Scales from Kyrgyzstan
12:02 am
A sketch of the head of Longisquama
The Lawrence-Journal World reports on the discovery of new specimens of the enigmatic reptile Longisquama. Longisquama (”long scale”) is known from only one site in Kyrgyzstan, primarily from one skeleton that preserves only the front half of its body, including some impressions of bizarre, hockey stick-shaped scales that seem to have sprouted from its back.
That one partial skeleton was described over thirty years ago by Alexander Sharov, and since then no additional bones of this strange animal have been described. Impressions of its distinctive scales, however, are unearthed at the Kyrgyzstan site from time to time, and it appears that a few more have come to light.
As referred to in the article, the scales of Longisquama have been brought up as examples of a transitional structure between scales and feathers, particularly by researchers who discount the theory (despite increasingly well-supported evidence) that birds are directly descended from theropod dinosaurs. It has long been suggested that Longisquama used its long scales as “wings” for gliding, and this hypothesis has clearly found favor among scientists who prefer a gliding-lizard type for their bird ancestors.
One of the difficulties with using Longisquama as a bird ancestor is that its relationships remain poorly understood. At a glance, its scales are vaguely feather-like and its pointed snout looks somewhat birdlike, but it is difficult to say whether these features share a deep structural similarity with those of birds or are simply converging on common solutions to the problems of living in, feeding from, and occasionally falling out of trees.
For my money, I doubt that Longisquama has much to do with the evolution of birds—I find the skeletal similarities between theropods and birds to be much more convincing. But, as an aficionado of little, prehistoric, arboreal reptiles, I’m glad to see Longisquama getting some attention.
There are a lot of good-sounding arguments as to why dinosaurs may have found “transitional” feathery scales useful (insulation, etc). If so, why aren’t there critters with such coverings making similar good use of them now?
Hi Pierce -
If you look at the “transitional” or “protofeather” covering on dinosaurs like Dilong or Sinosauropteryx, it resembles nothing so much as the fur that nearly every mammal is making good use of today. That’s not to say that it was actually the same as hair, but it seems to have some similarity of form and probably offered similar benefits for warmth, display, etc.
As to why currently scaled critters aren’t modifying their scales to similar effect, it is hard to say, since we don’t exactly know why the first feathers (or hair, for that matter) evolved. It may be that today’s reptiles are outcompeted by already-outfitted birds and mammals in niches where such coverings would provide a distinct advantage. It may also be that certain groups have modified their scales for other purposes that make evolving fur or feathers (or something else) less likely in the absence of strong selective pressures–turtles, for instance, have set up their scales for specific, protective purposes, and lizards and snakes are characterized by overlapping scales that are quite different from those of dinosaurs or other reptiles. Crocodilians, whose scales are probably most like those of ancestral dinosaurs, specialize in an aquatic lifestyle where extraneous coverings would put them at a distinct disadvantage.
Integument is only a thermoregulatory advantage to endotherms. If you dress a lizard in fur, it just prevents it from using the sun to heat up.
Only a handful of small endothermic form lack integument of some sort, such as naked mole rats. Clearly, anyone that would benefit from insulation already has it.
[…] Sharovipteryx was discovered by paleontologist Alexander Sharov, who found it and another amazing reptile in 1965 while collecting Triassic insect fossils in Kyrgyzstan. The only known specimen of Sharovipteryx shows that it was a slender, lizard-like animal with short arms and very long legs. It is the oldest animal known to have a gliding membrane attached to its limbs, a trait that would later evolve in supreme vertebrate aerialists like pterosaurs and bats. But while pterosaurs and bats expanded their arms and fingers to support their wings, Sharovipteryx is the only known animal to have evolved leg-based wings. […]
Hi Matt,
Once again I’ve nicked a drawing from your blog (credited of course!). Hope it’s okay!
cheers,
Neil