Delving into Sloth Dentitions with Development

There are two basic types of sloths alive today: two-toed and three-toed. Two-toed sloths include two species in the genus Choloepus that live in tropical rain forests from Central America to Bolivia and southern Brazil. Three-toes sloths include four species in the genus Bradypus that inhabit tropical rain forests from Central America through northern Argentina. In many respects, they are very similar to one another; they both spend most of their time hanging upside down from trees eating leaves and relying on their slow movements and camouflage to keep them safe from predators such as eagles and jaguars. Their common names are basically accurate, but I like to refer to them as two-fingered and three-fingered sloths because it is their hands that differ in digit number rather than their feet. (Both types of sloths have three toes on their back legs.)

Fig. 1 Sloths.jpg
The two types of modern tree sloths. Photos by D. Croft. Reuse permitted under CC BY-NC-SA 2.0.

Two- and three-fingered sloths differ from one another in other respects, too. Two-fingered sloths are more omnivorous than three-fingered sloths, consuming fruits and animal matter in addition to leaves – the latter being the sole food source of three-fingered sloths. In three-fingered sloths, algae grow in horizontal cracks in the hair, while in two-fingered sloths, they grow in longitudinal grooves. Studies of the evolutionary relationships of these sloths indicate that they are not closely related one another despite their superficial similarities. Although the precise evolutionary relationships of three-toed sloths remain unclear (see below), it is likely that they diverged from two-fingered sloths at least 25 million years ago. Three-fingered sloths are currently placed in their own family, Bradypodidae, whereas two-fingered sloths belong to the family Megalonychidae, the most widely-distributed sloth family during the Pleistocene Epoch (see also my blog post on Megalonyx).

For paleomammalogists like me, another conspicuous difference between two-fingered and three-fingered sloths are their teeth: two-fingered sloths have a pair of large, pointed, shelf-sharpening teeth at the front of their mouth that resemble the canine teeth of dogs, cats, and a variety of other mammals. Three-toed sloths lack these canine-like teeth, with the result that all of their teeth look pretty much the same. Despite these differences in tooth form, both types of sloths have the same number of teeth in total: five pairs in their upper jaw and four in their lower. Thus, a long-standing question for paleontologists has been whether the very different-looking front teeth in these sloths are evolutionarily equivalent (homologous) to one another. In other words, did they evolve from the same tooth in the common ancestor of all sloths or a different one?

Fig. 2 Sloth Skulls.jpg
Occlusal views of the upper (above) and lower (below) teeth of a two-fingered sloth (Choloepus didactylus, CMNH 18972) and a three-fingered sloth (Bradypus tridactylus, FMNH 21551), front end toward the right, not to same scale. Note that the front upper teeth of the three-fingered sloth are small, peg-like, and close to the rest of the teeth while those of the two-fingered sloth are large, triangular, and separated by a large gap (technically known as a diastema). Photos by D. Croft. Reuse permitted under CC BY-NC-SA 2.0.

To answer this type of question, a paleomammalogist typically refers to the fossil record. If a group of mammals has a good fossil record that samples many species that lived throughout much of the Cenozoic Era (the past 66 million years), it is often possible to trace changes in particular teeth through time to determine which have been lost and how those that remain have changed in form. Unfortunately, sloths do not have such a fossil record. Yes, there are many wonderful sloth fossils from the past 20 million years, but the oldest sloth skull is barely 30 million years old, and it has no more teeth than modern tree sloths. It may even have had one fewer pair. Thus, it and other early sloths are not very useful for trying to figure out which teeth are represented in modern tree sloths. So what’s a paleomammalogist to do?

Study how sloth teeth develop.

This is simple in concept, but difficult in practice. That’s because you can’t just corral a few pregnant sloths and track the teeth of their developing young in utero over a period of months. You have to comb through museum collections to find what specimens you can – a relatively small sample of fetal sloths of different ages – and then CT scan them to try to reconstruct the entire sequence. This invariably leaves some holes in the data but can still result in novel discoveries. A study of this type was published earlier this year, and here is the short version of what they found out…

Fig. 3 Sloth Devo.jpg
Three-dimensional reconstructions of skulls of juvenile three-fingered sloths (Bradypus variegatus), based on CT scans. The developing teeth are highlighted in purple (upper teeth) and green (lower teeth). The small area of red at the front of the skull (toward the left) is a bone called the premaxilla. The authors of this study used the sizes, positions, and timing of appearances (and sometimes disappearances) of these teeth to determine which teeth they represent. From Hautier et al. 2016:fig. 2.

Perhaps most interestingly, the front teeth in the upper jaws of the two types of living sloths do not seem to be evolutionary equivalent (homologous) to one another. In two-fingered sloths, these teeth appear to represent adult (permanent) canine teeth. In three-fingered sloths, they are deciduous canine teeth that are never replaced. This probably accounts for why they look so different in the two types of tree sloths. Moreover, if these teeth really do represent retained deciduous teeth in three-fingered sloths, it lends support to the idea that three-fingered sloths are pedomorphic, the technical term for species that retain juvenile characteristics of their ancestors. Other pedomorphic features that have been noted in these sloths include their very short snouts and certain aspects of their bony ear. This, in turn, could explain why most phylogenetic analyses have found them to be close to the base of the sloth evolutionary radiation; if these and other supposedly ancestral features are actually evolutionary reversals, then three-toed sloths could have diverged much later within another family of sloths.

The remaining teeth appear to be the same in the two types of modern tree sloths. However, only the last upper and lower teeth are probably true molars; the rest – except for the upper teeth in front, of course – are probably premolars. Putting this all together, the dental formula for a three-fingered sloth appears to be:

dC-P2-P3-P4-M1 / p2-p3-p4-m1

In the above formula, the “d” prefix indicates deciduous, c, p, and m represent canine, premolar, and molar, respectively, and upper teeth are in upper case whereas lower teeth are lower case.

The dental formula of a two-fingered sloth appears to be:

C-P2-P3-P4-M1 / p2-p3-p4-m1

Thus, sloths have lost all of their incisor teeth through evolutionary time, as well as the first pair of premolars (P1/p2) and the last two pairs of molars (M2/m2, and M3/m3).

Looking at these dental formulae, it is now clear why the upper and lower canine-like (caniniform) teeth of two-fingered sloths have a different positional relationship to one another than they do in other mammals (see photos below). In mammals with true canine teeth, such as dogs, baboons, and peccaries, the lower canines slide past the front edge of the upper canines when the jaws are closed. In two-fingered sloths, the opposite occurs: the lower caniniform teeth slide along the backsides of the upper ones. We now know the reason for this: only the upper caniniforms are true canine teeth; the lower ones are premolars that simply look like canines. They could technically be described as caniniform premolars. And yes, other types of teeth can also be caniniform. The sharp, canine-like upper teeth of hyraxes, for instance, are caniniform incisors.

Fig. 4 Canines.jpg
Side views of the skulls of a two-fingered sloth (Choloepus didactylus, CMNH 18972), a red fox, (Vulpes vulpes; CMNH 22075), a baboon (Papio anubis; CMNH 21972), and a collard peccary (Pecari tajacu; CMNH 17905). Note how the upper canine-like teeth of the sloth pass in front of the lower canine-like teeth, whereas the reverse is true in other mammals. Specimens not to same scale. Photos by D. Croft. Reuse permitted under CC BY-NC-SA 2.0.

Mammals like xenarthrans that have few, relatively simple teeth pose many challenges for paleontologists. Reducing the number of teeth and simplifying their structure is like losing information about a mammal’s evolutionary relationships and its paleoecology (such as diet). Fortunately, as illustrated here, some of this information isn’t entirely lost; it is just waiting to be uncovered.

References and Further Reading:

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