Xenodens, a weird little shark-toothed mosasaur from Morocco
What Mesozoic marine reptiles tell us about specialization, diversity, and mass extinction
Xenodens calminechari scavenges on a dead plesiosaur north of what is now Casablanca, Morocco, 66 million years ago. Art by Andrey Atuchin.
66 million years ago, the end of the Cretaceous period, shallow seas flooded northern Morocco, extending inland as far as the edge of the Sahara. An extraordinary diversity of marine reptiles lived in these waters, including mosasaurs- a family of giant marine lizards, as well as plesiosaurs and huge marine turtles. It’s one of the most diverse Mesozoic marine faunas we know of (and contains a surprising number of dinosaurs as well). And yet, new species of marine reptile keep emerging. One of these is a strange little species of mosasaur. The new species, named Xenodens, is unlike any mosasaur (or reptile) we’ve seen before. It had bladelike teeth packed edge to edge to make a cutting surface, like certain sharks. It seems that just before the Chicxulub asteroid impact that ended the Cretaceous, mosasaurs were still diversifying- but that wasn’t enough to save them. Ironically, their very diversity may have worked against them.
Composite photo of 7 meter long skeleton of Mosasaurus beaugei, late Maastrichtian of Morocco, 66 Mya. Note flipper-like limbs; the kinked tail that supported a tail fluke.
Mosasaurs are marine reptiles from the age of dinosaurs. They weren’t dinosaurs themselves, but lizards, possibly relatives of monitor lizards and snakes. 100 million years ago, they invaded the seas. The first mosasaurs were small, about a meter in length,a and restricted to shallow, nearshore environments. Around 94 million years ago, a mass extinction hit at the end of the Cenomanian. The Cenomanian-Turonian extinction is probably caused by a volcanic eruption and in marine ecosystems, it’s marked by widespread low oxygen conditions- an anoxic event named OAE2, Oceanic Anoxic Event 2 (it was one of a series of such events that hit in the Cretaceous).
Ichthyosaurs and other marine species went extinct, and mosasaurs evolved to take their place. They quickly became more specialized for marine life, and much, much bigger. They evolved flipper-like limbs, a fluked tail. Imagine a Komodo dragon crossed with a shark, and stretched out to 12 meters or more, and that’s a mosasaur. They evolved the ability to exploit the open seas and the deep ocean. Probably, like modern elephant seals and sperm whales, mosasaurs were able to dive thousands of feet in search of prey.
Mosasaurs were one of just a handful of tetrapod lineages (whales, sea cows, modern sea snakes, the extinct palaeophiid sea snakes, plesiosaurs and ichthyosaurs) to become fully specialized for a marine existence, neither needing to return to land, nor able to. The reason for this is that- unlike crocodiles, turtles, dinosaurs, or birds- lizards have repeatedly evolved live birth. In species like short-horned lizards, alligator lizards, rattlesnakes, and Jackson’s chameleons, eggs are retained and hatch inside the mother. Mosasaurs evolved a similar strategy, which meant they could give birth at sea, rather than returning to land.
By the end of the Cretaceous period, 30 million years after their initial appearance, mosasaurs had diversified to exploit a huge range of niches. The giant Mosasaurus was a fish eater, adapted to dive deep in search of large prey, like a sperm whale. Halisaurus, a little mosasaur with lots of small, sharp teeth, probably hunted small fish like a porpoise or a seal. Globidens evolved short, blunt teeth, which are variously shaped like marbles, acorns, or buttons, depending on the species. These teeth were specialized to crush mollusks, like a sea otter or a walrus. And the monstrous Prognathodon had teeth shaped like those of a killer whale- it was an apex predator, eating fish, plesiosaurs, marine turtles… and other mosasaurs.
To this mix of specialized species, we now can add another, weird little Xenodens, with its sharky jaws.
The holotype of Xenodens calminechari, unprepared.
The teeth of Xenodens aren’t like anything we’ve seen in mosasaurs before, or lizards. For that matter, I’ve struggled to find anything like them in any other reptile.
Prepared holotype upper jaw of Xenodens
Closeup of the teeth of of Xenodens
The only place I had seen teeth like this was years ago, working on my family’s boat, halibut fishing in the Gulf of Alaska, when we’d pull up sharks on the longline. We’d get sleek little dogfish, and these huge, matte-black sleeper sharks, rolling slowly in the water, their green cats-eyes gleaming in the light. In their jaws were these squarish teeth, packed edge to edge to created a serrated edge, like a sawblade.
Lower jaws of the dogfish Squalus acanthias. Courtesy Ross Robertson, STRI.
Dogfish and their relatives, the sleeper sharks, use these tooth arrangements to slice meat.
A dogfish can slice a fish in half; a sleeper shark can carve huge, circular bolts of meat out of its prey. Often, working on the boats, one would see halibut, hooked on the line and unable to escape the sleeper sharks, with big semicircular cuts on their sides, and huge chunks of flesh cut out. It looked like someone had taken a giant ice cream scoop and scooped it out. Even big fish were bit almost in half. The cookie-cutter sharks, which are related to sleeper sharks, take this feeding strategy even further, taking chunks of meat out of marine mammals, although they have triangular teeth rather than the rectangular ones in dogfish, sleepers, and Xenodens.
Reconstruction of the skull.
By analogy with dogfish, Xenodens may have done something similar with its teeth. Our best guess is that the specialized teeth served to allow it to cut large fish into bite-sized pieces. Perhaps they could also scavenge from large carcasses, as dogfish and sleepers sometimes do, although we can’t rule out other feeding strategies or other prey. Xenodens could probably take a huge range of foods, like modern dogfish, but the key purpose of those bladelike teeth was probably to let the animal take much larger prey than it otherwise could have.
The ability to cut into large animals was a useful adaptation because Xenodens was a very small mosasaur- about the size of a small porpoise (it’s not impossible we’ll eventually find larger individuals- juvenile mosasaurs are common in the fauna, and the specimen may not be full grown- but it probably didn’t get that big, since its relatives were also small). So these bladelike teeth let it to punch above its weight and eat large things despite its diminutive size, occupying a niche that’s distinct from anything any other mosasaur does.
Curiously, some oddities of the teeth- they’re laterally compressed, short but broad, and tooth bases fuse together- suggest affinities with another small mosasaur, Carinodens, which has crushing, rather than cutting teeth. What Carinodens did is another question. The teeth are for crushing but the jaws are fairly slender and weak, which to me suggests specialization for prey wth a hard but fairly weak and brittle shell- maybe crustaceans, or small ammonites.
Xenodens shown with Homo sapiens for scale.
So what’s it all mean?
Xenodens is one more piece of evidence showing that the very end of the Cretaceous, the late Maastrichtian, was a time of high diversity.
Over the past two decades, work in Morocco has revealed an astonishingly diverse mosasaur fauna, dating to 66 million years ago. This fauna lies just below the Cretaceous-Paleogene (K-Pg) boundary, immediately before the Chicxulub asteroid impact struck Mexico, and wiped out dinosaurs and ≥ 95% of everything else on the planet. Morocco gives us a picture of a marine ecosystem just before the asteroid hit, and there’s an incredible array of species there, with more being found all the time.
It’s been suggested that marine ecosystems were stressed before the asteroid hit, and low in diversity. This, the story goes, made them vulnerable to disruption. But that’s not at all what we see.
Diversity was as high as it had ever been in the marine ecosystem. There were a huge range of mosasaurs; there are more mosasaurs in the late Maastrichtian than at any time in their history. There were also plesiosaurs, giant marine turtles, and a remarkable diversity of bony fish (like the huge, saber-toothed Enchodus) and sharks. All these animals- mosasaurs, plesiosaurs, big fish- lie at the top of the food chain- so their diversity implies a high diversity and abundance of prey items, things like small fish, squid, ammonites, crustaceans, clams, snails, and so on.
It seems unlikely that low diversity made the Late Cretaceous ecosystem vulnerable to extinction.
Which raises a question. Is it possible that instead, that high diversity might have made the ecosystem more vulnerable to extinction, rather than less?
It’s just sort of assumed that high diversity is good, low diversity is bad, so low diversity ecosystems are somehow more vulnerable than high diversity ones. But is that really the case? To play devil’s advocate, could higher diversity and more complex ecosystems actually make things more vulnerable to environmental catastrophes?
After all, ecosystems pack in a lot of species, at least in part, by having them evolve to be more specialized. The Galapagos can support many Darwin’s finch species because each specializes on different stuff (insects, small seeds, big seeds, cactus flowers) and different habitats (lowlands, upland forest, mangroves), which limits competition between them.
Mosasaurs did the same. They evolved diverse teeth, jaws, bodies and life histories to specialize with different feeding strategies, and different prey- big fish, small fish, mollusks, marine reptiles. They probably specialized in where they fed and lived in space- the upper water column or the ocean depths, coastal shelves or open ocean, tropics or polar latitudes, soforth.
So evolving more species is probably possible because of specialization.
Mosasaurs had a huge range of tooth sizes and shapes, designed to crush, cut, and pierce prey, suggesting a diversity of foods and feeding strategies.
But in specializing, species become dependent on a narrower range of foods and habitats. So specialization makes them vulnerable to a disruption of the food chain, or destruction of habitats.
To the extent you rely on one kind of prey, if it becomes rare or goes extinct, you’re vulnerable. When you specialize in exploiting one small region of the world, or one habitat type, if it becomes hostile, you’re at risk. Generalists may compete less effectively for any given prey item or habitat, but they can switch between foods if one prey item is rare, or move to a different area if food becomes rare in one habitat . That’s probably why a lot of things that survived the asteroid impact- many mammals, alligators, turtles, fish- seem to be fairly generalized, omnivorous species, not hyper-specialists.
The implication here is that to the extent high diversity is created by specialization, highly diverse ecosystems could be more vulnerable to catastrophic mass extinction.
And highly diverse groups of animals might also suffer high rates of extinction. It’s striking that many groups hit hard by the asteroid impact were diverse- mosasaurs, dinosaurs, birds, pterosaurs, mammals, flowers, ammonites. Having lots of species didn’t help much. Groups that seem to have been less affected- alligators, alligator turtles and softshell turtles, champsosaurs, gars, bowfin, paddlefish, sturgeon, clams, conifers, ginkgo- were far less diverse, with few species. Moreover, diverse groups like dinosaurs and mosasaurs, mammals, ammonites and flowers seem to turn over rapidly, quickly evolving new genera and species as old ones go extinct. Evolution of groups like alligators, conifers and ginkgos seems to be far more conservative. High diversity may be associated with high rates of turnover and extinction, especially during catastrophes.
Last, during the asteroid impact, at least in marine ecosystems, it’s higher latitudes- which are less diverse- that seem to be least affected.
None of this is to argue that mosasaurs or tyrannosaurs would necessarily have survived if they had fewer and more generalist species. Sitting at the top of the food chain, with high food requirements, it may not have mattered how many species they had. Whether there was one species of Mosasaurus or a dozen, the genus probably wasn’t going to make it. My point is just that diversity by itself might not protect ecosystems and groups of animals, and might even hurt. The idea that a decline in species-level richness contributed to the end-Cretaceous extinction seems plausible, but it might be wrong.
It’s possible, for example, that if mammals had fewer species and more generalists, they might have suffered lower extinction rates. And there’s little reason to think if there were two or three times as many species in the Maastrichtian, the extinction would have been less severe; we might actually have seen a higher percentage of species go extinct. That same specialization that in the short term promotes survival might leave organisms vulnerable to a sudden change in the long term.
You can find the paper describing Xenodens here: Xenodens calminechari gen. et sp. nov., a bizarre mosasaurid (Mosasauridae, Squamata) with shark-like cutting teeth from the upper Maastrichtian of Morocco, North Africa
References
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Media
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