Part of the reason so many ordinary people doubt the evolutionary link between feathered dinosaurs and birds is because when they think of the word "dinosaur," they picture enormous beasts like Brachiosaurus and Tyrannosaurus 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 feathered dinosaur pictures
Closer to the Jurassic and Cretaceous periods, though, the visual referents are a lot different. For decades, 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 bits of avian anatomy. Unlike larger dinosaurs, these smaller theropods tend to be unusually well-preserved, and many such fossils have been found completely intact (which is more than can be said for the average sauropod).
Feathered Dinosaurs, Birds and Evolution
What do these fossils tell us about the evolution of prehistoric 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, but it's still not clear if this was a true bird (as some experts claim) or a very small, and not very aerodynamic, theropod dinosaur. (In fact, a new study claims that the feathers of Archaeopteryx weren't strong enough to sustain extended bursts of flight.)
The problem is, the subsequent discovery of other small, feathered dinosaurs that lived at the same time as Archaeopteryx--such as Epidendrosaurus, Pedopenna and Xiaotingia--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.
Feathered Dinosaurs up in Trees
There are, however, a few complications with this story. Although the "ground up" theory of bird evolution is widely accepted by paleontologists, we have strong evidence that feathered dinosaurs 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 feathered dinosaurs led an arboreal (tree-dwelling) lifestyle? Paleontologists often abstract prehistoric behavior from the lifestyles of similarly proportioned modern creatures. For example, 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 Feathered Dinosaurs
Another problem with tracing the exact course of dinosaur-bird evolution is that so many likely ancestors technically belonged to different families. While all feathered dinosaurs that we know of 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' (it's even possible that juvenile tyrannosaurs had a fine feather coating). The key thing is, all these creatures resembled each other more closely than they resembled the typical genera 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 dinosaurs seems to have been remarkably adaptable. Paleontologists have yet to discover any meat-eating ornithopods (these dinosaurs were strictly vegetarian), but at least two feathered "therizinosaur" theropods--Incisivosaurus and Falcarius--appear to have been plant eaters, and the large, ostrich-like ornithomimids were probably omnivorous.
The Feathered Dinosaurs of Liaoning
Every now and then, paleontologists stumble across a fossil treasure trove that forever changes the public's perception of dinosaurs. Such was the case in the early 1990's, when researchers uncovered rich fossil deposits in Liaoning, a northeastern province of China. All of the fossils date from about 130 million years ago, making Liaoning a spectacular window into the early Cretaceous period.
Although Liaoning has yielded fossilized insects, fish and mammals, among other creatures, it has become best known for its small, feathered dinosaurs. To date, paleontologists have uncovered dozens of exceptionally well-preserved fossils of feathered theropods, accounting for over a dozen separate genera. (You can often recognize a Liaoning dino-bird from its name; witness the "sino," meaning "Chinese," in Sinornithosaurus, Sinosauropteryx and Sinovenator.)
Since Liaoning's fossil deposits represent a mere snapshot in the long history of dinosaurs, their discovery has raised the possibility that more dinosaurs were feathered than scientists have ever dreamed--and that the evolution of dinosaurs into birds was not a one-time, linear process. In fact, it's very possible that dinosaurs evolved into what we would recognize as "birds" numerous times over the course of a hundred million years--with only one branch surviving into the modern era and yielding those familiar pigeons, sparrows, penguins and eagles.
Here's a list of the most notable feathered dinosaurs; just click on the links for more information.
Archaeopteryx This ancient, flying reptile was about the size of a modern pigeon.
Caudipteryx A birdlike dinosaur that changed the views of paleontologists.
Chirostenotes This birdlike dinosaur has been known by three different names.
Confuciusornis Guess what country this proto-bird was discovered in?
Eosinopteryx A tiny feathered dinosaur of the late Jurassic period.
Epidendrosaurus Did this tiny dino-bird spend its life up a tree?
Falcarius A bizarre, feathered theropod from North America.
Incisivosaurus This buck-toothed dinosaur was the Cretaceous equivalent of a beaver.
Khaan Few small mammals dared face the wrath of this dinosaur.
Mei The current record-holder for "shortest dinosaur name."
Microraptor This tiny proto-bird had four wings rather than two.
Mononykus This dinosaur may have dug into termite mounds for its lunch.
Nomingia This small dinosaur had a peacock-like tail.
Oviraptor Turns out that this "egg thief" got a bad rap.
Pedopenna One of the earliest known dino-birds.
Protarchaeopteryx "Before Archaeopteryx?" It actually lived millions of years later.
Rahonavis Was it a raptor-like bird, or a bird-like raptor?
Scansoriopteryx This early proto-bird probably lived in trees.
Shuvuuia Scientists can't decide if it was a dinosaur or a bird.
Sinornithosaurus A typical dino-bird of the early Cretaceous.
Sinosauropteryx The first dinosaur proven to have feathers.
Sinovenator This "Chinese hunter" preyed on its fellow dino-birds.
Xiaotingia This feathered dinosaur predated Archaeopteryx.