Wednesday, 4 November 2015

Go Teeth!

At a conference recently, a well-known palaeontologist made a passionate plea for us to appreciate  teeth more as tools for palaeontological analysis: "go teeth!" he yelled as a conclusion, hoping to leave us rooting for them.

I wasn't shouting anything nearly so positive after a recent visit to the dentist, the place where smiles go to die. Once you've been told of all the horrible things happening in your mouth, it's difficult to muster enthusiasm for those unreliable enamel time-bombs ticking away in your cranium, reminding you of your imperfection and mortality. However, teeth are massively important in the study of vertebrates, particularly mammals with their fabulous heterodont gnashers.

Heterodonty means we possess different kinds of teeth, whereas being homodont (having only one tooth type) is common in many non-mammals. We have two sets of teeth in our lives - a condition known as diphyodonty. Humans have 20 baby teeth (deciduous) and 32 adult teeth (permanent) - children don't have a full set of molars.
This is the nightmare scenario in a child's face before
they replace their deciduous set of teeth. The permanent set
are ready to erupt. Image via Wikipedia: Henry (Vandyke
Carter
, Henry Gray (1918)).

Dental formulas are used as a shorthand for describing how many of each tooth can be found in a mouth. They look like gobbledigook at first, but are quite straightforward once you get the hang of them. The formula lists the number of each type of tooth in one half of the jaw, incisors.canines.premolars.molars.

So humans have the dental formula:

2.1.2.3
2.1.2.3

Above the line is the upper jaw, below the line is the lower jaw. So we have:

2x incisors. 1x canine. 2x premolars. 3x molars (Upper jaw)
2x incisors. 1x canine. 2x premolars. 3x molars (Lower jaw)

So to get the full formula, you have to multiply all that by two = 32.

Dental formulas are for one half of the jaw, so
the picture above is 2.1.2.3 (Image: Wikipedia)
That's all very well in theory. I lay back on the wipe-clean, reclining chair of gob-doom, and the dentist had no sooner looked into my mouth than she crooned like a mating eider duck, tapped on one of my front teeth and exclaimed "oh we have ourselves a baby tooth!"

It turns out that I am a dental mutant*. Not only did my baby canine tooth never fall out, but the beautiful little sharp permanent canine that should have replaced it had moved next door instead, replacing my second incisor. This explained what I've long referred to as my "wonky smile". Both my father and brother have the same strange quirk in their dental arrangement.

So the "Panciroli smile" has this formula (on just one side, the other is "normal"):

1.2.2.3
2.1.2.3

"Normally when this kind of thing happens you end up with the canine growing out of the gum above the tooth," my dentist continued, poking around enthusiatically. "You usually need a dentist to fix it. For some reason your second incisor just never appeared, and they all just grew in nice and straight. Amazing!"

I suppose it is a little amazing, and lucky. This is a mutation my father - and possibly many generations before him - passed to me and my sibling. While I mourned the loss of the incisor I never had, and the permanence of this uneven grin on my face, I was left wondering about the possible mutants out there which could mislead palaeontologists studying their remains.
Imagine if a future being was excavating on earth, and the only human fossil they uncovered was a member of my family?
And what if all they found was the upper right jaw, complete with two canines?

They might think humans naturally had this dental formula:

1.2.2.3
1.2.2.3

Probably not. Nevertheless, teeth do deserve their (ocassionally resented) place at the heart of palaeontology, especially in mammal research. As I gaze down the microscope at my latest specimen - an individual straddling the boundary between reptile and mammal - I have a growing appreciation of all that a tooth can tell us about diet and life history.

Go teeth indeed.

Oligokyphus teeth down a microscope. (Image by me, taken at University of Cambridge)




*It's worth remembering that we are all, in fact, mutants. That's just how biology works.




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