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Just in time for Halloween, here's a creature from the past that could give you nightmares: a crocodile-like reptile, as long as a school bus and weighing about as much as a small whale, that may have snacked on small dinosaurs.
In a 25 October report published online by Science at the Science Express website (http://www.sciencexpress.org), Paul C. Sereno of the University of Chicago and colleagues describe newly discovered fossil skulls and partial skeletons of this 110 million-year old giant from Niger, Sarcosuchus imperator.
Specimens of Sarcosuchus (pronounced SARK-oh-SOOK-us) were first discovered by French geologists in the Ténéré Desert in Niger in 1964. Researchers have found isolated bones of this animal strewn about the site since then, but its anatomy, lifestyle, growth, and kinship with other crocodilians have remained largely enigmatic until now.
"This new material gives us a good look at hyper giant crocodiles--there's been rampant speculation about what they looked like and where they fit in the croc family tree, but no one had enough of the skull and skeleton to really nail any the of true croc giants down until now," says Sereno.
The Science researchers confirmed that "massive" is one of the distinguishing characteristics of Sarcosuchus, using skull length to estimate its maximum body length at approximately 11-12 meters, 37-40 feet and eight metric tons.
The giant crocodilian was covered from head to mid tail in an overlapping armor of scutes (bony plates embedded in the skin), each sporting annual growth rings that the researchers used to estimate the age of a given animal.
Thin sections from scutes, taken from a fossil individual that had reached about 80 percent of its estimated adult size, show around 40 growth rings. This may mean that the animals took 50 to 60 years to reach their full size, says the Science study team.
This surprising longevity for a crocodilian suggests that Sarcosuchus attained its mammoth proportions by extending the duration of its growing time, rather than speeding up its rate of growth like most dinosaurs, says Sereno.
The new Sarcosuchus skulls have long snouts, comprising about 75 percent of total skull length, that are relatively broad compared to the needle nose of the living gharial (a modern crocodilian from India) and other ancient, narrow-snouted crocodilians like Pholidosaurus and Terminonaris.
Features in the skull and jaw link Sarcosuchus with Pholidosaurus and Terminonaris on the reptile family tree, according to the Science report. Yet unlike Pholidosaurus and Terminonaris, found in marine deposits, Sarcosuchus fossils come from river deposits, located about 100 miles from any ancient coastline.
These two crocodilian cousins were also specialized fish-eaters, while Sarcosuchus' anatomy points to a more generalized diet. Sarcosuchus had a substantial overbite, with its upper jaw overhanging the front teeth of the lower jaw. All the teeth of the lower jaw fit inside the upper tooth rows during a bite, rather than interlocking like the teeth of an exclusively fish-feeding jaw. Many of Sarcosuchus' teeth themselves are stout, smooth, and rounded, more suited for puncturing and crushing, says Sereno, who believes the giant may have included dinosaurs in its meal plan.
The Sarcosuchus skull also features a large nasal bulla, a bulbous bony growth on the end of its snout, "like a toilet bowl," jokes Sereno. The bulla appears to increase in size dramatically upon the animal's maturity.
Sereno and colleagues solicited opinions on the bulla's function from a number of reptile researchers. Bulla size seems constant throughout the Sarcosuchus sample, with no obvious differences that could be attributed to males and females, so it's unlikely to be involved in any type of sexual selection, according to Sereno. Some scientists suggested the bulla was used to improve directional smell in the stiff-necked reptiles, unable to turn their heads towards interesting scents as mammals do. The bulla may have also enhanced vocalizations.
"We're still wondering what it's for," admits Sereno. "Crocodilians are among the most vocal reptiles, so I wouldn't doubt that it may have been involved in both sound and smell."
Sarcosuchus' era on the earth was a rich time for crocodilians. The researchers have uncovered six different species of crocodilian reptiles at the Niger site, ranging from the modest to the monstrous, and including a bite-sized crocodile that Sereno describes as "not much more than an Oreo cookie" to Sarcosuchus.
"That's the fascinating thing about crocodile evolution. It seems like modern crocodiles have been trimmed at each end of their size range, with the little ones and the big ones disappearing," says Sereno.
The other members of the research team include Hans C.E. Larsson of Yale University, New Haven, Connecticut, Christian A. Sidor at New York Colleges of Osteopathic Medicine, Old Westbury, New York, and Boubé Gado of Institut de Recherches en Sciences Humaines, Niamey, Niger Republic. This research was supported in part by the David and Lucile Packard Foundation, the National Geographic Society, and the Pritzker Foundation.
Journal
Science