|Photo of the Berlin Specimen by |
H. Raab (User:Vesta) under creative commons license.
Since 1861 eleven more specimens have been found. While the animal is covered with feathers so that it is instantly reminiscent of a raven-sized bird, the differences were enough that almost immediately it was seen as a transitional fossil between reptiles and birds. As much as Archaeopteryx is ingrained in our imaginations as the first bird, just as we saw with the mammals, there’s a degree of disagreement as to exactly what constituted a bird back in the Jurassic. But I don’t think it’s anything like as complicated as the story of mammals.
Archaeopteryx was certainly an early bird (or at least an extremely bird-like reptile), but whether or not it was truly ancestral to modern birds is debated. Discoveries of other avians in Jurassic rocks of China may be as much as 10 million years older than the Bavarian specimens, which are dated to about 150 million years ago. Other candidates have been suggested as early bird ancestors as well. Their existence leads to the idea that some other lineage might be the one that led more directly to modern birds.
Studies of the feathers have suggested that Archaeopteryx was incapable of true flight, but I think that’s a minority view. It lacks a backward-pointing toe, which modern perching birds have, so it might have been more of a ground-dweller, like a chicken. With only 12 specimens, some represented by only a few bones, it’s challenging to reconstruct the daily life of Archaeopteryx.
Many fossils have been found of dinosaurs that are definitely not birds, but which have feathers. So you can’t use the presence or absence of feathers as a simple criterion to draw the line between reptiles and birds. There must have been transitional species, and Archaeopteryx was probably one of them. If a scientist could go back to the Jurassic and examine a live one, he or she might well classify the creature as neither a reptile nor a bird, but as something else – like some of those early mammal ancestors we’ve heard about.
One evolutionary trend that seems clear among bird-like dinosaurs and birds themselves is their shrinking size and evolution of a lightweight skeleton, presumably to reduce weight for flight and to increase maneuverability generally. I have a couple links below to some recent papers on the topic of size in dinosaurs and early birds. Obviously there are some pretty big birds today, including ostriches, but the increase in size of birds may be a development of relatively more recent times, the past 60 million years or so as mammals and birds began to fill niches vacated by dinosaurs killed in the extinction event 65 million years ago.
Today, I think there is no doubt whatsoever that birds are descended from theropod dinosaurs. In some ways of looking at it, as I’m sure you have heard, birds ARE dinosaurs – evolved and modified, and it really becomes a semantic issue of definition – exactly where IS the line between them? It may be clear today, but it definitely was not clear back in the Jurassic.
And even though technically Archaeopteryx is generally seen as not really the ancestor to modern birds, if you want to think of it as the first bird, I’d say you’re only going to get in trouble calling it that if you get into a debate with a real specialist in this field.
And whichever particular group might be the ultimate ancestor to modern birds, there is no doubt that by the late Jurassic, we had animals that a modern time traveler would have called birds. Strange birds, maybe, but birds nonetheless.
—Richard I. Gibson
Photo of the Berlin Specimen by H. Raab (User:Vesta) under creative commons license.