A 2-metre-long reptile that flourished in North American waterways shortly after an asteroid wiped out most dinosaurs 66 million years ago has been identified as the oldest known representative of the alligator group.
Scientists have uncovered bones from about 15 individuals of the previously unknown species, which probably ambush-hunted near freshwater channels and may have sheltered in burrows.
The newly named Mandrasuchus milleri looked like a smaller version of a modern alligator, with pointed front teeth for seizing prey and rounded, molar-like back teeth that could crush shellfish and other hard-bodied prey.
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Combined, these characteristics may help explain its success in a dying world. This success may then have paved the way for the widespread expansion of alligators, says at the Denver Museum of Nature & Science in Colorado.
鈥淲e have a bunch of these specimens, so they were quite abundant,鈥� she says. 鈥淎nd so to have a large-bodied apex predator not just living, but actually thriving, so soon after such an apocalyptic event is really telling.鈥�
The asteroid strike at the end of the Cretaceous Period led to the extinction of all non-avian dinosaurs. Crocodilians, however, fared considerably better: multiple lineages, including the 鈥溾�� and the gharials, survived. The alligatorids survived, too. They are divided into two subfamilies. The 鈥渁lligatorines鈥� include the two living species of alligator 鈥� the Chinese and American. The other subfamily is the 鈥渃aimaninae鈥�, which includes the six known species of caimans.
Freshwater habitats may have offered some protection from the devastation, giving semiaquatic crocodilians a safer refuge, while their ability to eat a wide range of foods meant they could cope when particular prey disappeared, says Sherman. 鈥淚f you鈥檙e not picky, you have a much better chance of surviving an extinction event, right?鈥�
Even so, exactly when and how the alligatorids branched off from other crocodilians, and then split into today鈥檚 variety of alligators and caimans, has remained muddy.
A team from the Denver Museum, including Sherman, may have uncovered an important piece of the puzzle. Around a decade ago, they began finding well-preserved fossils of an unknown crocodilian at Corral Bluffs, a fossil site in central Colorado. The team has since recovered hundreds of bones, teeth and bony skin plates, or osteoderms, says team member .
Close examination and micro-CT scans of selected fossils suggested they belonged to a crocodilian with a short, broad, alligator-like snout and pointed front teeth that gave way to bulbous back 鈥渂utton鈥� teeth. 鈥淚t was probably just eating anything it could get its mouth on,鈥� Sherman says.
After looking at 330 anatomical features 鈥� including tooth numbers, skull proportions and the shapes and positions of individual bones 鈥� and comparing them with the same features in nearly 150 living and extinct crocodilians, the researchers argue that M. milleri was an alligatorine. Analyses of fossil pollen and radiometrically dated volcanic ash revealed that the oldest skulls date to a mere 350,000 years after the asteroid strike, making M. milleri the oldest known member of its subfamily.
Several M. milleri skeletons were found with bones still connected, including one inside what may have been a burrow. Burrowing could have sheltered the animals from harsh conditions after the asteroid strike, Sherman explains.
The name combines the Greek mandra, meaning an enclosed space or corral, in reference to Corral Bluffs, with Suchus, the Greek name for the crocodile-headed Egyptian god Sobek; milleri honours Ian Miller, who used to work at the museum, and who found many of the specimens.
鈥�Mandrasuchus milleri provides a rare insight into how alligatorids diversified almost immediately after the end-Cretaceous extinction,鈥� says at New York Institute of Technology in Long Island. 鈥淭he fossils are compelling.鈥�
at the University of S茫o Paulo, Brazil, agrees. 鈥淭he fossils of Mandrasuchus are beautifully preserved, and that鈥檚 always exciting,鈥� he says. 鈥淭hey improve our understanding of the group鈥檚 morphology and, ultimately, its evolution.鈥�
Journal of Vertebrate Paleontology
