A new species of Carinodens from the end Cretaceous of Morocco and mosasaur hyperdiversity

Recently my colleagues and I published a third species of Carinodens from Morocco, adding to the already extraordinary diversity of the phosphates. We keep finding new genera, and new species of known genera.

Carinodens belgicus tooth, late Maastrichtian of Morocco

There are lots of strange mosasaurs, but Carinodens is bizarre even by mosasaur standards. It’s adapted to do  something rather different than the other mosasasaurs. For that matter, it’s not like any other marine tetrapod.

Carinodens has short, blunt crowns covered in coarse bumps and ridges, which is typical of durophages— animals adapted to eating hard prey like mollusks and crustaceans- rays, horn sharks, and so on. The teeth are unusual though, in being narrow side-to-side and packed into a long row, so that the crushing surface is very long and narrow.

This is rather a strange way to build a durophage.

Another odd feature of Carinodens is that the jaws are long and slender, which limits the bite force they could produce. This isunlike, say, Globidens which has massive jaws that can generate high bite forces. Carinodens was crushing something, something hard but apparently fairly fragile- maybe small thin-shelled ammonites like Hoplitoceras, or crustaceans?

Up until now there have been three known species of Carinodens— C. belgicus, from Belgium, C. palistinicus, from Jordan, and C. minalmamar, from Morocco. Good skull material isn’t known for any of these animals, unfortunately— there’s not a single complete, well-preserved skull known.

But the distinctive teeth of the Carinodens sort are found around the world in the Maastrichtian, in Europe, the East Coast of North America, Africa, and South America. It was clearly widespread and successful.

Holotype left maxilla and right dentary (reversed) of Carinodens acrodon

Now a fourth species has emerged from the late Maastrichtian phosphates of Morocco, which we recently described and named as Carinodens acrodon. The most obvious difference between C. acrodon and the other species are the teeth. Where other Carinodens have very low, rectangular teeth, the teeth of C. acrodon are taller, more triangular, shaped a bit like a Hershey’s kiss.

A number of other subtle differences in the dentary show that it’s distinct from C. belgicus, or C. minalmamar. So we may have as many as three different species coexisting in Morocco at the same time — C. minalmamar, C. belgicus, and C. acrodon (it’s a bit complicated by the fact that some of the Moroccan jaws originally referred to C. belgicus may represent adult C. minalmamar, based on similarities of the teeth) but isolated teeth from the phosphates hint that C. belgicus is there). 

The diagnostic utility of mosasaur teeth is, well, a complicated problem.

My colleague Nathalie Bardet thinks they’re potentially diagnostic down to species level; I feel the variability (ontogenetic, individual, and along the jaw) makes this complicated and that they’re useful to fairly low taxonomic level, often to genus level, but only very rarely to species. I think you’re on much firmer ground by looking for characters in other elements- jaws, frontals, parietals, etc. Fortunately, our new taxon has a good dentary, so we can compare not just tooth shape but features of the jaw, and it’s pretty clearly a distinct beast. So, up to three species of Carinodens in one place and one time.

This is yet another example of the hyperdiversity we seen in late Maastrichtian mosasaurs. Why have one species, when you can have three? Mosasaurs are like that, they’ve never seen a niche they don’t want to carve into multiple niches.  Mosasaurus, for example, seems to be represented by three or more taxa in the late Maastrichtian- Mosasaurus hoffmanni in the Netherlands, M. beaugei in Morocco, and M. maximus in New Jersey.

There are different approaches to dealing with the diversity of fossils. Generally speaking, paleontologists can be “lumpers” who lump many fossils together into a few species, or “splitters” who tend to split them apart. This is often seen as a sort of philosophical difference.

I think these aren’t arbitrary differences in that one approach is going to be right and one is wrong- either these morphologies represent animals that interbred, or animals that didn’t. It’s clear that in certain cases, paleontologists have gone too far with splitting (something like a dozen Triceratops species have been named, which seems implausible) and in other cases too far with lumping (T. rex, long treated as one species, is clearly multiple species, but just how many is going to take a long time to figure out).

In general, though, I suspect we’ve been too conservative.

In the case of mosasaurs, there is just a lot more variability in these things than you expect in species: we end up with a lot of species because they were really very diverse, not just because we like naming species (although it is fun.

Mosasaurs were splitters.

This should not be surprising given that modern squamates are highly diverse, with around 12,000 species. If you prefer to use modern whales as an analogue, there are today over 100 cetacean species,. So it shouldn’t be surprising if mosasaurs had comparable diversity. Speciation is just what evolution does— species tend to make more species; extinction wipes them out, they speciate again.

And the differences between species are often quite subtle- try telling a crow from a raven at a distance. But the key thing in taxonomy isn’t whether differences are subtle, but whether they’re consistent. The differences in size, beak shape, wing shape, tail shape, feathering etc. between crows and ravens are all subtle but consistent. In fossils, unfortunately, many useful features like plumage or scalation patterns, behavior, etc. just don’t preserve. We also tend to have small sample sizes, which makes comparing variation in populations difficult.

Another curious phenomenon we see in the Maastrichtian is that we see a very primitive species (Carinodens acrodon is more primitive in terms of the tooth shape than any Carinodens yet)— coexisting alongside derived ones. Rather than replacing the primitive species, the new, derived lineages appear alongside it and coexist with it. The new species are (presumably) doing something different ecologically- niche partitioning- so over time, the appearance of these advanced lineages alongside primitive ones tends to drive diversity up. 

We see similar examples of this elsewhere in nature- humans coexist in Africa alongside far more primitive chimpanzees, and monkeys. T. rex coexisted in the Hell Creek alongside the far more primitive Nanotyrannus. Birds coexisted with their dinosaur relatives. The advanced forms don’t always replace the primitive one, instead they fork. One species moves starts to move into a new niche and becomes more and more derived, while the other hangs out in the old niche, not evolving very quickly. So in the Maastrichtian we see advanced mosasaurs alongside much more primitive morphologies. Some lineages stay in their existing niches for millions of years, subpopulations branch out into new niches, figuring out ways to pack more diversity into the same ecosystem.

Carinodens palestinicus, showing the “butterfly” shape of the first metacarpal and phalanges of the first digit

Plotosaurus bennisoni, showing “butterfly” shape of the first metacarpal and phalanges of the first digit

The origins of Carinodens are unclear. Traditionally it was allied with Globidens, but I think the similarities are superficial and probably convergent, they have to do with specialization of the teeth to crush hard prey. All durophages have blunt teeth.

I think postcrania may be more conservative and reliable than tooth shape. The only Carinodens with postcrania, C. palistinicus, has flukes that are similar to Mosasaurus— lots of extra joints in the fingers, and the bones of the first digit have little processes sticking off to give them a sort of butterfly shape. So Carinodens seems to allied with the derived mosasaurs, Mosasaurini, but otherwise its relationships are unclear.

Another issue is the relationship of Carinodens to the bizarre Xenodens.

In drafting the original Xenodens paper, we considered the possibility that Xenodens might be a juvenile Carinodens. In the end, we decided they were probably different— but the absence of a good Carinodens maxilla made comparisons difficult.

Carinodens acrodon (top) versus Xenodens calminechari (below)

Now we have a Carinodens maxilla, it’s clear these are very different animals— Carinodens has fewer, much larger teeth, and a much higher degree of heterodonty. Carinodens lacks the highly derived tooth implantation and elongate replacement pits seen in Xenodens; the overall shape of the maxilla is more elongate.  These are very different beasts.  Which raises the question- is Xenodens even related to Carinodens at all? Honestly I’m not sure.

Xenodens has very low, broad, laterally compressed teeth, similar to Carinodens, so it seemed logical to think that they were related; they’re both relatively small mosasaurs as well. It’s a simple, logical hypothesis.

But that doesn’t mean its right.

The new, primitive Carinodens actually looks more different from Xenodens in the teeth, than the derived C. belgicus and C. minalmamar do. That’s a hint that maybe the strange teeth of Xenodens are convergent on those of Carinodens? Who knows.

On the other hand, both Xenodens and Carinodens seem to have similar tooth implantation, and the teeth have short pedicels compared to other mosasaurines. So for now, I’m continuing to place Xenodens with Carinodens, but only tentatively, and only because I just have no idea what else to do with it. More material of Xenodens has emerged (some, unfortunately, in private collections) but like with Carinodens, we so far haven’t found any good skull material. Sooner or later (at this rate, probably later) more material will turn up and hopefully shed light on these problems.

Hyperdiversity

At the risk of being repetitive (since I write articles about this every few months it seems) Morocco has extraordinarily high mosasaur diversity.

We’ve now described most of the common species, probably- Thalassotitan, Eremiasaurus, Halisaurus arambourgi, Mosasaurus beaugei, Gavialimimus. These six species probably make up 90% of the mosasaurs you see. Now we’re working on the rarer stuff— Khinjaria, Hainosaurus, Stelladens, Xenodens, new Carinodens species. We are nowhere even close to describing all the mosasaurs; new ones keep emerging. This is actually pretty typical of how species are distributed in an ecosystem— in any given ecosystem, a few species dominate. Most are rare. So as you sample more and more, you find more species.

Why so many species? We may be sampling a biodiversity hotspot, in space and time. Environmental conditions- upwelling of bottom waters, full of phosphate and other minerals to fuel plankton growth- would have fertilized the seas and created high productivity. We’re also at low latitudes where there’s intense sun, year round. Between these, food was incredibly abundant, year-round.

Then there’s time. By the end of the Cretaceous mosasaurs had spent 25 million years radiating. Sampling the youngest mosasaurs, we’re sampling the fauna that’s had the most opportunity to diversify and move into new niches. The Turonian faunas, about 25 million years earlier, are far lower in diversity, with only a few species, perhaps because these species lived just after a mass extinction at the end of the Cenomanian, the Cenomanian-Turonian Boundary event that’s associated with OAE2. They hadn’t had time to move into all these niches (the fish, cephalopods and crustaceans they fed on were probably lower in diversity after the extinction as well, leaving fewer niches available).

But there’s also a possibility that there’s just something different about mosasaurs themselves— that they were good at specializing and speciating, for reasons unknown. We see this in certain modern clades- snakes among lizards, for example, are about 4,000 species, far and away the most diverse squamate group. In other clades, we see subclades- percomorphs among bony fish, passerines among birds, beetles among insects- that are hyper-diverse, and comprise many or most of the species.

Mosasaurs seem to be similar. They’re just good at evolving species. They’re master speciators, supremely effective at exploring niches.

What drives this, I don’t know.

It’s possible that the highly specialized dentition of mosasaurs- which seem to have evolved a much wider range of shapes than plesiosaurs and ichthyosaurs ever did- helped them speciate. Or is it that some other adaptation allowed them to move into lots of niches, and forced dental diversity to increase?

Even within mosasaurs, some groups were far more diverse; the vast majority of Maastrichtian mosasaur diversity- including Carinodens, Xenodens, Stelladens- is in the Mosasaurinae. Mosasaurs were highly speciose, and within them, the Mosasaurinae are the most speciose subclade. Something about them gave them an edge, and made them more diverse and abundant than other mosasaurs.

Trying to describe all this diversity is a daunting task; we’re still trying to figure out just how many species we have, and write them up. Some days I despair of ever getting through them all (to say nothing of the diversity of elasmosaurs, marine turtles, fish, and dinosaurs). So I’m trying to be ambitious and describe a couple new species per year. It’s a bit addictive in a way— mosasaurs are a bit like Pokemon, you gotta catch em all.

Species by species, paper by paper, we’re slowly trying to remove the veil of mystery, and shed light on this phenomenally diverse ecosystem from just before the demise of the mosasaurs, dinosaurs, and everything else at the end of the Cretaceous.

Why? Well, if I knew exactly what we were going to find, it wouldn’t be discovery. But there’s more— much, much more— to come.

One down, on to the next.