limbs is also an interesting outcome, suggesting tighter coupling of the evolution of these two parts of the body than usually realized."
The findings additionally negate many 1950s reconstructions of dinosaurs, including the fictional "mutant dinosaur" Godzilla, always shown with a horizontal backbone. An artist today might draw predatory, prey-grabbing Godzilla with more of a crouched position, in keeping with that of actual dinosaurs that had somewhat similar behaviors.