Fossil Friday – mammoth jaw

2014-09-19aIt’s Fossil Friday, and in what I intend to be a regular feature we’ll look at different specimens in the Western Science Center collection. To kick off, we’ll examine a mammoth jaw to show that not not everything in the Valley of the Mastodons is a mastodon!

The image above is an oblique view, with the front of the jaw to the left. Mammoths (and their close relatives, the elephants) have massive but remarkably short lower jaws. The example above is missing the condyles that articulate with the cranium, but is otherwise complete and has almost its entire length preserved. This particular specimen is from a Columbian mammoth (Mammuthus columbi), and was collected from Late Pleistocene deposits during the Diamond Valley Lake Project.

Here’s the same jaw from directly above (dorsal view), with the front of the jaw at the top:

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The two roughly oval patches with the ridges and grooves cutting across them are the teeth. Here’s an oblique close-up of the right one:

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One thing that you may find a bit curious is that in this huge jaw there are only two teeth, one on each side (although, to be fair, each tooth is enormous). That is a function of the unique method of tooth replacement found in elephants and their relatives. Unlike most mammals, elephants grow in their teeth one at a time, and as each tooth wears down it gradually moves forward in the jaw and falls out. As this happens, the next tooth in line moves forward and becomes functional, so the teeth act as if they’re on a conveyer belt. Because the new tooth starts to become functional before the older one falls out, at most points in its life an elephant will have approximately one and a half functional teeth in each half of its jaw at any one time.

There are two points in an elephant’s life cycle that are exceptions to this general rule. A very young elephant that has only just had its first tooth erupt has, of course, only one functional tooth instead of one and a half. Moreover, an elephant only ever grows a total of six teeth in each half of its jaw, three premolars and three molars (excluding the tusks in the upper jaw, which are also teeth). That means that an elderly elephant that has already lost its first five teeth will only have a single, very large tooth remaining in each half-jaw.

That’s exactly what we see in the mammoth jaw shown here. The enormous teeth on each side of the jaw are the lower 3rd molars, which had almost completely erupted but still had a lot of wear to go. The three premolars and the first two molars have already worn down and fallen out, making this a fully mature mammoth. We can even compare this to modern elephants to get an estimate of how old this mammoth was when it died. Assuming that mammoths and elephants grew and replaced their teeth at the same rate (and there’s pretty good evidence that they did), and assuming that their diets were equally abrasive (less certain, but not unreasonable), the Diamond Valley Lake mammoth was probably at least 40 years old when it died.

 

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