Part of the reason so many ordinary people doubt the evolutionary link between dinosaurs and birds is because when they think of the word "dinosaur," they picture enormous beasts like Brachiosaurus and T. Rex, and when they think of the word "bird," they picture harmless, rodent-sized pigeons and robins (and perhaps the occasional eagle or penguin). See a gallery of dino-bird pictures
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Closer to where the action is, though, the visual referents are a lot different. For decades now, paleontologists have been digging up small, birdlike theropods (the same family of two-legged dinosaurs that includes tyrannosaurs and raptors) bearing unmistakable evidence of feathers, wishbones, and other avian anatomy. Unlike larger dinosaurs, these smaller theropods tend to be unusually well-preserved, and many such fossils are found completely intact (which is more than can be said for the average sauropod).
Dinosaurs, Birds and Evolution
What do these fossils tell us about the evolution of birds from dinosaurs? Well, for starters, it's impossible to pin down a single "missing link" between these two types of animals. For a while, scientists believed the 150-million-year-old Archaeopteryx was the indisputable transitional form; it's still not clear if this was a true bird (as some experts claim) or a very small, and not very airworthy, theropod dinosaur.
The problem is, the subsequent discovery of other dino-birds that lived at the same time as Archaeopteryx--such as Epidendrosaurus and Pedopenna--has muddied the picture considerably, and there's no ruling out the possibility that future paleontologists will unearth dino-birds from as far back as the Triassic period. In addition, it's far from clear that all these feathered theropods were closely related: evolution has a way of repeating its jokes, and feathers (and wishbones) may well have evolved multiple times.
To show how tricky this issue is, here's the standard picture of bird evolution: small, running theropods (for the sake of argument, let's say raptors) evolved feathers as a way of keeping warm and attracting mates. As these feathers grew larger and more ornate, they provided an unexpected bonus: a split-second of extra "lift" when their owner pounced on prey or ran away from larger predators. Multiply this scenario by countless generations, and you have a solid theory for the origin of avian flight.
Dino-Birds in the Trees
There are, however, a few complications. Although this "ground up" story of bird evolution is widely accepted by paleontologists, we have strong evidence that dino-birds like Scansoriopteryx and Microraptor spent most of their lives in trees. In addition, Microraptor appears to have had wings on both its front and back limbs--making it more like a gliding squirrel than a modern bird. Did feathered flight begin when these tree-dwelling dinosaurs' young accidentally fell out of the perch?
In any event, how do we know that these dino-birds led an arboreal lifestyle? Paleontologists often abstract prehistoric behavior from the lifestyles of similarly proportioned modern creatures. The long middle fingers of Epidendrosaurus look uncannily like the claws of some South American lemurs, whose sole function is to pry insects out of tree bark!
Too Many Theropods
Another problem with tracing the exact course of dinosaur-bird evolution is that so many likely ancestors technically belonged to different families. While all of the dino-birds were true theropods, some are classified as raptors, some as oviraptors, some as troodonts, some as ornithomimids and some as, well, your guess is as good as the experts'. The key thing is, all these creatures resembled each other more closely than they resembled the typical genuses in their extended families (for example, Sinornithosaurus looks a lot more like the troodont Sinovenator than it does its fellow raptor Deinonychus).
Further complicating matters, the behavior of small, feathered theropods seems to have been remarkably adaptable. Paleontologists have yet to discover any meat-eating ornithopods (these dinosaurs were strictly vegetarian), but at least two theropod dino-birds--Incisivosaurus and Falcarius--appear to have been plant eaters. The dietary habits of many other dino-birds are a matter of ongoing debate; some appear to have been insectivores, while most others (like their larger cousins, the ornithomimids) were probably omnivorous.
Next Page: The Dino-Birds of Liaoning, and a List of Notable Genuses
