Mononykus (Pavel Riha) |
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).
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 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.
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