Fossilized remains of a bizarre, dog-sized predatory dinosaur were recently recovered on the island of Madagascar. The discovery, funded in part by the National Science Foundation (NSF), was announced this week in the journal Nature by a team of researchers led by paleontologist Scott Sampson of the University of Utah. Matthew Carrano and Catherine Forster from the State University of New York at Stony Brook co-authored the paper.
These fossils, which date to the Late Cretaceous period (about 65-70 million years ago), represent a dinosaur new to science, dubbed Masiakasaurus knopfleri. Masiakasaurus was relatively small, as dinosaurs go, with a total body length of 1.6-2.0 meters, much of which consisted of its long neck and tail. The total mass of this small carnivore would have been approximately 35 kilograms (80 lbs.), roughly that of a German Shepherd dog.
"Scott Sampson and the NSF-supported team of U.S. scientists working in Madagascar continue to reveal startling new vertebrate fossils," says H. Richard Lane, geology and paleontology program director in NSF's division of earth sciences, which funded the research. "Masiakasaurus is one of several such recent discoveries by this prolific team, with, I'm sure, more to come."
Masiakasaurus is based on a number of isolated bones from several individuals. The great majority of these fossils were recovered from a single site. Included in the collection are parts of the jaws and about 40 percent of the remainder of the skeleton, with some bones being represented by multiple examples.
The most bizarre aspect of this theropod dinosaur is its extremely specialized teeth and jaws. The first tooth of the lower jaw is oriented almost horizontally, projecting forward instead of upward. Subsequent teeth angle increasingly upward until the sixth tooth; from this point backward, all the teeth point straight up. The teeth themselves are also unique. Whereas the teeth at the back of the jaw are typical of theropods-being flattened and serrated-those at the front are longer and almost conical, with hooked tips and only tiny serrations. These features are otherwise unknown among theropod dinosaurs, which tend to have teeth of the same type front and back.
"When we dug up the first lower jaw bone, we weren’t even sure it belonged to a dinosaur!" says Sampson. It was only after we compared it with the lower jaws of other carnivorous dinosaurs that we became convinced as to the nature of the owner. Certain features at the back of the jaw are unmistakably theropod."
Masiakasaurus shared its island home with at least one other carnivorous dinosaur, Majungatholus atopus. At 7-9 meters in length, Majungatholus was the top predator of the time, likely feeding on the massive long-necked sauropod dinosaurs also found there.
The diet of the smaller cousin, Masiakasaurus, with its unique teeth and jaws, is much less certain. There are a few species of living mammals-including various shrews, as well as a group of South American marsupials known as caenolestids-that may provide an analogue. These mammals possess a similar dental set up, with elongate, conical, forward-projecting teeth up front. In virtually all cases, the front teeth are used for grasping and piercing rather than tearing and slicing, and the prey generally consists of insects.
The jaws of Masiakasaurus suggest a similar feeding strategy, with the front teeth used to capture and manipulate animal prey, and the blade-like rear teeth then slicing the victim into bite-sized chunks. As to the nature of the preferred prey of this little dinosaurian carnivore, potential candidates include insects, fish, lizards, snakes, and small mammals.
Masiakasaurus and Majungatholus, the two known Malagasy theropods, are members of an enigmatic group known as abelisauroids, and recovered only on Southern Hemisphere landmasses. In particular, the fossils of Masiakasaurus share a number of specialized characteristics with predatory dinosaurs found in Argentina and India. This finding indicates that a previously unrecognized radiation of small-bodied predatory dinosaurs spread across much of the southern hemisphere toward the end of dinosaur times, paralleling the Late Cretaceous radiation's of small-bodied theropods (such as dromaeosaurids and ornithomimids) in the northern hemisphere.
In addition, the broad geographic distribution of these small-bodied theropods parallels that of their larger-bodied cousins, the abelisaurids, a finding that may have implications for plate tectonics, the theory that landmasses shift their relative positions as they move slowly across the face of the earth.
Madagascar was once part of the southern supercontinent Gondwana, which fragmented during the Mesozoic heyday of dinosaurs. The known geographic distribution of abelisauroid theropods large and small is consistent with a recently proposed geophysical hypothesis that Gondwanan landmasses retained connections well into the Late Cretaceous, much longer than previously thought. "If so", Sampson added, "dinosaurs and other land animals may have been able to travel the vast distances between South America and India-Madagascar because the two regions remained connected via intervening land masses."
Funding for the Madagascar project has also been provided by the National Geographic Society, with additional support from The Dinosaur Society and the Field Museum of Natural History, Chicago.
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