Birds of a Feather - Part 1

by James Lamb, Paleontologist

The pair of fighting dinosaurs tumbled across the sidewalk in front of me, bringing me to an abrupt halt. Screeching, flipping tail over talon, grasping at each other with their sharp, three-toed hind feet and stabbing at each other with their mouths, they didn’t even pause when they rolled over a vertical drop twice their own body length. Finally they separated, each lying on its back, and continued to screech ferociously at one another. It was just in time too, as the drop off the curb had brought them perilously close to the street. Well, at least I didn’t have to jump in and break them up before they got in traffic. Then, as the two White-throated sparrows flew off to nurse their wounds in relative safety, I couldn’t help but think how glad I was that these little feathered dinosaurs were so tiny compared to their now extinct cousins. Imagine if they were the size of Deinonychus, the villains of Jurassic Park (incorrectly called Velociraptor in the movie) - no one would ever go outside.

At this point you are perhaps wondering if you read the previous paragraph correctly. Did I really mean that birds are little feathered dinosaurs? Well, yes. As amazing as it seems, birds are essentially the only group of dinosaurs to survive the global catastrophe 65 million years ago when something like 65% of all life on Earth became extinct. Dinosaurs had been the most successful terrestrial vertebrates ever; a group that dominated terrestrial ecosystems for 150 million years. And, except for birds, they’re all gone. But – dinosaurs? Really? Let me start the story at the beginning, always a good place to start.

The oldest known bird, Archaeopteryx, lived about 145-150 million years ago. The first specimen was discovered in 1861 and right from the beginning stirred controversy because of its mixture of avian (bird-like) features such as feathers, and more “reptile-like” features such as teeth and clawed, three-fingered hands. There were scientists even then who thought that the best match for bird ancestors were the dinosaurs. At the time though, there was only the one Archaeopteryx specimen and no particularly good match amongst dinosaurs for many features of the skeleton then known only in birds. The ancestry of birds remained in a sort of limbo for the next 100 years, with various groups of reptiles favored by different paleontologists. All that changed in 1969 with the publication of the formal description of Deinonychus. Deinonychus broke the mold of the old stereotype that dinosaurs were slow, lumbering, dim-witted animals with “extinction” stamped on their bony foreheads. Not only does the skeleton imply speed and grace, they had a sickle-shaped claw on the second toe of each hind foot that could be rotated through 180 degrees. The claw was held off the ground when running and then rotated into slashing/killing position once the prey had been captured. One of my college professors used to refer to Deinonychus and its kin as “Bobcats with a switchblade on each foot”. Perhaps equally as interesting were the skeletal similarities to birds.

One of the things that is unique about birds is the special arrangement of bones in the wrist that allows them to fold their hand, that is, the wing, which is mostly formed from an elongated hand, up against their body, and rotate the hand/wing properly to achieve the flight stroke. Deinonychus had this same feature. Deinonychus also possessed what until then was a uniquely bird characteristic called the “triosseal canal”, a trough formed by three bones in the shoulder region that allows the flight muscles that attach to the sternum, the breastbone, to loop over the shoulder like a pulley and power the recovery stroke of the wing during flight. Deinonychus appears to have evolved these features to use its long and mobile arms in capturing prey. Finding these features in Deinonychus started a flurry of renewed interest in the dinosaur/bird connection, and over the next couple of decades, as new dinosaur discoveries were made, and more Archaeopteryx and other early birds were found, paleontologists documented over 100 skeletal features shared uniquely in the animal world by only dinosaurs and birds.

Then came the fossils from dinosaur-aged lake deposits in China. The chances of any particular organism being preserved in the rock record are not good. The main reason for this is that most animals don’t die in an environment where they will be quickly covered up by rock-forming sediments such as sand or clay. The ancient freshwater lake deposits in China are an exception. Animals were buried quickly in fine-grained sediment that not only preserved the skeleton in near life position, but also so effectively sealed the fossil from the air that traces of soft tissue are often preserved. Many of the fossils coming from these deposits are of feathered dinosaurs. That is, traces of feathers, in some cases surrounding the skeleton in a sort of halo are preserved as a carbonaceous film. Not only did scanning electron microscope images show that these have the highly distinctive structure of modern bird feathers, they also have the same chemical signature as feathers. Feathers are made from a tough structural protein called keratin. Hair, fingernails and claws are other examples of biological structures made from keratin. However in the animal world only feathers are made solely of a single type of keratin, called beta-keratin.

Next week: Birds of a Feather Part 2

Contributor Profile:

James Lamb is a native Birminghamster with a nearly life-long interest in fossils. He collected his first fossil when he was 5 years old. Friends and family members assure him he has not matured much since then. James is curator of paleontology at McWane Science Center, and would like to one day see the absolute treasure trove of Alabama's fossil heritage revealed to the public. When not at work he wishes he were in the field digging up fossils. At home he enjoys reading, jogging, woodworking, and carving. He has been informed that he is in the habit of telling atrocious puns, but this comes as a surprise to him. James describes himself as a "science nerd".