Why are palaeontologists so interested in mosasaur teeth?

Mosasaurs have a special place in my heart. I worked on them for my Master’s degree, but I also re-visited them as a PhD student. This post explores how mosasaur teeth became some of the most thoroughly studied among any reptile, and how the findings from the debates surrounding them have inadvertently re-shaped our understanding of dental evolution. This is also a story about the origin of my PhD thesis!

I did my Masters degree at the University of Alberta on mosasaurs: giant marine lizards from the Late Cretaceous. They were formidable animals. Some were up to 15 meters long, had paddles for limbs, and a mouth lined with big teeth on the jawbones AND on the roof of the mouth (on the pterygoid bones). They were definitely at the top of the marine food chain during the last 20 million years or so of the Cretaceous period while the dinosaurs were stomping around on land.

Mosasaurs

A collection of mosasaur skulls I’ve seen during my days as a mosasaur specialist. Top right: Mosasaurus from the Royal Tyrrell Museum in Drumheller, Alberta; middle: an adult and juvenile specimens of Plotosaurus from the natural history museum collections at Berkeley; bottom: Prognathodon from the Royal Tyrrell Museum in Drumheller.

 

Clidastes skeletons

An illustration of a mosasaur skeleton, showing the typical body plan of a mosasaur.

Mosasaurs were not dinosaurs, however. They were true lizards. By that I mean that their family tree was just a branch on the larger lizard tree of life. But where exactly the mosasaur branch fits relative to the other major lizard groups has been an incredibly polarizing issue. Some think that mosasaurs are most closely related to monitor lizards (things like the Komodo dragon), but others contend that mosasaurs are actually marine relatives of snakes (and from this came an idea that the common ancestor of snakes and mosasaurs came from the water)! You might be wondering why I would bring this up in a blog called ToothPics, but it turns out that one of the most important “battlefields” where palaeontologists have been debating the evolutionary relationships of mosasaurs actually concerns their teeth!

But don’t take my word for it. Here’s a quick list of scientific studies that have looked closely at the teeth of mosasaurs since 1997:

(1) Lee (1997): The phylogeny of varanoid lizards and the affinities of snakes.

(2) Lee (1997): On snake-like dentition in mosasaurian lizards.

(3) Zaher and Rieppel (1999): Tooth implantation and replacement in squamates, with special reference to mosasaur lizards and snakes.

(4) Caldwell, Budney, and Lamoureux (2003): Histology of tooth attachment tissues in the Late Cretaceous mosasaurid Platecarpus.

(5) Rieppel and Kearney (2005): Tooth replacement in the Late Cretaceous mosasaur Clidastes.

(6) Budney, Caldwell, and Albino (2006): Tooth histology in the Cretaceous snake Dinilysia, with a review of amniote dental attachment tissues.

(7) Kearney and Rieppel (2006): An investigation into the occurrence of plicidentine in the teeth of squamate reptiles.

(8) Caldwell (2007): Ontogeny, anatomy and attachment of the dentition in mosasaurs (Mosasauridae: Squamata).

(9) Luan and colleagues (2009): The mosasaur tooth attachment apparatus as paradigm for the evolution of the gnathostome periodontium.

(10) Chinsamy, Tunoglu, and Thomas (2012): Dental microsctructure and geochemistry of Mosasaurus hoffmanni (Squamata: Mosasauridae) from the Late Cretaceous of Turkey.

(11) Liu and colleagues (2016): Varanoid tooth eruption and implantation modes in a Late Cretaceous mosasaur.

(12) LeBlanc, Lamoureux, and Caldwell (2017): Mosasaurs and snakes have a periodontal ligament: timing and extent of calcification, not tissue complexity, determines tooth attachment mode in reptiles.

 

So why are there so many studies on mosasaur teeth? Some of the titles of these papers make some bold claims (my personal favourite is the title for #9) and it boils down to two points about their teeth that prompted palaeontologists to really duke it out.

 

Mosasaur tooth replacement: more like snakes or monitor lizards?

As I’ve mentioned before, most reptiles continually replace their teeth throughout their lives, and mosasaurs were no exception. Some skeletons of these critters preserve two or more generations of teeth being formed at a single tooth position. Now multiply that by about 60 total tooth positions in their mouth (not including that second tooth row on the pterygoid bones) and you have…a lot of teeth!

Snakes and monitor lizards also replace their teeth continually, but the ways in which the replacement teeth form are very different. In monitor lizards (and pretty much every other kind of lizard, minus snakes), the teeth form upright and gradually migrate to eventually replace the old ones.

liu et al heloderma tooth replacement

Tooth replacement in the lizard Heloderma, a close relative of the monitor lizards [modified from two figures in Liu et al. (2016)]. The developing replacement teeth form vertically, a feature common to most lizards.

On the other hand, snake teeth develop nearly horizontally; migrate towards the tooth they are going to replace, and then somehow rotate into a more vertical position before they erupt into the mouth.

 

liu et al snake tooth replacement

Tooth replacement in a snake, in this case a python [modified from a figure in Liu et al. (2016)]. Snakes are unique in how the developing teeth (red arrows) form in a reclined position, then gradually rotate forward as they come into their final positions in the mouth.

With such an obvious difference between these two kinds of tooth replacement, many scientists have pondered if mosasaurs replaced their teeth more like monitor lizards or snakes, the results of which might tell us something about their relationships to these two lizard lineages.

And that’s exactly what palaeontologists have been doing over the last 20 years. One of the first was Michael Lee, who in 1997 reported that mosasaurs replaced their teeth more like snakes, with reclined replacement teeth that eventually rotated into position. According to Lee, this was one of the key features that united mosasaurs with snakes!

 

mosasaurus tooth replacement

Closeups of a Mosasaurus jaw I saw in Brussels. The reclined replacement teeth (red arrows) are similar to the ones Lee reported in his 1997 studies. Does this mean mosasaurs replaced their teeth like snakes??

However, 1997 was not the end of the story. Lee’s work was followed by two responses: one by Hussam Zaher and Olivier Rieppel in 1999, and another by Olivier Rieppel and Maureen Kearney in 2005. These studies took exception to Lee’s link between mosasaur and snake tooth replacement and showed something completely different:

Mosasaur tooth replacement

The responses by Zaher, Rieppel, and Kearney pointed out that when the replacement teeth are well preserved in mosasaur jaws, the teeth are held in a vertical position, like this one in a mosasaur jaw from Brussels. If that’s the case, then mosasaurs don’t replace their teeth in a similar way to snakes.

These two responses didn’t agree with Lee’s idea that the teeth of mosasaurs developed horizontally and gradually rotated vertically as they erupted into the mouth. They noticed that when the teeth were lying down in the jaws of fossil mosasaurs, they looked like they had fallen over after the animal had died and the soft tissues decayed- a phenomenon that had occurred post-mortem.

Moreover, when they looked at fossil mosasaur jaws that were better preserved, the teeth were sitting vertically in the jaws, like those of a monitor lizard. This was later corroborated by Michael Caldwell in a 2007 study of some more mosasaur fossils. When taken together, this alternative explanation suggests that mosasaur teeth developed more like those of other lizards than those of snakes.

Postmortem tooth replacement.jpg

Here’s an upper jaw of a mosasaur (upside-down for the picture) where you can see the REAL (left two arrows) and ALTERED (right two arrows) orientations of the replacement teeth. The one on the far right even rotated all the way around and is now backwards! All of that motion happens post-mortem, when the soft tissue surrounding the developing teeth decays.

It’s important to make a distinction though. The answer to the mosasaur tooth replacement question can only tell us if mosasaurs replaced their teeth more like snakes or not. The alternative explanation, the one I think is the most plausible, only reveals that mosasaur teeth developed similar to those of other lizards, including monitors. This is called a shared ancestral feature (a symplesiomorphy), and does not reveal that mosasaurs replaced their teeth like monitor lizards specifically. There are some similarities between them for sure (some of which I’m not covering here…and there are also many differences I have not covered), but the angle of the replacement teeth is a one-sided question. Even if Lee’s 1997 work on mosasaur tooth replacement may have been challenged by new data, it doesn’t mean that mosasaurs are automatically more closely related to monitor lizards.

It’s also important to note that the debate over this has carried on for nearly 20 years following his work, so even if it seems like a cut-and-dry, problem-solved type question, this serves as a reminder of how science works: by testing and re-testing hypotheses until the strongest one is left standing. That being said, I think it’s now safe to say that mosasaur teeth didn’t form in the same weird way as they do in snakes.

 

Mosasaur tooth attachment: shifting a century-long paradigm and kick-starting my thesis 

Lee’s 1997 study made another claim: that mosasaur teeth were held in tooth sockets, a condition called thecodonty. Thecodonty is a term usually reserved for the teeth of mammals and crocodilians, where the teeth are deeply implanted into discrete sockets along the jaws. Nearly all other toothed animals are supposed to lack thecodonty, but according to Lee, mosasaurs and snakes actually have discrete sockets, a feature they supposedly share to the exclusion of other lizards. Chalk another one up for the snake-mosasaur link, right?

Mosasaur and snake tooth sockets

Snakes and mosasaurs have teeth with spongy bases that are usually fused to the jaws. In Lee’s view, the depressions in which the teeth sit in snakes and mosasaurs are true tooth sockets and so we should call these thecodont.

While Lee was focused on the relationships of a particular group of lizards, this seemingly simple claim has had a profound effect on how we look at tooth evolution. In order to address this, it’s important to remember what’s implied when Lee argued that mosasaur teeth were thecodont, like those of mammals and crocs. The teeth of mammals and crocs aren’t just slotted into holes in the jaws, but are suspended in bony sockets that are actually formed by the tooth itself! There’s also a complex suite of tissues that hold the tooth in place. For the longest time we assumed that the complex dental tissues of mammals and crocodilians was unique to them and must have evolved at separate times in these two groups (seeing as how a crocodile and a mammal are separated by over 300 million years of evolutionary family history). But what did they evolve from? Looking at mosasaur teeth from Lee’s perspective spawned a new realm of study in palaeontology: one that involves histology (the study of tissues), tooth anatomy, developmental biology, and evolutionary biology. And many people that wanted to hold on to the conventional wisdom of late 19th century comparative anatomists did not like some of the answers…

Enter the debate over mosasaur dental tissues and the start of my PhD thesis.

The question is deceptively simple, but the answer is remarkably complex: should we call the stuff that anchors a mosasaur or a snake tooth to the jaws the same stuff that anchors a mammal’s tooth to its socket? If we should, then that means that the complex tooth tissues are not unique to mammals and crocs, but could be ancient (which is exactly what many subsequent studies have claimed). If we shouldn’t, then we need to give a separate name to the bony mass of stuff that fuses a lizard’s tooth to the jaws (this was the prevailing view since the late 1800’s) and thus it is fundamentally different from the tooth anchoring tissues in mammals and crocs.

Eremiasaurus tooth attachment

Scientists and dentists alike have no problem recognizing the crown tissues enamel and dentine in mosasaur teeth. Where it gets confusing is when people slap a name onto the spongy base of a mosasaur tooth. Bone pedestal? Bone of attachment? Osteocementum? Mineralized periodontal ligament? These are all terms you’ll find in the scientific literature that describe the tooth base in a mosasaur and they all imply different evolutionary and developmental scenarios.

What followed was a series of studies that sat on either side of the proverbial line in the sand. Were the tooth attachment tissues in mammals and crocs truly unique, or could you find them in other animals, like mosasaurs? The question was visited many, many times by palaeontologists and developmental biologists who cut up fossil teeth and jawbones and looked at the microscopic details of how the teeth were attached to the jaws.

This question also started me on my toothy quest for knowledge in 2011 when I started my PhD. In fact, at its core, my entire PhD thesis was based on this question, exploring tooth attachment and tooth development in all kinds of extinct animals, spanning 300 million years of evolutionary history!

mosasaur vs croc tooth tissues

With one word, Lee’s 1997 study scoffed at convention wisdom regarding the identity of reptile tooth tissues and started a polarizing debate about what to call the spongy stuff that fuses mosasaur teeth to the jaws. Could you equate it to any of the tissues in mammal and croc teeth? It’s a question that gets revisited again and again in vertebrate palaeontology.

Coincidentally, one of the breakthroughs for my PhD thesis was a study by my Master’s supervisor, Mike Caldwell and colleagues Lisa Budney, and Denis Lamoureux in 2003 (see the list of papers at the start of the post). In it, they argued that they could identify the same three tissues that anchor the teeth to the jaws in a mosasaur (cementum, periodontal ligament, and alveolar/socket bone) as he could in mammals and crocs. They did this by cutting open a mosasaur jaw and looking at thin sections of the tooth root and the socket. This spawned a series of responses (and responses to responses) by other palaeontologists for several years afterwards, but it started me on my quest to see if I could find those same tissues in thin sections of other kinds of reptiles and maybe even in the precursors to mammals, the synapsids! But more on that in later posts…

Like I said in the beginning, I have a soft spot for mosasaurs. They’ve shaped the direction I decided to take for my academic career, even today. The study of mosasaur teeth started as a potential tool to decipher the evolutionary relationships of these giant marine lizards, but it has since expanded beyond this question. This is particularly true for the study of tooth tissues. The initial debate has given way to a whole new set of questions, with researchers like myself pondering the inner workings of the teeth of dinosaurs, early mammals and their kin, early reptiles, and other extinct critters. I even had a chance to revisit mosasaur tooth tissues back in 2017 and contribute my own thoughts to “the big debate”. I will probably link back to this post in future entries, just to remind readers (and myself) just how important mosasaurs have been to the larger, tooth-related questions we now pursue.

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