BSCI392
11-21-07
Pterosaurs

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Avemetatarsalia: (Triassic - Recent)

Avemetatarsalia : Decoupling of fore and hindlimb function for bipedalism or powered flight. The Late Triassic (but pre-extinction event) Scleromochlus is the most basal known avemetatarsalian and gives a decent idea of what an unspecialized one might have looked like. Note, its snout-vent length is only about 10 cm.

Avemetatarsalian diversity includes two major groups, along with some minor taxa. The major groups are:


Peteinosaurus (Late Triassic) from Frankmuster


Pterosauria: It flies, but it isn't a bird and it ain't a dinosaur.

  • Controversy 1: Active flight or gliding: The body proportions of pterosaurs are generally similar to those of actively flying birds. Nevertheless, it was long argued that pterosaurs could not have been active fliers for two reasons:

    As more pterosaur fossils became known, the weight of opinion started to turn in favor of actively flying pterosaurs since:

    This is not surprising when you consider that most pterosaurs were small animals upon whom the requirements of flight placed high energy demands. Heat loss must have been a danger. This discovery cast doubt on whether pterosaurs were cold-blooded at all.

    Thus, the issue seems to be decided in favor of active flight.

  • Controversy 2: Bipedal or quadrupedal.

    Bill Clinton look-alike David Unwin of Humboldt University in Berlin has made strong arguments for the traditional view of and seems to have carried the day. Some specific points.
    But there was another issue. If pterosaurs were quadrupedal, how did they stand? There was an early assumption that the pterosaur forelimb was structured such that the humerus had to project laterally from the shoulder girdle, as it did when the creature was flying. Indeed, quadrupedal pterosaurs were reconstructed in this manner in high-profile media packages like Walking with Dinosaurs. The problem, as Unwin pointed out, was that if the creature lifted either hand from the ground, its center of mass would be outside of the resulting support triangle and require forelimb tracks to be farther apart than hindlimb tracks - not what was seen. Biomechanical computer modelling revealed that the most plausible stance is one in which the forelimbs are held almost erect.

    Evolutionary trends: During pterosaur evolution, various groups have developed many interesting features. In this review, we sample a very primitive and a rather derived pterosaur.

    Pterodactyloids: Derived pterosaurs (Late Jurassic - Cretaceous)


    Anhanguera (Cretaceous)

  • Anhanguera: Has many characters acquired in the general course of pterosaur evolution:
  • Size: The largest pterosaurs lived after the appeance of birds. Note Quetzalcoatlus, a giant pterosaur from the Late Creatceous. A possible reason involves intense competition with birds for the small - medium-sized flying niche.

    Some pterodactyloid specializations

    Crests: Several lineages of pterodactyloids support large head crests of varying shape. What we see are the bony components, but occasional soft-tissue fossils tell us that these were continued in soft-tissue, yielding crests that were in some cases quite astounding.

    What were they for? A structure like that in a flying animal simply couldn't have not had an aerodynamic effect. Perhaps these functioned as control surfaces. At the same time, it seems probable that they would have served as display structures as well. E.G. Nyctosaurus long known as the crestless pterosaur of central North America.

    We now have fossils preserving soft tissue of this taxon showing a very bizarre crest.

    WTF??: It's hard to see how the creature could have flown in anything but still air with this on its head. Stranger yet, some individuals from sites where you would expect soft tissue preservation lack the crest. Could it have been sexually dimorphic or deciduous like antlers?

    Skimming - NOT: Speculation begets speculation. The apparent analogy between Rhamphorhynchus and the black skimmer is already problematic even though the animals are the same size. Extending the analogy to a large pterosaur, even one with a strongly compressed jaw like Thalassodromeus, is perilous, but that has not stopped people.

    New biomechanical research now indicates that Thalassodromeus was incapable of achieving the energy output necessary to perform this feat. No surprise considering the scaling issues involved.

    Sieving:

    Creatures like Pterodaustro, with its hundreds of long slender teeth, indicate that some pterosaurs made a living sieving food from the water in the manner of modern flamingoes.

    Walking: An odd thing about pterosaur trackways - they don't show up until the Late Jurassic. Indeed, the specializations of the hindlimbs and feet of primitive pterosaurs leads one to wonder how they would have managed on land. Current speculation indicates that perhaps they couldn't, but were stuck clinging to tree branches when not flying. Thus, part of the pterodactyloid success story is one of "the conquest of the ground."

    Toothlessness:

    Originally, pterosaurs had teeth. One group of pterodactyloids, including the well-known Pteranodon (etymology = "wings - no teeth") saved weight by evolving a toothless beak. Ironically, our popular culture can't stand it, such that toothy Pteranodons constantly pop up in media. Fortunately, nature has come to the rescue with the discovery of Ludodactylus (etymology = "toy - wing"). A pterosaur with a Pteranodon- like crest and big teeth.