Freed to Fly Again
Through CT scans that can show sub-millimeter details, an imprisoned fossil reveals its secrets
Part of the charm of this new species is that it is so unlike anything else. Only two other types of gliding reptiles are known from the Triassic and they are rather different. One, Sharovipteryx mirabilis or "Sharov's miraculous wing," was named after the Russian paleontologist Alexei Sharov, who described the only known specimen in 1971. It is a strange creature, with short front legs, very long hind legs and a long tail. Sharovipteryx has an especially odd anatomy, with a tiny front membrane stretched between its body and short front legs and another, much larger, triangular wing (similar to that of the Concorde supersonic jet) stretched between the tail and the long hind legs.
According to an aerodynamic analysis by Gareth J. Dyke of University College Dublin and colleagues in the Journal of Evolutionary Biology in 2006, the small front wing acted like a canard to control pitch. Speed was controlled by the reptile's spreading or closing its hind legs which in turn tautened or loosened the elastic membrane between them. Sharovipteryx glided in a manner similar to a delta-wing jet—not at all like Mecistotrachelos but in a completely original fashion.
The other type of gliding reptile is best represented by the species Icarosaurus siefkeri and its close relatives. Discovered in an abandoned quarry in New Jersey by three teenaged amateur paleontologists, Icarosaurus siefkeri wasnamed both for the mythical flyer Icarus and for one of the discoverers, Alfred Siefker. Icarosaurus has a short neck and feet that are not strongly hooked like those of Mecistotrachelos. Perching on a small branch would have required muscular effort for Icarosaurus.
Both Mecistotrachelos and Icarosaurus glided on membranous wings supported by elongated ribs that could be spread laterally, but the two species had some key differences in the shape and curvature of their gliding structures. When Icarosaurus spread its ribs, the shape of the bones dictated a concave lower surface. The wings thus had a fixed curvature, or camber, in the language of aeronautics. Because of this, its aerial maneuvers were probably less controlled and less subtle than those of Mecistotrachelos. In contrast, Mecistotrachelos's ribs are straight in the portions farther away from the backbone, and the animal could have changed camber with muscles that moved the first rib on each side up or down.
A puzzling question is how—or whether—rib-gliders like Mecistotrachelos were able to breathe while gliding. Because the ribs had to remain outstretched to provide lift, they could not have moved much to pump air in and out of the lungs. Fraser guesses that Mecistotrachelos probably held its breath while in flight. Although the species is too primitive to be considered a lizard, he says, it is important to realize that "most lizards today do not breathe while they locomote—or at least there is very inefficient breathing going on—due to Carrier's constraint."
Carrier's constraint is named for David R. Carrier at the University of Utah in Salt Lake City, who observed that the typical sprawling gait of a lizard restricts the animal's ability to breathe while running or walking. As it moves, a lizard bends its body first to one side to take a step with one front leg and the opposite hind leg simultaneously. Then the animal bends its body the other way to take the next step. The lung on the bent side of the body is compressed, which squeezes the air into the opposite lung, and the next step reverses the process. Used air is shunted back and forth, and the creature never gets a chance to take a new breath. Carrier's constraint explains why these reptiles tend to have much less stamina than similar-sized mammals: Lizards simply run out of oxygen to fuel their muscles. Most are limited to short bursts of muscle-powered movement before they go into oxygen debt and must rest. By contrast, Fraser notes, "we don't think these gliders propelled themselves through the air by wiggling back and forth as if they were walking."
Some lizards take in oxygen while they are moving by gulping air into their mouths and throat sacs, a mechanism called gular pumping, and Mecistotrachelos might have done the same. It would not have needed as much oxygen while gliding as creatures require when running or walking. Fraser admits, "I am not sure how this animal with its very narrow thorax would have breathed even when the ribs were not in use for gliding."
Basic questions about the biology of these animals and the mechanics of rib-gliding remain mysterious. Apparently, this bizarre adaptation evolved twice, once in Icarosaurus and once in the unrelated Mecistotrachelos. Hind-leg gliding evolved independently in the Sharovipteryx lineage. Who would have expected gliding to be so prevalent in the Triassic? Who would have thought there were so many ways to evolve gliding anatomy?
This beautiful species represents a deliciously new kind of animal. In my years as a student of past life on Earth, I have come to believe that evolution, viewed over the span of eons, is more innovative than the most inspired human designer. Again and again, the record of the past has revealed extraordinary creatures and startling adaptations to ecological pressures no one envisioned. In this case, human cleverness, itself a product of evolution, has built a machine that parts the curtains of time and stone to show us a wonder-full, otherwise invisible, being.
- Dyke, G. J., R. L. Nudds and J. M. V. Rayner. 2006. Flight of Sharovipteryx: the world's first delta-winged glider. Journal of Evolutionary Biology 19:1040-1043.
- Fraser, N. C., P. E. Olden, A. C. Dooley and T. R. Ryan. 2007. A new gliding tetrapod (Diapsida: Archosauromorpha) from the Upper Triassic (Carnian) of Virginia. Journal of Vertebrate Paleontology 27:261-265.
» Post Comment