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Rick Robinson

Jun 30th 2021

Ancient Lizards With Shark Teeth Swam Cretaceous-Era Seas

Sixty-six million years ago, a small but terrifying sea monster swam the shallow sea that covered what is now North Africa. It was a mosasaur, a member of an extinct family of ancient lizards related to modern-day snakes and monitor lizards.

Much larger mosasaurs also swam the seas of the late Cretaceous era, some of them approaching the size of today’s sperm whales. But as Live Science reports, newly discovered Xenodens calminechari, a species in the mosasaur family, was only porpoise-sized but had a mouthful of unique, sharklike teeth that could slice through flesh like a saw.

Sawtoothed Ancient Lizards

Only one fossil of Xenodens calminechari is known — appropriately enough, an upper jaw lined with a sawblade of teeth that was found by miners in Morocco. The scientific name of these ancient lizards commemorates their most distinctive feature and the fossil’s origin; the name combines the Greek for “strange tooth” and Arabic for “like a saw.”

As Science News explains, these teeth are unique not only among ancient lizards but also across the whole range of tetrapods, which includes amphibians, reptiles, birds and mammals. What they do resemble are shark teeth, particularly those of the dogfish shark.

These teeth can “cut large bolts of flesh out as they scavenge,” paleontologist Nick Longrich, lead researcher on the team studying Xenodens calminechari, told Science News. This could have allowed this small but deadly sea monster to not only strip flesh from carcasses but also hunt much larger prey.

Slice and Dice

We are only beginning to understand this toothy mosasaur, and much remains uncertain. Paulina Jiménez-Huidobro, a paleontologist not involved with the research, suggested to Science News that the teeth could have been used to “slice and dice” crustaceans. However, she questioned the comparison to sharks’ feeding methods, due to differences in structure (and thus munching action) between shark jaws and mosasaur jaws.

But even if we don’t yet fully understand how Xenodens calminechari hunted and ate its food, its unique dentition already tells us much about the late Cretaceous ecosystem in which it lived. According to Smithsonian magazine, the first mosasaurs appeared some 120 million years ago, more than 50 million years before the fossilized specimen that was found in Morocco.

Soon afterward, the mosasaurs would be wiped out by the same asteroid that killed off the dinosaurs. But right up until that final disaster, they seemed to be thriving. According to Longrich, the prehistoric sea that covered North Africa was a rich and varied ecosystem. And a deadly one — “the most dangerous seas in the world,” said Longrich, adding that “predator diversity there was unlike anything seen anywhere else on the planet.”

The Hazards of Specialization

Writing for his own blog, Longrich expanded on the variety and specialization of this ancient ecosystem. There has been a longstanding suggestion in biology circles that the late Cretaceous era (just before the dinosaur-killer asteroid hit Earth) was already ecologically stressed, with a reduced variety of species that left it especially vulnerable to disruption.

But the seas covering North Africa, where Xenodens calminechari lived, were biologically rich and varied at just this time. Yet, the very richness and variety of the ecosystem might have also made it vulnerable to disruption.

The reason is that a wide variety of species goes along with a high degree of specialization among those species. Instead of relatively few species of general-purpose predators that could hunt and scarf down whatever prey were available, a multitude of highly specialized predators were each optimized to hunt some particular prey.

This means they were not well-adapted to hunt anything else if their ideal prey suddenly became scarce or extinct. Thus, as Longrich suggests, when the asteroid hit Earth and disrupted the world’s ecosystems, species like Xenodens calminechari — and many others — were unable to find fallback options to feed on. The result: a cascade of extinctions that put an end to the Cretaceous world and cleared the way for the age of mammals.

Much, much more work will be needed to fill in and adjust this picture, but it is very typical of biology that a single fossil with saw-like teeth can shed new light on one of the most dramatic episodes in the history of life on Earth.

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