June 19, 2009
As I have written about many times here on Dinosaur Tracking, paleontologists presently have an overwhelming amount of evidence that birds are living dinosaurs. That doesn’t mean that everything about the dinosaur-to-bird transition is well-understood, though. For years scientists have been faced with a puzzle involving the hands of living birds and bird-like dinosaurs. The dinosaurs most closely related to birds appeared to have a thumb and two fingers (digits I-II-III) while studies of the embryonic development of birds showed that they have fingers II-III-IV. This difference would have to be accounted for, and a bizarre new theropod dinosaur described by an international team of scientists in yesterday’s edition of Nature provides a crucial clue to this fossil puzzle.
The new dinosaur, named Limusaurus inextricabilis, is so strange that I almost don’t know where to start describing it. It lived about 156 million to 161 million years ago in what is now western China, and it was one of the dinosaurs that became mired in the famous “Dinosaur Death Trap” featured by National Geographic. Yet while scientists were able to identify it as a ceratosaur, one of the early groups of theropod dinosaurs, it was like no other ceratosaur they had ever seen. Instead of packing a mouthful of sharp teeth, like Ceratosaurus, Limusaurus did not have a tooth in its entire mouth! A pile of stones was found in its stomach region that probably ground up food inside its gut, and it is likely that this theropod dinosaur was actually a herbivore.
What is making headlines, though, is that Limusaurus had at least one feature that is very important to understanding how avian dinosaurs (i.e. birds) evolved. Since we know that theropod dinosaurs evolved from five-fingered ancestors, it has long been assumed that, to end up with digits I, II and III, they lost two fingers: their pinky and ring fingers (or digits IV-V). In this way the fingers were reduced and lost, probably due to changes during embryological development, from the outermost finger moving in. What Limusaurus shows, though, is that after some dinosaurs lost their pinky they began to lose their thumb.
This is not what would have been expected, but it is clear that Limusaurus has a greatly reduced thumb and an enlarged second digit. In this way the second finger functionally became like a thumb, but what if Limusaurus was just an oddball? We can’t know if it was directly ancestral to any other dinosaurs, but the paleontologists then looked at the hands and fingers of other dinosaurs more closely related to birds (the coelurosaurus) and found their answer not in the finger bones, but in the wrist bones. The finger bones of these later dinosaurs alone might make it hard to tell if they were really I-II-III or II-III-IV, but the wrist bones provided a clearer picture. The wrist bones of dinosaurs more closely related to birds did not change as much as the fingers. They retained signs that the modified finger bones they were attached to were really II-III-IV, and this finally makes sense of both the fossil and embryological evidence.
Now keep in mind that Limusaurus is probably not directly ancestral to the dinosaurs that gave rise to birds. It is not a “missing link” (and the phrase “missing link” itself is more confusing than helpful when thinking about evolution). What the skeleton of Limusaurus suggests, though, is that there was a significant shift in hand shape going on among ceratosaurs during the Jurassic, and Limusaurus provides a window into how this change occurred. If the hypothesis of the authors is correct, and there is much reason to think it is, then we should expect to find other theropod dinosaurs with similar hand anatomy that link some ceratosaurs to tetanuran dinosaurs, the group to which coelurosaurs (and hence birds) belong.
There is much more to discuss about Limusaurus than any one blogger can cover, though, so have a look at what some other science bloggers have to say about this new find:
Sign up for our free email newsletter and receive the best stories from Smithsonian.com each week.