Birds – from emu to hummingbird – are the sole surviving dinosaur lineage. You probably knew that, you smart cookie. But you might think that any real family resemblance has been lost to evolutionary changes. In fact, if you know what to look for you can spot plenty of dinosaur traits in your average ol' bird.
Compared to the amazing variety of vertebrate life on this planet, birds stand out. They are the only animals with feathers, for starters, not to mention other specializations like a complex air sac system that keeps their bones light while making them especially efficient at drawing oxygen from the air. But it’s only when we look into the depths of prehistory that the truth becomes clear: Many of the characteristics that we think of as unique to birds were also common among their unlucky, extinct cousins – the non-avian dinosaurs that haunt museum halls.
Take the most lucky bone in the skeleton, for example. Today’s fuzzy dinosaurs have a modified set of clavicles called a furcula – the wishbone you break at the end of your Thanksgiving gorge-fest. But some non-avian dinosaurs had a smaller, boomerang-shaped version, even though they weren’t using their arms to flap around. The small, toothless Oviraptor was one of the first dinosaurs to be found with a furcula, but flesh-ripping giants like the Allosaurus and Tyrannosaurus had the bowed bone, too.
Imagine the kinds of wishes you could make on that bad boy.
Specialized air sacs were also thought to have been uniquely avian at one time. These internal balloons – which not only help make bird skeletons light, but also make bird breathing superior to our simple in-and-out method – run out of the bird respiratory system to indent or even invade bones.
Because these structures weave their way into bone, their presence is preserved long after soft tissue disintegrates. That's how paleontologists were able to find them in two other dinosaur groups – the “beast-footed” theropods and the long-necked sauropods. This respiratory system may have helped those dinosaurs reach humongous sizes, but the adaptation proved especially useful for birds: Flying has a strict set of weight constraints, and better breathing is a boon when you spend life flapping around the sky.
The most telling hallmark of birds’ dinosaurian ancestry, however, is their feathers. Since 1996 paleontologists have uncovered a wealth of fossils from all over the world that have left no doubt that many non-avian dinosaurs were clad in protofeathers or similar, feather-like structures. The simplest of these are tubes that would have given dinosaurs like Compsognathus the look of a newly-hatched chick. Others are more complex, branching and carrying locking barbs, laying out the evolution of the feather from a singular wisp into an asymmetric structure capable of providing lift.
So far, feather forerunners have been found among the closest relatives of the first birds – think raptors – as well as tyrannosaurs, ostrich mimic dinosaurs and other members of the theropod family. Then there are dinosaurs like Kulindadromeus – a dinosaur as far from birds as possible, but with what look to be protofeathers of its own. This raises two possibilities. Either some kind of fuzz was present in the last common ancestor of dinosaurs – lost in some groups, retained in others – or fluff evolved over and over again in dinosaur history.
Regardless of which turns out to be true, it’s clear that feathers did not first evolve for flight. Protofeathers evolved for insulation and display at the outset were eventually co-opted to other uses -- like flapping to give clawed feet a better grip on inclined surfaces and, eventually, flight. Soaring through the air was an unexpected happenstance that rose out of feathers spreading far and wide through the dinosaur family tree. Think about that the next time you see a turkey vulture lilting overhead or a pigeon frantically fluttering off the curb. Those birds are the kin of Velociraptor, and the Age of Dinosaurs continues to this day.