David Schwimmer, a paleontologist at Columbus State University in Georgia, said he was familiar with Sereno's discovery and was "thrilled with it" because it helps fill in the picture of giant crocs, which appeared repeatedly in evolutionary history. Schwimmer is an expert on a giant croc genus named Deinosuchus, which was prevalent in North America.
Sereno and his colleagues announced their discovery October 25 at National Geographic headquarters in Washington, D.C. The research expedition to Niger last year was funded in part by the National Geographic Society.
The announcement was made in conjunction with the publication of a scientific report on Sarcosuchus by the journal Science, which posted the paper on its Science Express Web site.
The co-authors of the Science paper are Sereno; Larsson, who is a postdoctoral fellow at Yale University and the University of Toronto; Christian Sidor of the New York College of Osteopathic Medicine in Old Westbury, New York; and Boubé Gado of the Institut de Recherches en Sciences Humaines in Naimey, Niger Republic.
Not a "Modern" Croc
SuperCroc lived in Africa during the Middle Cretaceous period, when broad rivers stretched across lush plains.
Larsson said Sarcosuchus was not from the same branch of the reptile family tree that gave rise to modern crocodilians, which consist of about 23 species that include alligators, crocodiles, caimans, and gharials. "It's not a modern croc, but they share an early common ancestor," he said.
The oldest precursors of crocodiles may have spent more time on land, but Sarcosuchus was a river dweller that appeared after early crocodilians had already split into two separate land-based and marine groups.
Sereno said Sarcosuchus probably spent most of its time underwater, "living an ambush lifestyle." Like the gharial, a large long-nosed crocodilian in India, the ancient croc had eye sockets that tilted upward, which helped it conceal its huge body underwater while scanning the river's edge.
Another distinctive feature that Sarcosuchus had in common with the gharial was a round bony protrusion at the tip of its snout that housed a large bowl-shaped inflatable nasal cavity, called a bulla. The function of the bulla isn't clear, but the researchers think it may have heightened the croc's sense of smell or enabled it to emit striking calls. Larsson said gharials are known to use the muscles around the bulla to make different kinds of sounds, especially for mating.
Sereno said some people have an erroneous image of crocodiles "as dumb, clumsy, silent creatures." But crocodiles "are anything but clumsy, and they communicate extensively by calling, even roaring and splashing," he said. "It looks as if Sarcosuchus did some of that too."
The narrow jaws of an adult Sarcosuchus housed more than 100 teeth, which Larsson likened to "railroad spikes." While the giant croc shared the water with large fish, its hearty teethwhich included bone-crushing incisorssuggest that Sarcosuchus "didn't seem limited to eating fish," Larsson said. Other prey may have been small dinosaurs and other terrestrial animals, such as turtles.
Overlapping rows of scutes covered the crocodile's body from head to tail, forming a tough protective armor. The scutes, like trees, have annual growth rings. By counting these rings in the fossilized scutes, the researchers estimated the giant croc's full life span as 50 to 60 years.
The fossils unearthed at Gadoufaoua included bones from four other croc species of varying sizes that lived at the same time as Sarcosuchus. One specimen was the three-inch (eight-centimeter) skull of a new species of dwarf croc.
The discovery of five ancient species existing side by side was especially interesting, Larsson said, because such diversity at a single site is seldom seen today. "Most modern crocs are relatively similar," he explained. "Perhaps for that reason, you rarely get more than one species at a particular location."
"The reason why we can get five [ancient] species at the same time," he added, "is because of differences in size and antomy. They were not eating the same thing or competing for the same resources."
Early forms of crocodiles first appeared about 230 million years ago, during the late Triassic, and diverged into "an amazing number of forms," said Larsson.
The few existing fossils show that these earliest crocs didn't much resemble the crocodiles we know today. "They were more dog- or cat-size, with elongated limbs like those of a gazelle or antelope," Larsson said. "The skull also was not crocodilian at all, but more 'dog-faced.'"
These earliest forms of crocodiles were succeeded in the late Triassic by a diverse group of terrestrials with squat bodies and more croc-like skulls (Crocodyliformes). In the early Jurassic, crocodilians split into two distinct groupsone group living in water (even sporting tail fins), the other on land. Crocs most like modern ones, with amphibian bodies and distinctive skulls, began emerging in the Early Cretaceous.
Although it's still uncertain, Sarcosuchus may have had some rivals throughout history that matched or exceeded it in size and weight.
As with dinosaurs, many branches of crocodilians spawned giants. "There are actually quite a few giant crocodilians," said Schwimmer. "The idea of really big crocs is a repeat theme in evolution."
Deinosuchus, the subject of much of Schwimmer's research, lived in the Late Cretaceous, which means it's younger than Sarcosuchus. The two species "were not closely related," Schwimmer noted.
The range of Deinosuchus was much of North America. It dwelled from New Jersey to Montana, and was especially common in Texas and Alabama. The first report of the species came in 1858, based on ancient teeth that were discovered in North Carolina, said Schwimmer, who has received research grants from the National Geographic Society.
A number of fairly complete skulls of Deinosuchus have been found, but "we haven't yet put together a full body reconstruction," said Schwimmer. Once that happens, he added, the analysis might show that Deinosuchus was similar in body size or even bigger than Sarcosuchus.
That conjecture is based in part on differences in the snouts of the two species. "Sarcosuchus had a long, narrow snout, so a lot of its length is in the snout," Schwimmer explained. "But Deinosuchus was broad-snouted, built more like an alligator, so a skull of the same length [as Sarcosuchus] would, based on proportional size, be an even bigger animal."
Why have giant crocs recurred throughout evolutionary history?
One reason, Schwimmer said, is because crocodilians have been primarily aquatic. Massive bodies, like those of whales, are especially suited to an aquatic environment because they can float, thereby diminishing the physical burden of size that would be more taxing on land.
Another advantage is that crocodiles have osteoderms, or skin armor, across their backs. These armor plates are embedded in the creatures' back tissue, helping to support the back "like an external flying buttress," Schwimmer said. Crocodiles also have very strong skulls, he added, "so they can bite hard and feed on big prey."
Research Supported by the National Geographic Society:
Paul Sereno is one of a distinguished group of scientists from around the globe, in fields ranging from astronomy to zoology, who have been awarded grants from the National Geographic Society. Here are some recent news stories about the work of other NGS grantees:
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The Society's support of groundbreaking scientific research began with the 1890 exploration and mapping of the Mount St. Elias region along the Alaska-Canada border and the discovery of Canada's highest peak, 19,524-foot (5,956-meter) Mount Logan.
The Society's Committee for Research and Exploration meets six times a year to review applications and award grants. About 280 grants are given each year.
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