Was Archaeopteryx a Bird or a Dinosaur

On the face of it, Archaeopteryx wasnt much different from any other feathered dinosaur of the Mesozoic Era: a small, sharp-toothed, two-legged, barely airworthy dino-bird that feasted on bugs and small lizards. Thanks to a conflux of historical circumstance, though, for the last century or so Archaeopteryx has persisted in the public imagination as the first true bird, even though this creature retained some distinctly reptilian characteristics--and almost certainly wasnt directly ancestral to any bird living today. (See also 10 Facts About Archaeopteryx and How Did Feathered Dinosaurs Learn to Fly?) Archaeopteryx Was Discovered Too Early to Be Fully Understood Every now and then, a fossil discovery hits the zeitgeist--that is, contemporary trends in prevailing thought--square on the head. That was the case with Archaeopteryx, the exquisitely preserved remains of which were unearthed barely two years after Charles Darwin published his masterwork, On The Origin of Species, in the mid-19th century. Simply put, evolution was in the air, and the 150-million-year-old Archaeopteryx specimens discovered in Germanys Solnhofen fossil beds appeared to capture the precise moment in the history of life when the very first birds evolved. The trouble is, all of this happened in the early 1860s, well before paleontology (or biology, for that matter) had become a fully modern science. At that time, only a handful of dinosaurs had been discovered, so there was limited scope for understanding and interpreting Archaeopteryx; for example, the vast Liaoning fossil beds in China, which have yielded numerous feathered dinosaurs of the late Cretaceous period, had yet to be excavated. None of this would have affected Archaeopteryxs standing as the first dino-bird, but it at least would have put this discovery in its proper context. Let's Weigh the Evidence: Was Archaeopteryx a Dinosaur or a Bird? Archaeopteryx is known in such detail, thanks to the dozen or so anatomically perfect Solnhofen fossils, that it offers a wealth of talking points when it comes to deciding if this creature was a dinosaur or a bird. Heres the evidence in favor of the bird interpretation: Size. Archaeopteryx adults weighed one or two pounds, max, about the size of a well-fed modern-day pigeon--and much less than the average meat-eating dinosaur. Feathers. Theres no doubt that Archaeopteryx was covered with feathers, and these feathers were structurally very similar (though not identical) to those of modern birds. Head and beak. The long, narrow, tapered head and beak of Archaeopteryx were also reminiscent of modern birds (though bear in mind that such similarities may be the result of convergent evolution). Now, the evidence in favor of the dinosaur interpretation: Tail. Archaeopteryx possessed a long, bony tail, a feature common to contemporary theropod dinosaurs but not seen in any birds, either extant or prehistoric. Teeth. Like its tail, the teeth of Archaeopteryx were similar to those of small, meat-eating dinosaurs. (Some later birds, like the Miocene Osteodontornis, did evolve tooth-like structures, but not true teeth.) Wing structure. A recent study of Archaeopteryx feathers and wings suggests that this animal was incapable of active, powered flight. (Of course, many modern birds, like penguins and chickens, cant fly either!) Some of the evidence vis-a-vis the classification of Archaeopteryx is much more ambiguous. For example, a recent study concludes that Archaeopteryx hatchlings required three years to attain adult size, a virtual eternity in the bird kingdom. What this implies is that the metabolism of Archaeopteryx wasnt classically warm-blooded; the trouble is, meat-eating dinosaurs as a whole were almost certainly endothermic, and modern birds are, as well. Make of this evidence what you will! Archaeopteryx Is Best Classified as a Transitional Form Given the evidence listed above, the most reasonable conclusion is that Archaeopteryx was a transitional form between early theropod dinosaurs and true birds (the popular term is  missing link, but a genus represented by a dozen intact fossils can hardly be classified as missing!) Even this seemingly uncontroversial theory is not without its pitfalls, however. The trouble is that Archaeopteryx lived 150 million years ago, during the late Jurassic period, whereas the dino-birds that almost certainly evolved into modern birds lived tens of millions of years later, during the early-to-late Cretaceous period. What are we to make of this? Well, evolution has a way of repeating its tricks--so its possible that populations of dinosaurs evolved into birds not once, but two or three times during the Mesozoic Era, and only one of these branches (presumably the last) persisted into our era and gave rise to modern birds. For example, we can identify at least one dead end in bird evolution: Microraptor, a mysterious, four-winged, feathered theropod that lived in early Cretaceous Asia. Since there are no four-winged birds alive today, it seems that Microraptor was an evolutionary experiment that--if youll forgive the pun--never quite took off!