Archosauria: (Permian - Recent)
The most recent common ancestor of crocodilians and birds. Archosaurs, ancestrally were medium-sized saurian predators who shared the Late Permian world with the dominant synapsids. After the Permo-Triassic extinction, they proliferated so that, by the middle Triassic, they had replaced synapsids as the dominant land vertebrates, both as predators and as herbivores.
They have a long list of synapomorphies, of which this course will only sample a few. Note that they are biologically very significant.
Archosaur locomotion: In stark contrast to creatures like lizards, the muscles that undulate the archosaurian torso attach not to the ribs, but to lateral projections of the vertebrae. Additionally, archosaurs hold their torsos straight when they move, relying on the limbs for propulsion.
Who are the archosaurs? There are two major groups.
In the case of Crurotarsi the calcaneum (heel) takes the form of a lever for the rotation of the foot on the shin. That action works most efficiently when the stance is more nearly erect.
For now, let's consider crurotarsi. These archosaurs took over land ecosystems during the Middle and Late Triassic only to suffer greatly in the Late Triassic extinction event, with only the lineage that gave rise to Crocodylia surviving.
As with synapsids, crurotarsans encompass a huge range of diversity. In sampling it we are, in effect, visiting a zoo of Late Triassic big game.:
Phytosauria: (Late Triassic)
Worst possible name, based on an early misinterpretation. Large long-snouted fresh water aquatic predators superficially much like modern crocodiles. They were different in that:
Aetosauria: (Late Triassic)
Rauisuchia: (Middle - Late Triassic)
Top predators of the Late Triassic.
Crocodylomorpha: (Late Triassic - Recent)
Consider what we have been seeing. Based on what we know, how would you reconstruct the locomotion and thermal metabolism of late Triassic rauisuchians and crocodylomorphs? You mght want to remember our earlier discussion of sail-backs:
The Late Triassic extinction created an ecological vacuum at the upper levels of the terrestrial food-chain. By the Middle Jurassic dinosaurs had occupied the major herbivore and carnivore niches. Crocodylomorphs weren't out of the picture, however. In the Jurassic, we see the adaptive radiation of Metasuchia (Early Jurassic - Recent). These creatures:
One interesting departure is Thalattosuchia: (Early Jurassic - Early Cretaceous) Sea-crocs!
Almost as soon as they invaded fresh-water, neosuchians began showing up in the shallow-marine niche, also. (In fact, more than one such invasion occurred during the Mesozoic.) Familiarity with the oceans is a recurrent trend in crocodylian evolution. Even modern crocodiles are at least slightly adapted to salt water, but the thalattosuchian like Metriorhynchus (skull at right compared to that of the modern Crocodylus cataphractus) represent a serious attempt to move into the oceans. In their most extreme forms, such as Geosaurus (above) they:
Fossil eusuchians seem generally to have had similar ecological, locomotor, and thermoregulatory adaptations as living crocodilians, but there are some specialists worth noting:
Crocodylia (Cretaceous - Recent): For the record, the monophyletic group Crocodylia (note spelling) = "The most recent common ancestor fo living crocodilians and all of its descendants." It includes: