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This ancient marine reptile had a bite like no other

Shortly before a mass extinction ended the Age of the Dinosaurs, a reptile-like carnivore with a mouth like a box cutter patrolled the warm seas that once covered strips of what is now North Africa. A recently described fossil of the beast living in the ocean reveals that its bite was different from that of any of its relatives, in water or on land.

The animal was a mosasaur, an extinct marine reptile related to snakes and lizards. Mosasaurs commonly had piercing, conical teeth to grasp slippery or flat prey, crushing teeth to shatter hard-shelled animals. But this new variety had short, serrated, square blades, tightly packed in series to form a knife edge. This razor-sharp mouth is unique among mosasaurs and even in the entire lineage of tetrapods, mainly vertebrates that include amphibians, reptiles, birds, and mammals.

The discovery, described Jan. 16 in Cretaceous Research, suggests that mosasaurs were evolving experimental physical traits and lifestyles until their abrupt extinction 66 million years ago.

Morocco’s phosphate miners have found the curious fossil: a piece of upper jaw splashed with teeth. The jaw came from a mosasaur that lived in the late Cretaceous period. Many mosasaurs were massive predators, some spreading more than a school bus. But this fossil belonged to an animal just over a meter long, determined Nick Longrich, a paleontologist at the University of Bath in England.

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Longrich says the small size of the animal is interesting, but that didn’t catch his eye. “Those teeth don’t look like anything I’ve seen in a lizard,” Longrich says. The team called the mosasaur Xenodens calminechari – Xenodens means "strange tooth;" calminechari in Arabic means "like a saw."

Longrich says the closest match for X. calminechari's teeth appears to be that of modern sharks, which can "cut large meat bolts while razing them," he says. “Probably these cut teeth allowed him to process a huge variety of food,” Longrich says, noting that gray sharks use their teeth to cut everything from fish to sea anemones. Despite its small size, X. calminechari may have been a predator of fairly large animals or ravaged its remains, as it shared its watery home with large fish and cephalopods, as well as long-sized killer whale plesiosaurs.

jaw and fossilized teeth of mosasaur Xenodens calminechariThe fossilized jaw of Xenodens calminechari was coated with a battery of razor-like sharp teeth, which are shown here on the sides (top and middle image) and the edge of the knife (bottom image) of the teeth.Nick Longrich

The fossil is "completely strange," says Paulina Jimenez-Huidobro, a paleontologist at the University of Bonn in Germany who did not participate in the research, noting that teeth can be used to "cut and dice" crustaceans, shells and everything. .

However, he has doubts about the comparison with shark feeding methods and the diet based solely on the shape of the teeth, due to the fundamental differences between how sharks and mosasaurs bite prey. Shark teeth cut into flesh on the jaws extending outward toward the prey regardless of the skull.

“This mechanism does not occur in mosasaurs, although Xenodens have a shark-like tooth shape,” says Jimenez-Huidobro. "No lizard can do that."

For Longrich, the discovery of X. calminechari helps paint a picture of the tropical seas of the late Cretaceous as full of biodiversity before a late-dinosaur-era asteroid reached Earth (SN: 17/06/20). The ocean ecosystem was thriving at the time, so there may have been dozens of species of mosasaurs in a single habitat, he said, and strange prototypes like Xenodens have not yet been produced.

“Mosasaurs were still experimenting with new ways of feeding, new morphologies, new lifestyles just before the asteroid fell,” Longrich says.

Learning more about this agile, barracuda-like sea lizard will reveal more of the evolutionary experimentation that was taking place at the time, but to understand this it is necessary to discover more fossil material, which could take some time, says Longrich. In half a dozen years of searching, he has only seen one fossil of this species.

"Finally, something has to come up, but it's a waiting game."

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