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Mastodon Surprise! An Interview with Alton Dooley

Replica skeleton of the holotype for <i>Mammut pacificus</i>, on display at the Western Science Center. Photo by A. Farke.
Replica skeleton of the holotype for Mammut pacificus, on display at the Western Science Center. Photo by A. Farke.

The mastodon–an Ice Age cousin of today’s elephants–is an icon of North American paleontology. Known to science for over 200 years, and studied by some of the greatest scientific minds, you might think that all of the “big questions” about this animal have been answered.

Yet, the wily mastodon is still spilling its secrets–including a previously unknown species that has been hiding in plain sight! Today, the “Pacific mastodon” (or Mammut pacificus) was named in a paper authored by Alton Dooley and colleagues and published in PeerJ. A few days before publication, I emailed Alton a couple of questions I had about the research. Here are his responses (marked AD). I think you’ll find this behind-the-scenes look quite interesting!

AF: I’ll admit that I was a bit skeptical of this new species at first because the American mastodon seems like something that was so well studied! How do you think Mastodon pacificus will be received by the broader scientific community?

AD: Hopefully you’re not skeptical any more! In fact, it took us a long time to come to the conclusion that we were working with a new species. But I think our data is very strong, and that while many people may initially be skeptical (they are scientists, after all), they will agree with our conclusions once they see the strength of our data.

There are a few things that may make some people initially have doubts about Mammut pacificus. As you mention, there is a perception (that I used to share) that the American mastodon is a thoroughly understood taxon. After all, it’s very common and was described almost 200 years ago. But as it turns out, a lot of the literature on mastodons focuses on paleoecological or Pleistocene extinction issues, or are focused on specimens from a small geographic area (although there are exceptions). The status of California mastodons was even worse, mostly because they are fairly rare and poorly preserved. More than half of all the California mastodons known were collected in the late 1990s from Diamond Valley Lake and haven’t been examined in detail until now. So, we really still have a lot to learn about mastodon anatomy and variability through time and space.

A second issue has to do with philosophical views of what constitutes species distinctions. The characters we use to distinguish between M. americanum and M. pacificus are most easily observed in large samples. As a result we see some slight overlap in morphology between the two taxa. This is fairly unusual in vertebrate paleontology; we’re often dealing with very small samples that show no morphological overlap in part because the samples are so small.

All that said, the case that the western mastodons form a distinct population is extremely well supported, and I think no one will dispute that. Given that this distinct population seems to have been stable through the entire Pleistocene at least, I think most people will agree with our interpretation of this population as a distinct species.

Has anyone recovered DNA from Mastodon pacificus? Is it worth trying?

AD: To my knowledge no one has recovered any DNA from Mammut pacificus. From what I understand of DNA preservation the odds of successful recovery at least from the Diamond Valley Lake specimens would be very low. I hope that eventually some genetic data is recovered, because I would love to know if there was any gene flow at all between M. pacificus and M. americanum, or if they had become completely genetically isolated.

It’s pretty cool that digital models of some of the specimens are available at MorphoSource. What are your hopes for use of these data?

AD: After some initial hesitation, I have completely adopted the view that scientists should strive to make as much of their work as possible accessible. That’s why we’re publishing in PeerJ and posting our digital models at MorphoSource and Sketchfab. Our hope is that other researchers can use this information to confirm our findings and build on our research. The genus Mammut is widely dispersed across North America, and travel to see important specimens can be challenging, particularly for students. With the publication of these models, researchers can quickly see what M. pacificus data might be available at the Western Science Center before arranging a trip. We also hope that there will be educational value to the open publication of these models.


Left side view of a skull of <i>Mammut pacificus</i>, from Dooley et al. 2019.
Left side view of a skull of Mammut pacificus, from Dooley et al. 2019.

This paper is a pretty powerful statement on the importance of collections housed in smaller, regional museums. What advantages did having many of the researchers and specimens at the Western Science Center offer for the project?

AD: I’ve spent my entire career in smaller museums. One thing I’ve found is that specimens that might be completely ignored in larger museums get a lot of attention if they’re in a smaller collection. At WSC, mastodons make up a significant part of our collection and so they figure heavily in our exhibits and outreach. This project actually originated as an effort to make an updated exhibit panel about one of our mastodons; while developing the panel I noticed anomalous tooth measurements in the specimen.

Related to this, in 2017 we opened an exhibit at WSC called Valley of the Mastodons, featuring the Diamond Valley Lake specimens. To kick it off we hosted a mini-conference in the week leading up to the opening, with about a dozen mastodon researchers as well as artists and science writers in attendance. We used some of our exhibit budget to cover costs for the attendees, and justified it by having much of the conference take place on the exhibit floor, including specimen examination, measurement, and scanning, and lectures. In hindsight that had a big impact on our paper, and it was a great success for the museum from the standpoint of outreach and publicity. But, we accomplished it by essentially turning over our whole operation to supporting the conference and exhibit for a week, which was possible because of our relatively small size.

Besides the WSC specimens, we examined a lot of M. americanum specimens from small museums across the Midwest. These places are a goldmine of potential data, but often researchers don’t make the effort to visit. Believe me, these museums will love to have you visit, and it will be well worth your time!

I should also point out, though, that the Diamond Valley Lake collection was housed at Western Science Center for eight years before it was studied. This is because there was no paleontologist on staff. So, small museums, if you have collections, try to find the money to hire someone to care for and study them. It really does pay off in the long run.

Femur (thigh bone) of <i>Mastodon pacificus</i>, from Dooley et al. 2019. CC-BY.
Femur (thigh bone) of Mastodon pacificus, from Dooley et al. 2019. CC-BY.

Was there a single “eureka” moment when you concluded that you had a new species? What tipped you over the edge into the new species camp? What was that like?

This project was about four years in the making, and took a lot of unusual turns. As I mentioned earlier, it didn’t start as a research project at all, but rather as an effort to make a new exhibit panel. I wanted to emphasize the large size of “Max”, our signature specimen (and now the holotype of M. pacificus), and to do that I chose to measure the 3rd molars. It turns out that Max’s 3rd molars are quite small and especially narrow, which was surprising. I dug through papers looking for additional length and width measurements of teeth (which were shockingly rare) and found a GSA abstract by Trayler and Dundas that made the same observation about a small sample of teeth from Rancho La Brea. I discussed what I was seeing with Eric Scott, and we concluded that tooth size in mastodons was not a reliable indicator of body size, and presented our results at a WAVP meeting. After that meeting, Greg McDonald provided us with additional mastodon m3 measurements that he had taken, and we started to see what looked like a real distinction between Southern California mastodons and those from the Midwest. We decided we needed more comparative data, so we launched a crowdfunding campaign to pay for a 2016 road trip for me and Brett Dooley to take tooth measurements at a number of western and midwestern museums.

At that point we firmly believed that the Southern California mastodons were a variant of Mammut americanum, either locally adapted to particular conditions or a “founder’s effect” population that was isolated late in the Pleistocene. I expected we would either find populations with similar tooth proportions to California because they lived in similar conditions, or that the range of California length:width ratios would overlap with, say, the upper 5-10% of the range of other mastodons. But as the data accumulated, we didn’t see either. Instead, there was practically no overlap at all between California and the rest of the country (except Idaho). Eric and I had a bunch of conversations trying to figure out what was going on (still using the assumption that everything was M. americanum) with no success. The last gasp was at the Valley of the Mastodons conference, where we presented our data up to that point. I really thought that at that conference someone would say “Oh, yeah, there’s a population you haven’t seen yet that looks just like these California specimens”, but that didn’t happen. So we were still stuck.

At some point a few weeks after the conference, Brett and I were discussing the data for perhaps the thousandth time, and I said something like “It’s funny. If we were looking at any other animal that had two geographically distinct populations with almost no overlap in their measurements, it would be easy to explain; they’re different because they’re different species. The only problem is that they’re mastodons and I’m a lumper.” That was probably my “eureka” moment, although I don’t think I really believed it yet. I debated the idea with my coauthors and other paleontologists to see if I was overlooking anything.

We finally decided that, if we really were dealing with different taxa, we should be seeing differences in other parts of the skeleton. The problem was that when we did our data collection trip I was only thinking about teeth, so we had only sporadically looked at postcranial specimens. I poured over my notes and photos and went back to the literature to find what data I could. Fortunately, most of the California specimens are at WSC, and most of the rest are in Los Angeles and San Diego, so I had ready access to those. When we found the differences in the sacrum and femoral proportions I finally really started to really believe we had a new taxon.

It was an interesting path for me. I’ve been involved in naming four other taxa during my career, which were based on a total of seven mostly fragmentary individuals between them. Mammut pacificus is based on far, far more data than any of those taxa (all of which are still considered valid), yet it took me a lot longer to get there, in part because I started from the assumption that it couldn’t be something new. Our starting assumptions and personal biases can have so much influence on our research, both positive and negative. I feel that in this case, the fact that we had to work so hard to convince ourselves made this a stronger paper in the long run.

Thanks, Alton, and congratulations to all of you on this new research!

Citation: Dooley AC, Scott E, Green J, Springer KB, Dooley BS, Smith GJ. 2019. Mammut pacificus sp. nov., a newly recognized species of mastodon from the Pleistocene of western North America. PeerJ 7:e6614 DOI 10.7717/peerj.6614

Full disclosure: Farke contributed to the original crowdfunding effort, and is a professional colleague of Dooley in the southern California area.

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