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Book Review

A look into the past…and into a gnarly set of teeth

Kear BP, Larsson D, Lindgren J, Kundra ́t M (2017) Exceptionally prolonged tooth formation in elasmosaurid plesiosaurians. PLoS ONE 12(2): e0172759. doi:10.1371/journal.pone.0172759

An anatomically incorrect sketch of an elasmosaurian plesiosaur. Sketched by Charles Robert Knight, 1897.

An anatomically incorrect sketch of an elasmosaurian plesiosaur. Sketched by Charles Robert Knight, 1897.

You’ve fallen through a time portal and find yourself in the Mesozoic era, 70 million years ago. If you’re fortunate enough to have landed near an ocean, you might be tempted to go for a swim. This would not be advisable. These waters aren’t only home to sharks, you see. Aquatic life at this point in time was dominated by ichthyosaurs, mosasaurs, and plesiosaurs—large reptiles with very large teeth.

Plesiosaurs, in particular, have captured our imaginations (see the Loch Ness Monster and Lake Champlain’s “Champ”) since they were first described in the early 1820s. Some short-necked plesiosaurs, like Liopleurodon, were truly terrifying apex predators, but other long-necked species (elasmosaurians) were equally proficient hunters that managed to capture and digest everything from clams and ammonites (early shelled cephalopods) to small bony fish.

Figure 1: Fossilized skulls of elasmosaurian plesiosaurs. Their heads were almost abnormally small compared to their bodies. Taken from Kear et al. (2017).

Figure 1: Fossilized skulls of elasmosaurian plesiosaurs. Their heads were almost abnormally small compared to their bodies. Taken from Kear et al. (2017).

We humans have evolved to be flexible with our diets and often take for granted the fact we mix meat with vegetables and grains. The mixed diet of plesiosaurs perplexed scientists because such a varied diet would have required flexibility in predation styles. Throughout the decades, multiple hypotheses about feeding strategies have been whittled down with morphological structural modeling, a technique that approximates the functional limits of a body part based on its form/shape. The common thread among all the ideas, though, was that the teeth had something to do with it. But just how did the teeth dictate diet?

This was the cue for Benjamin Kear and his team from Uppsala University in Sweden to enter the picture. He and his team set about finding fossilized plesiosaur jaws that could be used for in-depth analysis—certainly not an easy task. Ultimately, his team used a subsample of 131 individual teeth to reconstruct a dental array. The original fossils, known as Scanisaurus (Fig. 1), had anisodont teeth, meaning all teeth had the same shape but were of different sizes and grew in unequally dense clusters depending on the jaw region. Like other plesiosaurs, this species was polyphydont (Fig. 2), meaning it continually replaced teeth that were damaged or broken (similar to modern sharks). What if there were something unique to plesiosaurs’ tooth replacement mechanism that allowed or even forced them to have such a varied diet?

Figure 2: Depiction of tooth replacement in an elasmosaurian plesiosaur. A. A small replacement beings forming in a separate pocket, or bony crypt. B. The mature tooth’s base is partially reabsorbed as the replacement tooth slides into place. C. Final maturation of the replacement tooth. Taken from Kear et al (2017).

Figure 2: Depiction of tooth replacement in an elasmosaurian plesiosaur. A. A small replacement beings forming in a separate pocket, or bony crypt. B. The mature tooth’s base is partially reabsorbed as the replacement tooth slides into place. C. Final maturation of the replacement tooth. Taken from Kear et al (2017).

To answer this question, Kear’s team selected specific teeth for mounting and sectioning to analyze their internal structures, while other teeth were subjected to X-ray microanalysis to determine the chemical compositions. Each analysis served to piece together the larger puzzle of how long teeth needed to form and the structural strength of the teeth throughout the cone-like shape.

The results were quite surprising. Teeth, like trees, have rings (Fig. 3K) that show increments of time when growth and chemical deposition happened. By tallying these rings, Kear and his colleagues determined the teeth had an average of 950 rings per tooth. Back calculating based on known rates, this placed plesiosaur tooth formation on a time scale of 2-3 years. Most other polyphydont vertebrates we know of only need 1-2 years to fully grow a replacement tooth! Kear also noted that structurally, a mature tooth would become increasingly fragile as part of its base was reabsorbed by the body to make room for the replacement (see Fig. 2B).

Figure 3: Scanisaurus tooth sections taken from multiple samples. Teeth were mounted in epoxy and sectioned into narrow slices. The white arrows in K highlight the growth rings, also known as the incremental growth lines of von Ebner. Taken from Kear et al. (2017).

Figure 3: Scanisaurus tooth sections taken from multiple samples. Teeth were mounted in epoxy and sectioned into narrow slices. The white arrows in K highlight the growth rings, also known as the incremental growth lines of von Ebner. Taken from Kear et al. (2017).

Given the long growth period for replacement teeth and the fragility of mature teeth, it starts becoming clearer why elasmosaurian plesiosaurs had such a varied diet. Their prey primarily consisted of animals that could either be easily caught, or quickly killed, thereby decreasing the chances of damaging their set of chompers. This shift to prey on organisms in the middle of the food chain may have also given the plesiosaurs access to an abundant nutritional supply, thereby allowing the species to become as prevalent as the fossil record shows today.

The dental details of a plesiosaur might not seem that exciting, but this type of reconstructive work helps morphologists understand what animals could and could not do and provides us with a wealth of information as we try to piece together the history of ecosystems. So, if you did take that unadvisable swim in than Mesozoic ocean and came across a plesiosaur, what other questions would you want to ask?

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