by Greg Mayer
Last weekend, a new exhibit opened at the American Museum of Natural History in New York: “Pterosaurs: Flight in the Age of Dinosaurs“. The New York Times had a piece on the making of the exhibit last week, and today their museum critic, Edward Rothstein, weighs in with his take on the pterosaurs. We’ve had occasion to favorably note Rothstien’s reviews previously here at WEIT, and his conclusion is that the exhibit is well worth seeing.
The exhibition is unusually compelling, given its directness and simplicity. In one sense, pterosaurs are quite familiar: Any image of the dinosaur age shows them ruling the skies. But as you work your way through this exhibition, they become confoundingly strange. Walking on wings! A fourth finger for flying! Crests larger than heads!
His review also considers how it is we come to know about the pterosaurs (‘pterosaur epistemology’), the serendipity of fossilization and discovery, and how small clues can be used to build up a more complete picture of the creature, noting, for example how a small mass of ejected bones (a gastric pellet), which might be overlooked, reveals what pterosaurs ate.
It reminds us of what exists before hypotheses accumulate, and what the paleontologist must accomplish, combining meticulous examination with speculative reconstruction. The pellet presents just a slightly more extreme version of how many pterosaur fossils are found. Some are seen here: jumbles of flattened bones and random filaments, gastric pellets spat out of some geological maw. …
Out of accidents, order takes shape; we see this to be as true of the paleontologist’s enterprise as it is of evolutionary change. The effect is to make us wonder which is more marvelous: the creatures themselves, or the ways they have been recreated?
The accompanying website is chock full of images, videos and information– go have a look. Here’s a nice summary video.
Some aspects of the reconstructions are speculative– we don’t really know what colors their crests were (although we do have evidence for the color of some Mesozoic reptiles). And, surprisingly to me, there is almost nothing about the “hairs”– called “pycnofibers”– that have been described in a number of pterosaurs. I’ve always thought the suggestion of pterosaurs being haired was very exciting, and, if true, a nice example of convergence, and evidence that pterosaurs were warm-blooded. The only mention I can find on the AMNH site concerns Jeholopterus, a small pterosaur with pycnofibers, seen in the following gif:
Pterosaurs are, of course, reptiles (and not dinosaurs!), and one of the three groups of tetrapods to have evolved true flight (as opposed to gliding, which has evolved many more times). Pterosaurs’ air foil is membranous skin, stretched along an enormously elongated 4th finger; bats, too, have a membranous wing, but it is supported by fingers 2 through 5; birds have a wing of feathers, which project not from elongated finger bones, but from a shortened and fused set of hand/finger bones. These structures are nicely illustrated in the following figure from Steve Gatesy and Kevin Middleton:
Powered flight is thus an excellent example of convergent evolution— the origin of similar structures as adaptations to similar conditions of existence. The wings, because they evolved independently, are said to be analogous (i.e. not derived from a common ancestor possessing wings), as is evident from the different nature of the air foil, and the different modifications of the bones involved in the wings of the three groups– the similarities are superficila nad functional. It also nicely shows the hierarchical nature of homology. The front limbs of bats, birds, and pterosaurs are homologous as limbs (i.e. derived from a common ancestor possessing front limbs), but not as wings. The common structures (humerus, radius, ulna, etc.) are homologous at the level of tetrapods, but the modifications of these structures as wings are separate evolutionary events.
The exhibit is temporary, and will be up through January 4, 2015. Be sure to put it on your list of things to see while in New York; it’s on mine!
Gatesy, S.M. and K.M. Middleton. 2007 Skeletal adaptations for flight. pp. 269-283 in Hall, B.K., ed., Fins into Limbs: Evolution, Development, and Transformation. University of Chicago Press, Chicago.