Biting into the Past

If you’re a mammal – and I suspect you are – your teeth are arguably the most important part of your body, at least to a palaeontologist like me.

You see, teeth are the hardest tissues in the body, and as a result often the only record we have of extinct mammals. Luckily, we can use mammal teeth to identify the species to which they belong.

Fossil teeth also provide a wealth of biological information that we can use to understand things about extinct mammals that we otherwise can’t observe (at least not without a time machine).

We start with what we’ve learned from studying the teeth of living mammals.

Mammals, of course, have a variety of diets. As a simple example, tigers eat deer, while horses eat grass. Mammal teeth thus come in a variety of characteristic shapes because they provide a variety of functions, including pulverizing, slicing, and grinding.

A) The varied shapes of the upper dentition (right) of a honey badger’s teeth (Mellivora capensis) reflect its generalist diet (Catalogue number USNM 175751); (B) The fairly uniform teeth (left) of a modern beaver (Castor canadensis) are similar to its extinct Pleistocene relative (right) indicating that this ancient animal also had an herbivorous diet. (Catalogue number: CMNFV 16407); (C) The robust, peg-like teeth (right) of the herbivorous Megalonyx jeffersoni, a Pleistocene giant ground sloth (left), enabled it to crush a diet of leaves, twigs, and perhaps also nuts (Catalogue number CMNFV 31778). Images: A) Left: public domain; Right: Danielle Fraser, © Canadian Museum of Nature. B) Left: © Steve Hersey (CC BY-SA 2.0); Right: Danielle Fraser, © Canadian Museum of Nature. C) Left: public domain; Right: Danielle Fraser, © Canadian Museum of Nature.

The fact that mammal teeth have characteristic shapes related to their function is lucky for palaeontologists because it enables us to immediately understand some very basic things about the biology of extinct mammals from their teeth, for example whether an ancient animal was an herbivore or carnivore.

However, it’s not always so clear cut.

The polar bear (Ursus maritimus) and raccoon (Procyon lotor) have very similar teeth but very different diets. Polar bears eat seals while raccoons are omnivores. So, how do we distinguish between species with relatively similar chompers?

Colourized, 3D surface scans of a polar bear tooth (lower left, first molar) and a raccoon tooth (lower right, first molar). Both teeth show a similar basic topography of peaks and valleys. These specimens are part of the collections of the Finnish Museum of Natural History. Scan data provided by Dr. Alistair Evans and Dr. Silvia Pineda-Munoz. Catalogue numbers HELU201 (polar bear) and HEL885 (raccoon). Image: Danielle Fraser, © Canadian Museum of Nature.

One way is to use high-tech, 3D imaging methods that enable us to identify minute differences in tooth shape that correspond to seal skewering, or in the case of urban raccoons, a diet of garbage.

This video shows a 3D rotating micro-CT scan of the right lower first molar of Parictis parvus, an early bear that lived around 38 million years ago. Scans such as these permit a detailed examination of the three-dimensional shape of fossil teeth. This specimen comes from the late Eocene Calf Creek Fauna of the Cypress Hills in Saskatchewan and is housed at the Royal Saskatchewan Museum. The scan data was captured at Carleton University by Dr. Fred Gaidies of the Department of Earth Sciences. Catalogue number RSM P661.1701. Image: Danielle Fraser, © Canadian Museum of Nature.

Teeth also record the chemical composition of what an animal eats and drinks during its life.

The chemical formula for water is H₂O; a molecule of water has two hydrogen atoms and one oxygen atom. But natural water actually comes in a range of different isotopic varieties. (An isotope is a natural variant of an element, such as oxygen, with an identical number of protons, but varying numbers of neutrons). The ratio of different oxygen isotopes in a body of water depends on many factors, including the water’s temperature and salinity. Thus, water from different sources, such as lakes, rivers or wetlands, have slightly different oxygen isotope compositions.

While teeth are growing, whenever an animal drinks water these oxygen isotope differences are incorporated into its tooth enamel. As a result, we can take samples of tooth enamel and learn a lot about what, and even where, an animal drank.

The parallel rows visible on the pronghorn (Antilocapra americana) molar (left) are areas where enamel was removed for oxygen isotope sampling. Blog author Danielle Fraser (right) samples a fossil tooth using a Dremel tool. Images: Danielle Fraser, © Canadian Museum of Nature (left), Marisa Gilbert, © Canadian Museum of Nature (right).

I hope that with this blog post I’ve given you something to chew on and convinced you that fossil mammal teeth are a lot more exciting than they initially sound — they certainly always give me a big smile when I find them!