The Saw-toothed Mosasaur with Shark-like Teeth

I originally planned to write this as a short entry for a paleontology mini-encyclopedia, but the text gradually became much longer than expected. It felt a bit excessive for a discussion forum, so I decided to expand it into a full article instead. As for what to call it—perhaps a “paleontology mega-encyclopedia”? (Just kidding.)

Xenodens calminechari

Reconstruction of Xenodens calminechari(由Andrey Atuchin繪製)

When people think of mosasaurs, they often imagine gigantic marine reptiles. Popular culture—especially the Jurassic film series—has reinforced this image of enormous ocean predators, even though the mosasaurs depicted on screen are at least twice the length of the largest real specimens known from fossils. In reality, mosasaurs were a diverse family, Mosasauridae, containing species of many different sizes.

Just as the cat family includes both massive lions and tigers as well as small domestic cats, mosasaurs also ranged widely in body size. The animal discussed here represents one of the smaller members of this group.

The mosasaur Xenodens calminechari was discovered in the phosphate mines of Morocco, deposits famous for yielding an abundance of Late Cretaceous fossils. The species lived during the final stage of the Cretaceous period. Fossils of this mosasaur indicate that adults reached only about 1.6 meters in length, making it one of the smallest mosasaurs known.

Phylogenetic analysis suggests that Xenodens belonged to a lineage related to globidensine mosasaurs, a group known for their rounded, ball-shaped teeth. These teeth functioned like crushing tools, allowing those species to break open hard-shelled prey such as mollusks or turtles.

However, Xenodens followed a very different evolutionary path. Instead of rounded crushing teeth, its teeth were shaped like thin blades. Even more remarkable, the teeth were packed extremely close together so that they formed a continuous saw-like cutting edge along the jaw.

This arrangement is extraordinarily unusual. In fact, a similar dental structure is known almost nowhere among reptiles or other tetrapods. The only clear modern comparison occurs in certain dogfish sharks, whose tightly packed teeth form a serrated cutting blade capable of slicing through flesh.

Anyone familiar with the feeding scars left by the cookiecutter shark can imagine how efficient such teeth are at cutting tissue. The shark leaves smooth, bowl-shaped wounds in much larger animals by slicing out plugs of flesh. A comparable mechanism may have existed in Xenodens.

Because of these blade-like teeth, researchers propose that this small mosasaur was adapted for cutting rather than crushing prey. It probably fed on typical small marine animals such as fish, crustaceans, and cephalopods. Yet its unusual dentition may also have allowed it to exploit larger food sources. For instance, when large marine reptiles—such as plesiosaurs or other mosasaurs—died and sank to the seafloor, Xenodens may have scavenged the carcasses, slicing pieces of flesh from the body. A reconstruction of such behavior even depicts Xenodens feeding on the remains of an elasmosaurid plesiosaur.

Overall, the discovery of Xenodens calminechari highlights the remarkable diversity of mosasaurs in the Late Cretaceous oceans. Within the same marine ecosystems, different mosasaur species evolved distinct body sizes and feeding strategies—some crushing shells, others tearing apart large prey, and still others developing specialized cutting dentitions. This ecological diversification likely reduced competition between species and allowed many different mosasaurs to coexist in the same habitats.

In short, the oceans of the Late Cretaceous were not dominated by a single type of mosasaur. Instead, they hosted a complex community of marine predators, each occupying its own ecological niche—among them a small but highly specialized hunter armed with a mouthful of shark-like blades.

Author: Bai Leng

Reference:

Longrich, N. R., Bardet, N., Schulp, A. S., Jalil, N. E. (2021). Xenodens calminechari gen. et sp. nov., a bizarre mosasaurid (Mosasauridae, Squamata) with shark-like cutting teeth from the upper Maastrichtian of Morocco, North Africa. Cretaceous Research.