Tall Tooth Tales

One feature that makes mammals distinct is having two sets of teeth: a first, temporary set that functions while a young animal is growing, and a second, more durable set that lasts the rest of the animal’s life. In placental mammals, the first set includes the incisors in front, the canines, and the premolars in back. These are variously referred to as baby teeth, milk teeth, or deciduous teeth (because they fall out). The second set includes all of these teeth as well as the molars, which erupt behind the premolars. These teeth are referred to as adult or permanent teeth, though they are only permanent if you take good care of them!

Pseudotypotherium carhuense.jpg
Partial right upper jaw of the mesothere notoungulate Pseudotypotherium (MLP 37-III-7-1 and MLP 37-III-7-5), anterior toward the right, illustrating several deciduous premolars (DP2, DP3, and DP4) and two permanent molars (M1 and M2). The last upper molar (M3) has fallen out of this specimen, but part of its socket (alveolus) is preserved behind M2, indicating that it was fully erupted when the animal died. Photo by D. Croft. Reuse permitted under CC BY-NC-SA 2.0.

The timing of eruption of the various permanent teeth varies greatly among different species. This is true even if you just focus on the chewing teeth (premolars and molars). The first molar typically erupts before the rest of these teeth, but what happens after that  runs the gamut. This has been the subject of many papers, because the sequence in which teeth erupt and the timing of tooth eruption relative to adult body size can provide information about both evolutionary relationships and ecology. A principal challenge, of course, is determining which characteristics were inherited from a common ancestor (and therefore provide evidence of relatedness) and which probably evolved for ecological reasons. Right now, scientists are still in the early stages of documenting how different species vary in tooth eruption patterns. Even doing that isn’t simple, as it requires having a large collection of specimens that died at varying ages. This is seldom the case for living species and rarely the case for extinct ones.

Given this, I was struck by a paper published earlier this year on tooth replacement in notoungulates, one of my favorite groups of South American mammals. There are many reasons notoungulates are notable, but their principal claim to fame is that they evolved high-crowned (hypsodont) molars long before ungulates in other parts of the world. What’s more, several groups took this to an extreme, evolving ever-growing molars – an adaptation that is unknown among modern ungulates and virtually unknown among their extinct relatives. (One extinct rhino evolved ever-growing molars.) Such “infinitely tall” teeth are the ultimate solution to dealing with extreme wear caused by eating abrasive vegetation close to the ground. Similar teeth have evolved many times in rodents, but notoungulates are the only group of ungulate-like mammals in which this trait has evolved multiple times (at least four).

Toxodont jaw hypsodonty prep.jpg
Side view of the left lower jaw of a toxodontid notoungulate, anterior to the left. The outer later of bone has been removed so that it is possible to see the full extent of the teeth in their sockets. Note that the molars (the three teeth to the right) extend far into the jaw and that no distinct tooth roots are visible on any of the teeth. This is typical of animals with ever-growing teeth. Specimen is on display in the Museo de La Plata. Photo by D. Croft. Reuse permitted under CC BY-NC-SA 2.0.

The study in question demonstrated that the evolution of ever-growing molars in notoungulates is correlated with a change in timing of when their permanent teeth erupt. In earlier members of the group, the permanent premolars finish erupting before the permanent molars do. In this regard, early notoungulates have something in common with humans: the last molar, dubbed the “wisdom tooth” in us, is the last tooth to erupt. This isn’t the case in the four groups of notoungulates that evolved ever-growing teeth. In these animals, the molars erupt earlier, with the result that some deciduous premolars are still present when the last molar (the wisdom tooth) erupts.

Cladogram.jpg
Diagram of notoungulate evolutionary relationships, calibrated to millions of years ago (along the bottom), showing the four families that evolved ever-growing molars. The red tooth symbol indicates when the family evolved this feature, while the red “E” indicates when the family evolved early-erupting molars (with the last molar – M3 – erupting before the permanent premolars). Modified from Gomes Rodrigues et al. (2017:fig. 3A).

 

How did the authors determine that the molars erupted early, as opposed to the alternative possible explanation: that the premolars erupted late? They relied on an earlier study that looked at skull size in relation to tooth eruption. Since an animal’s skull gets larger with age (until adulthood), its relative size can be used as a proxy for developmental age. In the two groups in which this could be studied, the molars were found to erupt at relatively smaller skull size, as one would expect if they were erupting at a younger age.

So… what does this apparent correlation between ever-growing cheek teeth and early eruption of the molars mean for the paleobiology of notoungulates?

We don’t know yet! Perhaps it has something to do with faster growth, but that is speculative at this point. The first step is documenting the pattern. The second step is figuring out what it means. We are somewhere between the first and second step, so stay turned for updates!

References Cited:

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