The many careers of crocodilians
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.
Here we see two major biological themes: The ability to bite chunks out of food effectively, and an accelerated metabolism.
- Antorbital fenestra.*
- Mandibular fenestra.*
- Serrated teeth.*
- Four chambered heart.
- Muscular diaphragm separating the thoracic and abdominal cavity.
- Nest building and parental care of young.
- Although not a proper synapomorphy, archosaurs inherited a double-row of scutes (ossified scales) down their backs.
Food processing: In lepidosaurs, we saw an evolutionary trend toward the ability to swallow larger and larger items. Animals that did this had to cope with a surface volume ratio problem: It took much longer to digest food. Thus, we got the proverbial python resting for a week after each meal.
- Archosaurs display the opposite trend, the ability to unlock nutrients quickly by reducing food to small bites before swallowing. (Though NOT chewing)
- This development seems to have coincided with an improved ability to get oxygen to the tissues
- A four chambered heart (review) is essential for this, but not sufficient. Getting blood to the lungs does little good if the lungs aren't receiving much air.
- Diaphragm actively pumps air into and out of lungs, working synergistically with ribs.
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.
- This simple development set the stage for the evolution of archosaurs into animals that could dedicate their lungs to breathing during bouts locomotion in a way that other saurians couldn't.
Who are the archosaurs? There are two major groups.
As their names indicate, synapomorphies of these groups have to do with features of their ankles and feet. what makes this interesting is that these features predispose members of both groups to evolve toward erect posture.
- Crurotarsi: Archosaurs more closely related to crocs than to birds.
- Avetarsalia: Archosaurs more closely related to birds than to crocs. Includes living birds and extinct dinosaurs and pterosaurs.
Crurutarsan (left) and avemetatarsalian (right) ankle schematics.
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:
- they were incapable of an upright stance
- they lacked a secondary palate. Instead, their nostrils migrated to a tall hump at the base of their snout.
- contemporary ancestors of crocodiles were not aquatic.
Aetosauria: (Late Triassic)
- Archosaurian herbivores with small heads and toothless upturned snouts.
- Most early archosaurs had two rows of bony scutes down their back. In aetosaurs, these were expanded into extensive armor, with additional rows along the sides and belly. In some genera, the side rows bore impressive spikes.
- Recent information describes communal aetosaur nest sites.
Rauisuchia: (Middle - Late Triassic)
Top predators of the Late Triassic.
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:
Eusuchia (Cretaceous - Recent): Within Neosuchia is the monophyletic group containing living crocodilians and their closest relatives. Characterized by:
- Tended to be larger than earlier crocodylomorphs
- Were characterized by the evolution of a secondary palate (convergently with cynodonts'. Note the choanae translated almost to the back of the toothrow in Araripesuchus below.)
- Encompassed a surprising range of ecological specializations. E.G.:
- Notosuchia: (Cretaceous) From the southern continents, omnivores and herbivores (!) with broad snouts and differentiation of the toothrow reminiscent of synapsids.
- Sebecia: (Cretaceous - Neogene) Large terrestrial predators with tall narrow snouts superficially resembling those of carnivorous dinosaurs. (Indeed, their serrated, blade-like teeth have been erroneously cited as evidence for post-Cretaceous non-avian carnivorous dinosarus.)
- Neosuchia: (Jurassic - Recent) Aquatic and semiaquatic. A radical departure for crocodylomorphs! Crocodylomorphs began to occupy the niche left vacant by the extinction of phytosaurs. Many fresh-water aquatic neosuchians resembled modern crocodilians tot he point that you would need expert knowledge and dissecting tools to see that they were different (E.G. the Late Jurassic Bernissartia below.)
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:
Thalattosuchian snouts resemble those of living fish specialist crocodylians like Tomistoma, so we assume they ordinarily preyed on smaller items, unlike other living crocs.
- lose their dermal armor
- reduce their limbs
- evolve an ichthyosaur-like tail bend.
Fossil eusuchians seem generally to have had similar ecological, locomotor, and thermoregulatory adaptations as living crocodilians, but there are some specialists worth noting:
- Flatheads: Long flat snouts lined with small conical teeth have evolved independently several times among eusuchians. We have already seen an extreme example in the ram-suspension feeding Stomatosuchus, but slightly tamer versions existed, too.
- Land predators: One eusuchian group, the Pristichampsinae (Paleogene) attempted to occupy the terrestrial predator niche vacated by the dinosaurs at the end of the Cretaceous. Like sebecians, they had relatively tall, narrow heads and blade-like teeth. Additionally, their claws were modified into hooves. Unlike sebecians, they were nover very large.
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:
- Gavialoidea (Gavials or gharials) (Cretaceous - Recent) Slender snouted fish-eaters.
- Alligatoroids (Caimans, alligators) (Cretaceous - Recent) Broad-snouted fresh water macropredators.
- Crocodiloids (Crocodiles, Tomistoma) (Cretaceous - Recent) Broad-snouted fresh - salt water macropredators.