Carl Zimmer has a nice summary of the increasing amount of fossil evidence for the evolution from dinosaurs to birds, surely one of the most fascinating facts of evolution. Interestingly, as the evidence mounts, the importance of Archaeopteryx, that classic fossil that caused such excitement when it was first discovered because it was the first direct evidence of the transition, has become diminished.
In this article, he also focuses on the unusually large size of birds’ brains, which implies that we should stop using the term ‘bird brain’ in a derogatory way. The bigger brains are believed to be necessary for them to have the vision and coordination necessary to fly. It looks like the dinosaur precursors to birds developed bigger brains before they developed the ability to fly.
Adrian Thomas at the department of zoology at Oxford University, who was not involved with the study, said the picture now is much more complicated than “dinosaurs couldn’t fly and Archaeopteryx could”.
“There were a whole group of more or less distantly related feathered dinosaurs, some were gliding down from trees, some were flapping, and it all seemed to be happening at the same time.
“Rather than a straight [evolutionary] path that led Archaeopteryx to birds, the picture now is that there were lots of dinosaurs exploiting the advantages of gliding and flight. The birds are the ones that carried on successfully to the present day,” Prof Thomas told BBC News.
The brain of Archaeopteryx was also rather small for a bird, further suggesting that it was not as important a transition fossil as was once believed.
“A pheasant’s brain looks like a nerve with a knot in it.”
-- Lorne Elliot, Canadian comedian
Rather than teaching us that Archaeopteryx is less important as a transitional fossil, I think this whole situation is giving us a better idea of what we should expect to see in transitional fossils. The idea that a particular fossil is going to be ‘the’ ancestor of a group doesn’t fit well with what we can see of the world around us — monotypic taxa are the exception, not the rule. There doesn’t seem to be any reason why evolution would have worked differently 100mya, so we shouldn’t expect to see a single lineage changing — rather, any fossil we find is most likely going to be part of a complex of species, and thus is unlikely to be “the” ancestor.