American Scientist

To the Editors:

I found Pat Shipman's article on fossil gliding lizards really interesting ("Freed to Fly Again," Marginalia, January-February). Dr. Shipman mentions that the fossil lizard from Virginia could not draw its longish neck back in an S-shape, the way herons do, even though having an extended neck would be less stable in flight. Although herons fly with their neck pulled back, most other birds that leave their long legs trailing behind as they fly have their long necks extended forward (storks, cranes, flamingos, ibises, spoonbills), so this posture must not be an inherently unstable flight configuration. For this gliding reptile, perhaps an extended neck counterbalanced the weight of a long tail trailing behind, helping keep the body horizontal, and perhaps increasing, rather than decreasing, stability.

Stewart Ware
College of William & Mary
Williamsburg, VA

To the Editors:

I read with interest Pat Shipman's article on Triassic gliding lizards. I was disappointed that she did not make some comparisons to the extant gliding lizard, Draco volans. Observing this species in Borneo, I was entranced at its ability to take off and maneuver to land accurately on a tree trunk some 5 meters or more away. This adaptation in both Mecistotrachelos and Draco seems to be a good example of convergence, assuming the two genera are unrelated.

Jon Reiskind
University of Florida
Gainesville, FL

To the Editors:

The article by Pat Shipman tells the tale of how technologies such as computed tomography (CT) can lead to the discovery of novel anatomy in fossil organisms. The particular discovery of the small reptile Mecistotrachelos by Nick Fraser and colleagues contributes yet another case of remarkable convergent evolution in the fossil record. However, contrary to Dr. Shipman's article, the discovery of a small reptile with movable ribs that likely supported a gliding membrane is neither novel nor enigmatic. The living lizard genus Draco, comprising more than 40 species, is completely overlooked.

While the discovery of another reptile converging on gliding morphology is "delicious," as indicated by Dr. Shipman, Mecistotrachelos and the anatomically similar, and also extinct, Icarosaurus are not a "new kind of animal." In fact, the genus Draco was described nearly 250 years ago by Carolus Linneaus. Furthermore, there is even a growing body of work, especially by University of California, Berkeley, professor Jim McGuire, on the diversity and biomechanics of the living genus Draco. New technologies may give us insight into the anatomy of extinct animals, but, in many cases, it is living animals that provide us with the ability to interpret the behavior and functional anatomy of the extinct forms.

David C. Blackburn
Harvard University

Dr. Shipman responds:

Several readers, including Drs. Blackburn and Reiskind, wished I had included information on Draco, the beautiful rib-gliding lizard that lives in Southeast Asia. I greatly envy Dr. Reiskind the experience of seeing Draco in the wild.

I decided not to discuss Draco in my article because I didn't want to foster the incorrect assumption that Mecistotrachelos is an ancestor of Draco. The taxonomic distinctions among the various rib-gliders are complicated. Mecistotrachelos is an archosauromorph, which makes it closer to crocodiles and dinosaurs than lizards. In contrast, Icarosaurus is a basal form that might be a lepidosauromorph (a very primitive lizard-like form) or might be an archosauromorph, depending on which taxonomist you talk to. Soon after the paper describing Mecistotrachelos came out, a group headed by Pi-Peng Li of Shenyang Normal University published a description of another new gliding lizard from the early Cretaceous of China, which represents yet another independent evolution of rib-gliding. I have no doubt that Nick Fraser and his colleagues will rely heavily on information from the living agamid lizards in the genus Draco as they continue to investigate the extraordinary locomotion of Mecistotrachelos, which would be completely unknown without micro-CT studies.

Dr. Ware raises an interesting consideration—that the long tail and large hind legs of Mecistotrachelos may have been counterbalanced by its unusually long neck. All I can say is that I wish I had thought of that! Even as a counterweight, the long straight neck of Mecistotrachelos must have demanded considerable neurological and muscular control in gliding.