Tuesday, March 20, 2007

Not another gliding reptile...

I haven't got hold of the paper yet, but PNAS is supposed to be carrying a contribution by Xu Xing and colleagues that describes a new lizard from the Early Cretaceous of Liaoning China. (Cue groans of 'oh no, not another spectacular fossil from China'). It's not just the preservation that is spectacular this time, but the splayed ribs supporting a skin membrane that look remarkably like wings. In fact that is their most likely function - not true flapping wings, but something that could be held open enabling the animal to glide.

For once though, China hasn't got there first. This is now the fourth group of reptiles to have independently evolved the same peculiar trait.

We have to go all the way back to the Late Permian, some 260 million years ago, for our first example. Back then it was Coelurosauravus and Weigeltisaurus from Germany and Madagascar respectively. United together in the family Coelurosauravidae it is assumed that these two taxa had a common ancestor in which the splayed-ribs-as-support-for-gliding-membrane adaptation first evolved, but these strange creatures disappeared almost as soon as they had arrived.

Only 40 millions years or so later (in the Late Triassic) it happened again. Icarosaurus and Kuehneosaurus from the US and Europe did pretty much the same thing - evolved this bizarre trait, spread out across the globe, then disappeared.
Another 60 million years later and we have Xianglong zhaoi, our brand new rib-splayed glider. Unlike it's predecessors Xianglong is a true lizard and yet it is not closely related enough to the living Draco (our fourth rib-glider; see movie) to share a gliding ancestor. We could be forgiven for thinking that there must be something about reptilian skin that stops them from making the (presumably) much simpler shortcut to a gliding membrane - just stretching some skin between the forelimbs and hindlimbs, as flying squirrels do. Only that's not true.

At the same time as Icarosaurus and Kuhneosaurus were taking in the Late Triassic air in what would become modern day Russia a creature called Sharovipteryx had made an appearence. Undoubtedly reptilian, although beyond that it's relationships are unclear, Sharovipteryx had developed not just a membrane between the two pairs of limbs, but another between the legs and tail and even one between the neck and arms. Overall this made a triangular-shaped membrane that is unique in nature. Although some authors have suggested Sharovipteryx may be the ancestral pterosaur it is more likely another oddball that disappeared as soon as it had arrived.

There are a number of mysteries here. Why did so many lineages go down the gliding route the 'hard way'? Why did none of the groups possessing this odd trait persist for for more than a few million years? Why do we have so many fossils of a group of animals that have such a low preservation potential?

Any ideas?

5 comments:

Sarda Sahney said...

Re: "Why did none of the groups possessing this odd trait persist for for more than a few million years?"

Since these animals are gliders living in an arboreal habitat they have fewer options then true flyers when times get tough. Perhaps they could not easily/quickly move to other forests when problems struck their habitat (eg fire, disease).

Malacoda said...

OK, but if they were so constrained how did they expand their geographic ranges? and if they were purely arboreal why do we have so many fossils? (It is an unlikely environment for preservation).

archosaurmusings said...

My guess would be that the very actof gliding makes them more susceptible to preservation, or to be more specific, more likely to have a whoopsie and end up in a lake.

Gliding in these kinds of things is not an exact science and their steering is not great. I would suggest that they are therfore more likely to (on occasion) drop into the odd place where then can be buried and preserved as opposed to terrestrial animals that won├Ąt take the plunge or true fliers that would not fall in.

Conjecture to be sure, but certainly possible. They might also be highly numerous, Draco, where it lives can be very common indeed as it really can outcompete others in some environments.

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