Ever since the announcement of an exquisitely-preserved specimen of the feathered dinosaur Microraptor gui in 2003, paleontologists have been debating how it might have flown and what relevance it might have to the origin of birds. How did it hold its legs? Could it really fly, or just glide? Is is representative of a stage in the origin of flight, or does it represent a different way of taking to the air? Answers to these questions depend on who you ask. Earlier this year a pair of papers appeared in the journal PNAS hypothesizing that the dinosaur held its hindlimbs out to the side—like a crocodile—to create a second set of wings behind the first.
According to a new commentary published in the same journal by American Museum of Natural History paleontologist Stephen Brusatte and colleague Jason Brougham, however, the authors of the recent Microraptor study made some significant errors. It all comes down to a few bits of esoteric anatomy about where the head of the femur (thighbone) articulates with the hip. In the original research paper published by David Alexander and colleagues, the scientists asserted that the hip of Microraptor—as well as all dromaeosaurid dinosaurs (roughly, "raptors" and their kin)—lacked two features of the pelvis called the supracetabular crest and antitrochanter, which normally constrain the flexibility of the hip socket. Without these features, Microraptor could have splayed its legs out to the side to glide.
Not so fast, say Brusatte and Brougham. Dromaeosaurid dinosaurs have supracetabular crests which are reduced in size, but their antitrochanters are actually enlarged in size, and these features would have prevented Microraptor from splaying its legs out in the manner that Alexander and co-authors proposed. This would have made the posture favored by Alexander and colleagues "anatomically implausible," says Brusatte, adding, "if the femur was held completely lateral to the body, then it would have been dislocated out of its socket." It does not matter whether the posture hypothesized by the other team of scientists would have made Microraptor a better glider. It simply could not have held its limbs in that position, Brusatte argues, "so flying a model with this posture tells us nothing about how the living animal could actually fly."
The reason for this difference between the scientists may be a result of the preservation of the dinosaur. The fact that the Microraptor hips Alexander and co-authors used were crushed flat means that they may have mistakenly thought the constraining features were absent. "Even though the fossils are crushed," Brusatte says, "it is still clear that they possessed supracetabular crests and antitrochanters." Furthermore, Microraptor was closely-related to the recently described dinosaur Hesperonychus, which was preserved with an uncrushed pelvis. In this dinosaur the constraining features are present, Brusatte observes. Although a peculiarity of its hip socket might have given the legs of Hesperonychus a little more flexibility, "there is no way that Hesperonychus could have splayed its legs completely laterally," says Brusatte.
It is noteworthy that the researchers who published the first PNAS paper have been long-time critics of the well-supported hypothesis that birds evolved from feathered dinosaurs. Their preference for a crocodile-like posture for the hindlimbs of Microraptor is more consistent with their previously stated idea that the first birds evolved from a yet-unidentified lineage of archosaurs.
Naturally, Alexander and his co-authors disagree with the criticisms of Brusatte and Brougham. They state that the hip specimen on which this entire argument hinges truly lacks the constraining features, and they suggest that other small dromaeosaurid dinosaurs lacked them as well. Frustratingly, however, the hip in question has not been extensively described in the accessible peer-reviewed literature. Paleontologist David Burnham featured it in his 2007 thesis and the image has been reproduced in a print-on-demand version of that thesis, but it has yet to be presented to the paleontological community through a detailed analysis published in a peer-reviewed journal. This step would have been essential for building a rigorous case for a sprawl-legged Microraptor, but it was not done in the PNAS study by Alexander, Burnham and their peers.
In the larger context of the origin of flight, though, it is unclear how significant Microraptor might be in investigating how the first birds evolved. Early birds already existed by the time Microraptor lived 120 million years ago, and it is possible that it was simply part of an array of small feathered dinosaurs that independently evolved the ability to glide. "It is unclear whether the gliding capabilities of Microraptor were an odd feature of this dinosaur only, or whether dromaeosaurids more broadly were capable of gliding," Brusatte says. How significant Microraptor is to the question of how the first birds evolved is something which will require further evidence, but as Brusatte summarizes, understanding the paleobiology of Microraptor will help place the evolution of its close relatives in context:
It is important to study Microraptor, but there are over 40 dromaeosaurids and troodontids—the closet relatives to birds—and these vary greatly in their size, feathery integument, and presumed lifestyle. It is no more fair to say that Microraptor is the key to understanding the origins of avian flight than to say that Deinonychus is. In order to argue that Microraptor's gliding ability was a precursor to the origin of flight, it must be demonstrated that the its gliding ability was retained by the immediate ancestors of birds. That is not certain, or even likely, based on current theropod phylogenies.
Alexander DE, Gong E, Martin LD, Burnham DA, & Falk AR (2010). Model tests of gliding with different hindwing configurations in the four-winged dromaeosaurid Microraptor gui. Proceedings of the National Academy of Sciences of the United States of America, 107 (7), 2972-6 PMID: 20133792
Alexander, D., Gong, E., Martin, L., Burnham, D., & Falk, A. (2010). Reply to Brougham and Brusatte: Overall anatomy confirms posture and flight model offers insight into the evolution of bird flight Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1007798107
Brougham J, & Brusatte SL (2010). Distorted Microraptor specimen is not ideal for understanding the origin of avian flight. Proceedings of the National Academy of Sciences of the United States of America PMID: 20864633