Dino-Birds - The Small, Feathered Dinosaurs

The Evolution of Feathered Dinosaurs, from Archaeopteryx to Xiaotingia

haplocheirus
The late Jurassic Haplocheirus was equipped with primitive feathers (Nobu Tamura).

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 hummingbirds, or perhaps the occasional eagle or penguin. (See a gallery of feathered dinosaur pictures and profiles and an article explaining why birds aren't dinosaur-sized.)

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, meat-eating 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 discovered completely intact (which is more than can be said for the average sauropod).

Types of Feathered Dinosaurs

So many dinosaurs of the later Mesozoic Era sported feathers that it's virtually impossible to pin down the exact definition of a true "dino-bird." These include:

Raptors. Despite what you saw in Jurassic Park, Velociraptor was almost certainly covered with feathers, as was the dinosaur it was modeled on, Deinonychus.

At this point, the discovery of a provably non-feathered raptor would be major news!

Ornithomimids. "Bird mimic" dinosaurs like Ornithomimus and Struthiomimus probably looked like giant ostriches, complete with feathers--if not all over their bodies, at least on certain regions.

Therizinosaurs. All of the dozen or so genera of this small family of bizarre, long-clawed, plant-eating theropods likely had feathers, though this has yet to be conclusively proven.

Troodonts and oviraptorosaurs. Typified by, you guessed it, the North American Troodon and the central Asian Oviraptor, virtually all of the members of this theropod family seem to have been covered with feathers.

Tyrannosaurs. Believe it or not, we have conclusive evidence that least some tyrannosaurs (like the recently discovered Yutyrannus) were feathered--and the same may hold for the juveniles of Tyrannosaurus Rex.

Avialan dinosaurs. Here's where paleontologists classify the feathered dinosaurs that don't fit in the above categories; the most famous avialan is Archaeopteryx.

Further complicating matters, we now have evidence that at least some genera of ornithopods, plant-eating dinosaurs unrelated to modern birds, had primitive feathers as well! (For more on this subject, see Why Did Dinosaurs Have Feathers?)

Which Feathered Dinosaurs Evolved Into Birds?

What do all of these genera 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.) For more, see Was Archaeopteryx a Bird or a Dinosaur?

The problem is, the subsequent discovery of other small, feathered dinosaurs that lived around 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 dating to as far back as the Triassic period. In addition, it's far from clear that all of these feathered theropods were closely related: evolution has a way of repeating its jokes, and feathers (and wishbones) may well have evolved multiple times. (For more on this subject, see How Did Feathered Dinosaurs Learn to Fly?

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The Feathered Dinosaurs of Liaoning

Every now and then, a treasure trove of fossils forever changes the public's perception of dinosaurs. Such was the case in the early 1990's, when researchers uncovered the rich deposits in Liaoning, a northeastern province of China. All of the fossils discovered here--including exceptionally well-preserved feathered theropods, accounting for over a dozen separate genera--date from about 130 million years ago, making Liaoning a spectacular window into the early Cretaceous period. (You can 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 165-million-year-old rule of the dinosaurs, their discovery raises 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, non-repeatable, linear process. In fact, it's very possible that dinosaurs evolved into what we would recognize as "birds" numerous times over the course of the Mesozoic Era--with only one branch surviving into the modern age and producing those pigeons, sparrows, penguins and eagles we all know and love.