At the time of the discovery, Sereno's team had identified the fossils as belonging to the genus Carcharodontosaurus.
This group of predatory, two-legged theropod dinosaurs was first described based on two fossil teeth recovered in the 1920s that have since been lost.
The only previously named species from the genus, C. saharicus, was described from a single fossil skull found by Sereno in Morocco in 1996.
Subtle differences in the new Niger skull suggested that it belonged to a distinct species—a suspicion now confirmed following detailed examination, Brusatte said.
"There are five or six key differences in the skull, the snout, and the bones that surround the brain," the researcher said.
Jerry Harris, a paleontologist at Dixie State College in Utah, agrees with the classification of the Niger fossil as a new species.
C. iguidensis appears to have been built for "running in and slashing at things with its teeth and waiting for the crippled animal to topple over," he added.
Evolution in Action
The study suggests that the newly described species is evidence for rapid evolution after shallow prehistoric seas inundated North Africa, separating Carcharodontosaurus populations in what is now Morocco and Niger.
The mid-Cretaceous period when these dinosaurs lived was marked by some of the warmest temperatures and highest sea levels in Earth's history, lead author Brusatte said.
"It looks like we're seeing evolution in action," he added. "We see it not only in Carcharodontosaurus but with other dinosaurs from Morocco and Niger from the same time period."
The find could also help researchers understand how the massive Carcharodontosaurus was able to share its watery territory with a variety of other huge, two-legged carnivores.
Researchers know, for example, that theropods from the Abelisaurid family and spinosaurs—sail-backed meat-eaters that may have grown up to 60 feet (18 meters) long—also prowled the ancient ecosystem.
"It seems there are a number of different very large theropods milling about. That's been an interesting puzzle," Dixie State's Harris said.
These top predators may have coexisted because they weren't in direct competition. Spinosaurs, for example, had crocodilelike heads that were loaded with teeth.
"That's the kind of snout you see in organisms that eat fish a lot," Harris said.
"Things like Carcharodontosaurus had much taller, more robust skulls, with teeth that are not nearly as tightly packed," he added.
"That's an adaptation you see in things which are hunting and tracking down land animals."
Free Email News Updates
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES