Oct 21 : Crocodilians — today's crocodiles, alligators, caimans and gharials and their many extinct relatives — have been a roaring success since debuting more than 240 million years ago. And new research identifies one important factor in their success — stubbornly keeping to warm climates throughout their history.
In fact, the researchers said, this trait may help explain why crocs as a group survived the asteroid strike 66 million years ago that doomed the dinosaurs, along with certain physiological characteristics that make them one tough bunch of reptiles.
The crocodilian evolutionary lineage, called Pseudosuchia, arose during the Triassic Period in the aftermath of Earth's worst mass extinction event at the end of the Permian, appearing on the scene millions of years before the first dinosaurs and mammals.
The researchers tracked the geographic range for crocs during their entire history, guided by an evolutionary tree of more than 400 extinct and living species. They used data from more than 7,000 fossils and geographic records of modern crocs such as the alligators of Florida, saltwater crocodiles of Australia, gharials of India, caimans of Brazil and more.
They combined this information with 52 global palaeoclimate reconstructions to determine the climatic conditions under which crocs lived over time. While other groups such as dinosaurs and mammals conquered a variety of climates, the study showed that crocs consistently kept to warm climes.
"Rather than continually adapting to new climatic conditions, they largely tracked suitable environments as these shifted geographically, expanding towards the poles during warmer periods and retreating towards the equator during cooler ones," said University College London vertebrate paleontologist Alfio Alessandro Chiarenza, lead author of the research published in the journal Science Advances.
For example, during an exceptionally warm moment in Earth's history around 50 million years ago, Chiarenza said, crocs inhabited high-latitude regions of North America, Europe and even Antarctica.
"One of the most striking patterns is the repeated expansion and contraction of suitable habitat in response to global temperature changes," Chiarenza said.
While living crocs are primarily semiaquatic ambush predators, their extinct relatives included a broad diversity of forms, University College London paleontologist and study co-author Philip Mannion said.
"They have occupied almost every conceivable ecological role," Chiarenza said.
There were fully land-dwelling apex predators like 30-foot-long (9-meter-long) Fasolasuchus, pug-snouted herbivores like Simosuchus, armored plant-eaters like Aetosaurus, mollusk-eaters like Kuttanacaiman, filter-feeders like Mourasuchus and ocean-going types like Metriorhynchus. During the age of dinosaurs, there also were semiaquatic crocs like Deinosuchus up to 40 feet (12 meters) long that may have rivaled Tyrannosaurus in mass.
"Modern crocodilians are only the surviving remnants of a once extraordinarily diverse evolutionary group," Chiarenza said.
DISASTER FROM ABOVE
The asteroid strike at a site called Chicxulub off the coast of Mexico's Yucatan Peninsula at the end of the Cretaceous unleashed a global environmental cataclysm. Three-quarters of Earth's species including the dinosaurs, aside from their bird descendants, vanished. The crocs survived.
"The Chicxulub impact caused a dramatic reduction in incoming sunlight, producing an impact winter and widespread environmental disruption. However, our climate simulations suggest that this cooling did not affect the availability of suitable climatic conditions equally for all groups," Chiarenza said.
"Under all but the most extreme impact scenario we tested, tropical and subtropical regions retained climatic conditions suitable" for the crocs, Chiarenza said.
This suggests that the longstanding association of crocs with warm and low-latitude environments may have placed some surviving populations in geographic refuges near the equator and in the subtropics where climatic conditions remained within their tolerances, Chiarenza said.
For dinosaurs, there was a collapse of suitable habitat under these impact conditions.
Crocs also possess physiological traits beneficial for times of environmental stress. They can enter states of dormancy to cope with harsh conditions — aestivation to survive extreme heat or drought, and brumation to withstand extreme cold. Crocs today, for instance, may dig burrows or encase themselves in hardened mud during severe droughts, slowing their heartbeat and breathing.
"Following the Chicxulub impact, when sunlight was dramatically reduced and terrestrial primary productivity collapsed, the ability to reduce energy expenditure and survive periods of limited food availability could have been extremely advantageous," Chiarenza said.
Crocs also are cold-blooded, while recent research points to at least some dinosaurs including Tyrannosaurus as warm-blooded. Baseline metabolic requirements for cold-blooded animals are much lower than for comparably sized warm-blooded animals.
"That can become a significant advantage when food becomes scarce. Crocodilians can survive extended periods without feeding," Chiarenza said. "But even if an animal can survive prolonged starvation, it still needs environmental conditions that remain within its physiological limits."