Fossil Friday – Old Man Mastodon

Closing our our Month of Mastodons is one of the more complete mastodon skulls from Diamond Valley Lake, a specimen that has several interesting features in addition to its relatively good preservation.

First, to explain what you’re looking at, here’s an annotated version:


The skull is laying on its right side, at a slight angle. Much of the braincase seems to be missing, although this area hasn’t been fully prepared, so there could be more there than there appears. The left dentary (lower jaw) is present, but disarticulated; it’s not clear if the right dentary is still in the bottom of the jacket.

So, what can we say about this skull? Well, for starters it looks pretty large, with a massive tusk. I haven’t measured it out yet, but I suspect that based on size this will probably be a male mastodon. Zooming in on the teeth gives us more information:

 Both upper 3rd molars are visible, as is the lower left 3rd molar. These teeth are in wear across their entire occlusal surfaces. Also notice the flat area outlined in red below, located in front of the upper left 3rd molar:

This is the closed-up socket for the 2nd molar, which has completely worn down and fallen out. With the 2nd molars gone and the 3rd molars heavily worn, this was a pretty old mastodon, most likely over 50 years old. He was much older than Max, who still had his 2nd molars, and in fact is the oldest mastodon I’ve seen from Diamond Valley Lake so far.

There is another interesting feature in these teeth, specifically in the lower 3rd molar. The red line below traces the outside edge of the chewing surface of the tooth (the “labial edge of the occlusal surface”):


The tooth is most heavily worn in the middle, and is higher both in the front and the back. This is the only time I’ve seen this wear pattern in a mastodon tooth, and I’m not sure how it formed. Here’s the issue: like other advanced proboscideans, mastodons replaced their teeth horizontally. That means the front of the tooth erupts first. Because of this,  at any given point in time after eruption, the more anterior parts of the tooth have been functional for longer than the more posterior parts. Because of this there should be a wear gradient, with the tooth being progressively less worn as you move from the front of the tooth to the back. And normally that’s exactly what we see in mastodons, mammoths, and other proboscideans with horizontal tooth replacement. So how did this tooth get most heavily worn in the middle? Was the tooth injured or deformed in some way? Did the mastodon’s diet change at some point, changing the wear rate? Did the mastodon suffer an injury to his jaw or jaw musculature that affected his ability to chew, altering the occlusion pattern of his teeth? (Max had multiple injuries to his lower jaw, so this type of injury may be common in male mastodons.) Or is there something else going on?

This mastodon skull is currently on display in the “Stories from Bones” exhibit at WSC, but if you haven’t seen it yet you’ll have to hurry; “Stories from Bones” closes on May 29.

With complete 3rd molars, this mastodon is also one of the specimens that is contributing data to our mastodon tooth research project, “Mastodons of Unusual Size“. We only have a few days left in our crowdfunding campaign to support this research, and we’re still well short of our goal, so donate today and help us understand mastodons a little better!

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