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:
- Dinosauria (Late Triassic - Recent)
- Pterosauria (Late Triassic - Cretaceous)
Pterosauria: It flies, but it isn't a bird and it ain't a dinosaur.
- Elongate 4th finger
- Pteroid bone
- Biological meaning:
For systematists, the big problem with pterosaurs is that we have no transitional proto-pterosaurs, so it is difficult to say with certainty who their closest relatives are. Serious contenders include:
Pterosaurs are known from Late Triassic through the end of the Cretaceous and on all continents except Antarctica (they are probably there too). Best finds from are from the late Jurassic of Germany (same time and place as Archaeopteryx, the first known bird-like theropod dinosaur) and Middle Cretaceous of Brazil.
- They are derived members of Protorosauria (this would place them outside of Archosauria!)
- They are sister taxon to Dinosauromorpha (Dinosaurs and their immediaate relatives). This is the general, but not universally held consensus.
Since there is no living analog to pterosaurs, and since in many cases their anatomy is poorly known, it is difficult to know how they lived.
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:
- Being reptiles, it was presumed that they were ecologically similar to modern reptiles - cold-blooded. If this were so, then it wass reasoned that they could not have possibly been able to generate anough energy for flight.
- Some of the earliest pterosaurs to be discovered were, in fact, very large and probably adapted for soaring, in the manner of modern pelagic birds. Based on previous lectures you should be able to figure out why.
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.
- Most pterosaurs were small, well within the size range of modern birds.
- The flight apparatus of pterosaurs indicated the presence of powerful, bird-like flight muscles. Indeed, the pterosaur sternum (breast-bone) was similar to that of a bird in size.
- Fur? We have one pterosaur, Sordes pilosus, fossil that seems to be covered with a fur-like integument. Note: Just because it looks superficially like fur does not mean that it was histologically identical. The congruence test definitely tells us that it could not be homologous with the fur of mammals. Nevertheless, if its function is analogous, then it would appear that pterosaurs were actively seeking to prevent heat exchange with the environment, not promote it, as cold-blooded creatures do.
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.
- Original presumption was that on the ground pterosaurs were quadrupeds.
- In the last 15 years, an alternate proposal suggested by Kevin Padian of Berkeley that pterosaurs, like the ancestral dinosaur, were bipeds. Perhaps he was taking an non-traditionalist approach and seeing how far he could push it. This was based in part on the recognition that in some pterosaurs the wing membrane seemed to end at the torso or upper thigh rather than encompassing the leg.
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)
- Sordes: from the Late Jurassic represents the primitive condition. Like it, many other pterosaurs had
Within this general body type, there are several groups with interesting specializations, including Rhamphorhynchus, a tern-sized Late Jurassic pterosaur with an extremely slender jaw and long conical teeth. It has been hypothesized on the basis of general similarity to have a life-style resembling that of the modern black skimmer.
- short necks
- short wrists
- long tails, with rudders at the ends.
- tail-membrane stretched between legs and supported by specialized fifth digits (note: illustration is wrong). Soles of feet point medially.
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.
- long neck
- short tail
- long wrists
- small - large size.
- Unique scapular attachment to the axial skeleton. In large pterosaurs, whose wings bore all of their body weight while flying, an energy saving strategy was developed:
Notarium (In Anhanguera). This allowed the axial skeleton to be supported effortlessly by the pectoral girdle.
- Up until the Late Jurassic, when dinosaurs became airborn, pterosaurs stayed within the size range of modern birds.
- During the Cretaceous, however birds occupied the smaller size niches of flying vertebrates, and pterosaurs, the larger ones. Indeed, some pterosaurs became VERY big, whereas none are known that were smaller than a pelican.
- One possible explanation (note the compounded speculations!) is that the aerodynamic efficiency of a feathered wing drops off at increased size, whereas that of a membranous wing does not. Thus, pterosaurs took refuge in the one size niche in which they could compete effectively with birds. This left open to them life-styles such as:
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.
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."
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.