Published August 4, 2010
Some dinosaurs, a new study suggests, knew very different crocs from today's reptiles—including a newfound fossil crocodile that might seem to have been designed by committee. (See pictures of the prehistoric "cat crocodile.")
According to the study, the cat-size Pakasuchus kapilimai had relatively long legs and a nose similar to a dog's. Perhaps weirdest of all, Pakasuchus—literally, "cat crocodile"—had mammal-like teeth that gave the crocodile a power previously unknown among reptiles: the ability to chew.
As a result, the 105-million-year-old crocodile, researchers say, was something of a stand-in for mammals in the mammal-poor, long-gone southern supercontinent of Gondwana, which included the Pakasuchus fossil site in what's now Tanzania.
According to lead study author Patrick O'Connor, Pakasuchus's "legs were longer and more slender than what you think of for modern crocodiles ... and it had a little more of an upright stance," the Ohio University paleontologist said.
Living among dinosaurs, the fossil crocodile—technically a crocodyliform—was also lankier and had fewer of the bony plates that armor modern crocodiles.
And unlike in modern crocodiles—which have nostrils positioned on the tops of their heads, for breathing while swimming—the nose of Pakasuchus was located at the tip of its snout, canine style.
"It was not like the crocodiles we know today, which are submerged much of the time. It was up, moving around on the land," said O'Connor, who received a National Geographic Society/Waitt Grant for this project. (The Society owns National Geographic News.)
Video: Mammal-like Crocodile Found
Mammal-like Teeth Gave Crocodile Chewing Power
Modern crocodiles have fairly undifferentiated, conical teeth specialized for a bite-and-gulp feeding style. But Pakasuchus had teeth resembling mammals' canines, premolars, and interlocking molars, says the study, to be published in tomorrow's issue of the journal Nature.
In addition, the lower jaw of Pakasuchus could slide back and forth. "Crocodiles alive today don't have a major sliding component to their jaw," O'Connor explained. "It's just a hinged joint that allows the jaw to move up and down."
Taken together, the crocodile's unique dentition and sliding jaw suggest Pakasuchus was able to chew and grind its food, a trait previously thought to be unique to mammals.
"Chewing is a mammalian characteristic, almost by definition," O'Connor said.
"Most people who do functional anatomy or paleontology reserve the term for mammals only," he said. Other animal groups are generally thought to lack the necessary anatomical features and coordinated muscle activity required for complex chewing, he added.
Pakasuchus was almost certainly capable of chewing, agreed Zhe-Xi Luo, a paleontologist and early-mammal expert at the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania.
"We used to think that it was not possible for crocodiles to really chew their food," said Luo, who was not involved in the research. "But [this study] shows that, yes, they could."
Crocodile Was Mammal Proxy in Land of Reptiles?
Pakasuchus likely fed on small lizards, insects, and the few primitive mammals that existed in Gondwana at the time. The animal's unique dentition may have allowed the fossil crocodile to occupy ecological niches filled by mammals in other parts of the world, the study says.
"This small-bodied animal occupied a dramatically different feeding niche than do modern crocodilians" study co-author Nancy Stevens, also of Ohio University, said in a statement. The crocodilian order includes crocodiles, alligators, caimans, gavials, and related extinct forms.
Pakasuchus belonged to a reptile family called the notosuchians, or "southern crocodiles." For this group, differentiated teeth may have been the key to success, said Steve Salisbury, a paleontologist at the University of Queensland in Australia.
Increasingly, fossil discoveries in former areas of Gondwana are revealing that the notosuchians boasted a very diverse range of species, said Salisbury, who also did not participate in the Pakasuchus-fossil research.
If it turns out that mammal-like teeth were typical among other notosuchians, "it may have opened up new opportunities that weren't possible with more simplified dentition," he added.
Salisbury adds that the discovery of Pakasuchus will likely prompt many scientists to go back and reexamine their fossil collections.
"Isolated teeth that people previously assumed belonged to mammals," he said, "may in fact belong to crocodiles."
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