Almost there...

 

It’s been several months since I posted an update, and quite a lot has happened in that time. In August the Western Science Center hosted the Valley of the Mastodons Symposium and Exhibit. A group of mastodon researchers, writers, and artists came together for a 3-day intensive look at California mastodons, culminating with the largest mastodon exhibit in history. One of the exhibit components is called “Mastodons of Unusual Size” and talks about this project, including an interactive where visitors measure casts of mastodon teeth and try to figure out if they came from California or Florida:

 

Since the symposium, I’ve made several follow-up visits to some museums, including San Diego, Rancho La Brea, and the Burke, to double check on some data. While on vacation, I also visited the University of Oregon Museum of Natural and Cultural History. Their small collection of mastodon remains look very much like those from California.

As we’ve gathered more data for this project, some of our long-held hypotheses have fallen to the wayside. For some time, I had believed that the narrow California teeth only began to be expressed around puberty, and was only present in the 3rd molars. It now appears that that was an artifact of our limited data on the premolars that are functional before puberty. After the Valley of the Mastodons Symposium, Jeremy Green provided us with a wealth of measurements of mastodon premolars from Florida. The new data, while still limited, indicates that California mastodons had narrow teeth though their entire lives. For example, below is a graph of lower 3rd premolar ratios (remember, higher values mean narrower teeth):

 

Another hypothesis that appears to be incorrect, and one I’ve favored for several years, is that the California mastodons are exhibiting a Founder Effect. This is a phenomenon that occurs when a small group is separated from the main population and begins evolving in isolation. If the initial members of the isolated group (the founders) happen to have characters that are very rare in the larger population, these characters can spread and become dominant in the isolated group.

The issue I’m having with the Founder Effect is a little esoteric, but it’s exemplified in this graph of upper 3rd molar ratios:

 

In a founder population, we would expect the amount of variation in the population to be less than the variation in the parent population. This makes sense, because the founded population is a subset of the parent population. But in our case, the California M3 ratios, while narrower that the rest of the country on average, are spread across a greater range of values. This seems to rule out a Founder Effect as a simple explanation for the California tooth morphology.

So, what happens now? Well, I have a follow-up visit to the Los Angeles County Museum scheduled for December, and I’m hoping to do an additional visit to the Santa Barbara Natural History Museum in December or January. In the meantime, we’ve started drafting out a manuscript, preparing our images, graphs, references, specimen lists, and background information while we continue to work out details of the nature of the variation. I’m hopeful for a submission by this coming spring. I’ll contine to post updates here as things progress.