Crocodilian Evolution

[lpetrich]

 Crocodilian - includes alligators, crocodiles, and reptiles closely related to these. "Alligator" is from Spanish "el lagarto" - "the lizard", and "crocodile" is from Ancient Greek krokodilos, a word for lizard, appearing in "ho krokodilos tou potamou" - "the lizard of the (Nile) river". Its name may be a compound that means "pebble worm".

Though they have a very lizardlike shape, they are nevertheless not strict-sense lizards by present-day taxonomic criteria. There are some details of their skulls that differ, for instance. Their lizardlike shape is evolutionary conservatism -- lizards, crocodilians, and salamanders all have body shapes not much modified from early tetrapods' body shapes.


One can distinguish an alligator and a (true) crocodile by their snouts - alligators have U-shaped ones and crocodiles have V-shaped ones. This difference in snouts reflects different preferences in prey. Crocodiles prefer smaller prey, like fish, while alligators sometimes catch prey larger than they themselves. But Mesozoic crocodilians had more variety than present-day ones.

What did ancient crocodiles eat? Study says as much as a snout can grab

"Several of these fossil groups had skulls and teeth wildly different from living species. This suggests that the way they fed also differed dramatically," said coauthor Eric Wilberg, an assistant professor in Stony Brook University's Department of Anatomical Sciences. Among these are a group of extinct crocs that lived in the oceans. While they had slender snouts similar to those of living gharials, their eyes were positioned more on the side of the head, and the part of the skull that houses the jaw muscles was enlarged. This suggests they were not ambush predators like modern crocodylians.

Another group consists almost exclusively of species that lived on land. These crocs had flattened, serrated teeth, like those of carnivorous dinosaurs, and eyes positioned more on the side of the head.

There is indirect evidence of what they ate: bite marks on bones. The inferred biters mostly have the sizes that one might expect from their prey sizes and inferred eating habits.

Some crocodile groups remain mysterious. No fossil bite marks exist for the stubby-faced crocs, whose complex teeth and weak jaws suggest they might have been plant eaters, or for the surfboard-headed ones, which had tiny teeth and may have sported pelican-like pouches under their long, wide jaws.

Journal paper: A synthetic approach for assessing the interplay of form and function in the crocodyliform snout | Zoological Journal of the Linnean Society | Oxford Academic

 

[lpetrich]

Present-day crocodilians do not look much different from their late Mesozoic ancestors. They are mostly semiaquatic ambush predators, though saltwater crocodiles (Crocodylus porosus) like open water.

Climate change created today's large crocodiles
noting journal article The multi-peak adaptive landscape of crocodylomorph body size evolution | BMC Evolutionary Biology | Full Text

Extinct crocodylomorphs—henceforth "crocs"—ranged from marine forms who lived a fully aquatic life to terrestrial herbivores with complex teeth that look like those seen in mammals today. There's much more ecological diversity on display in the fossil record than what you see in modern crocodylians.

The oldest crocs, alive during the Triassic Period, were small, rarely more than 1 meter long.

At first glance, the fossil record might seem to suggest that crocs evolved from this small start to be larger over time. That would fit with a general trend biologists have identifed: Animals tend to evolve over time to be bigger. This pattern is known as Cope's rule. Scientists have spotted this tendency in mammals, dinosaurs and pterosaurs.

So the authors of that paper decided to test that hypothesis. They found a more complicated picture.

We divided crocs into three ecological categories: land-based, semi-aquatic and fully aquatic. We found that terrestrial crocs are significantly smaller than semi-aquatic and marine ones. Other researchers have documented similar findings for mammals, with marine species larger than their terrestrial relatives. Think of walruses and sea lions, which are much bigger than their cousins: cats, wolves and dogs.

There weren't any clear trends when one takes all the crocodilians together, but when they looked at Crocodylia, they found a trend over the Cenozoic - a trend toward larger body size. This was correlated with decreasing overall ocean temperature. So as the Earth cooled over the Cenozoic, crocodilians became both larger and confined to areas that stayed warm - tropical and nearby areas.

In the end, the large size of modern crocs, although a symbol of strength and power, might mean that this group is instead ecologically fragile, facing a long-term process of extinction. And today, the few croc species left are facing an extra challenge related to environmental changes, this time due to human-made destruction of their ecosystems.

 

[lpetrich]

Scientists reconstruct genome of common ancestor of crocodiles, birds, dinosaurs
noting journal paper Three crocodilian genomes reveal ancestral patterns of evolution among archosaurs | Science

Richard E. (Ed) Green, lead author of the crocodilian genome paper and an assistant professor of biomolecular engineering at UC Santa Cruz, said the slow evolutionary rate in the crocodilian lineage was helpful in reconstructing the genome of the common ancestor.

"The ticking of the molecular clock in the crocodilians is much slower than in other lineages we're used to looking at, like mammals, which means we can see back into their past more cleanly," Green said.

An interesting feature of their published family tree is that the ancestors of present-day crocodilians diverged relatively recently. From the fossil record, according to the journal paper, "Although crocodilians diverged from birds more than 240 million years ago (Ma), animals with morphology unambiguously similar to the extant crocodilian families (Alligatoridae, Crocodylidae, and Gavialidae) first appear in the fossil record between 80 and 90 Ma." - in the middle of the Cretaceous.

The slow rate of genome evolution in crocodilians also enabled the researchers to probe population sizes further into the past than is possible for faster evolving lineages, he said. They found that crocodile and gharial populations experienced sharp declines during the most recent ice age. Alligators, which inhabit more temperate latitudes than other crocodilians, showed a continuous decline in population throughout the Pleistocene epoch.

"Big-bodied, cold-blooded reptiles would have found the Earth a more hospitable place during warm periods like the Pliocene, and the cooling trend of the Pleistocene must have been bad news for crocodilians," Green said.