Below is a “lab note” from the Jaws of Life Project on experiment.com. Access the note and support the project here.
How do we know what teeth we have when we find a jaw or an isolated tooth?
Let’s start with a simple, yet common case: The lower jaw of a deer or sheep. For those of you that spend much time hiking, deer jaws and bones are fairly easy to find. These jaws can be valuable for a study like this because they are so common and don’t really have a lot of value. People usually want skulls to put on their shelves, not jaw fragments with a couple of teeth.
What would you expect to find?
For most plant-eating animals, you would expect five or six cheek teeth in the jaw. These are the heavy grinding teeth behind the canines. In herbavores like deer and horses, there usually isn’t a canine – or at least not a big one. There is, however, typically a gap between the cheek teeth and the incisors called the diastema. In a mature adult, there should be three molars and two or three premolars. If you count the cheek teeth, and there are fewer than six, it’s usually the third molar that’s missing. It generally hasn’t come in yet, even though the animal may already be able to reproduce.
Start with counting the cheek teeth. Also, check to see if there are any empty tooth sockets, which indicate missing teeth.
It’s often very helpful to start by looking at the teeth from the top. That is, looking down on the grinding, or occlusal, surface like in the photo below.
The jaw above had six teeth, three molars and three premolars. You can also see that the teeth are worn down a little from eating. Unworn (not chewed with) teeth appear bulbous, but these have flat surfaces and sharp edges. This was a mature adult and the tooth on the far left is the third molar.
Molars do not have baby teeth that fall out before they come in. So if you can see that there is a baby tooth, then the tooth you’re looking at is a premolar.
Molars also tend to be more complex than premolars. They have more cusps (bumps) and crests (sharp ridges) than the premolars. If you look at the deer above, you’ll see that the tooth on the far right (a premolar) is less complex than the tooth on the far left (a molar).
Here is the same teeth as the deer above, but from a fossil horse. This is a photograph of a cast. No fossils were harmed in the taking of this photograph.
The Mesohippus and the deer have the ‘W’-shape on each tooth in common. That is, there are two connected V-shaped crests on all the premolars and the first and second molars. These crests help cut up and grint the plants they eat.
If you count the crests, you’ll notice that the third molar has three connected V-shaped crests. This is another common characteristic of the third molar. It’s extra long.
The elk third molar in the photo above shows the three big V-shaped crests on the third molar. The V-shaped crests in the modern horse are a little less obvious then they are in deer and elk, which reflects their different dietary preferences, but there is no giant lobe (extra crest) on either of the molars already present in this jaw. (There’s a little knob on the back of the molars, but that will wear away pretty quickly once the horse’s other molars come in fully.)
But what if you don’t find the whole jaw. What if all you find is this?
The great thing about mammals is that we can often identify the species of the animal from an isolated tooth. This scenario is even better because we get two teeth!
If you look at the occlusal surface of the teeth, you immediately see that the tooth closer to the mouth as two V-shaped crests whereas the furthest back tooth has two big V-shaped crests and a pronounced almost-V-shaped lobe at the back. This is the third molar.
This jaw, with only two teeth, and be used for chemical studies using current thinking because it has a very lovely third molar to work with. This jaw is also helpful for this project because we can compare what we learn from the third molar with what we measure from the second molar.
Looking for that third lobe can help us when all we have is a pile of isolated teeth (which is unfortunately a fairly common scenario). It can be very challenging, especially in horses, to take an isolated tooth and know what tooth position it is, with the third molar being a notable exception.
This is great if we limit ourselves to third molars, but there aren’t always third molars present. And that’s why we need to systematically compare the results and interpretations based on analysis of the third molar with those from the other teeth. One way or another, we need to know if we can use other teeth and get similar results especially for those situations when third molars simply aren’t present.