All crocodiles and alligators today are semi-aquatic hunters, with relatively long, flat skulls, and a new study may have uncovered the key evolutionary logic behind this. Although this is not the ideal skull shape for biting prey, it is the one that works best underwater.
The conclusion, which appears in a new work by Brazilian researchers and foreign colleagues, is based on tomograms of animal heads, including living (modern) species and those that existed in the age of dinosaurs. The research was published last month in the scientific journal Proceedings of the Royal Society B.
The comparison between modern and ancient animals is important because the current skull shape of these reptiles, although it varies relatively little between living species, is only one part of a much greater diversity of morphologies (forms).
In Brazil, during the Cretaceous, for example – that is to say the final phase of the reign of dinosaurs, between 143 million years ago and 66 million years ago – many members of the group had skulls similar to those of carnivorous dinosaurs. And, like them, they also hunted on land, and not in an aquatic environment.
The study team, which includes Fabiano Vidoi Iori, from the Pedro Candolo Paleontology Museum (Uchoa, inland São Paulo), Fresia Ricardi-Branco, from Unicamp (State University of Campinas) and Ismar Carvalho, from UFRJ (Federal University of Rio de Janeiro), explains that it is possible to divide the morphological diversity of animal skulls into two main format types.
On one side are the dome-shaped, short-sided skull species, whose “classic” morphology virtually disappeared after the age of dinosaurs.
On the other hand, we have the current “crocodile face” (and alligator face) typical of modern animals, which is broad and flat. This morphology was also present in the age of dinosaurs. The question is what evolutionary processes ultimately produced these different patterns.
Apparently, the flattened version of the skull and snout is more efficient at moving through water, providing less resistance when the animal swims. But there was also the idea that it might help alligators and crocodiles better resist the forces generated by twisting the snout when the animal feeds.
It was in an attempt to resolve this doubt that Brazilian paleontologists and their foreign colleagues subjected the animal skulls to tomography sessions. Detailed images of bone structure allow scientists to reconstruct, with a reasonable degree of reliability, the distribution and function of muscles. This is because bones have specific areas of muscle insertion within their own structure, which provide clues to how each animal bites and chews its prey, for example.
On the modern species side, the team included the American alligator (Mississippi alligator), the Nile crocodile (Crocodylus niloticus) and the dwarf caiman or paguá caiman (Paleosuchus palpebrosusonly Brazilian species of the trio). All are semi-aquatic carnivores.
Among the animals of the dinosaur era, the sample consisted of three terrestrial species found in the interior of São Paulo. Among them are the super predator Baurusuchus saloensismeasuring up to four meters long, and two smaller animals: the Montealtosuchus arrudacamposiwhich probably captured small prey, and the Caipirasuchus paulistanuspossibly herbivorous (something not seen among its modern relatives).
The analysis showed that there was a significant difference in the technique of attacking prey: Extinct animals likely “bite and pull,” while current animals rely on tactics such as the so-called death spin, in which the reptile holds the victim and spins the body in the water to kill it. But more important is the fact that extinct land animals had greater muscular efficiency and skulls more capable of resisting the effort of biting (and, in the case of herbivorous species, chewing). In practice, these are cranial structures that are more resistant to fractures caused by stress, for example.
The research therefore indicates that the evolution of semi-aquatic crocodiles and alligators required some sort of trade-off based on cost-benefit. To develop skulls that facilitated movement in water, it was necessary to lose the efficiency of the muscles and the safety of the bite in relative terms.
Apparently, it was worth it, because land species with powerful skulls ultimately didn’t survive during the mass extinction that wiped out most dinosaurs (except birds) 66 million years ago.
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