April 26, 2016

Posted by cwidga

To paraphrase Larry Agenbroad, the former director of the Mammoth Site in Hot Springs, SD, Mammoth taxonomy is confused, and confusing. And it has been this way for a long while. Henry Fairfield Osborn, a giant of North American vertebrate paleontology dedicated decades of his life (and that of his assistants) to the production of a 1600-page, 2-volume, tome describing the Proboscidea, published posthumously in 1942. Through a specimen-by-specimen analysis, he described 16 species of North American mammoths across 3 genera. Since that time, North American mammoth species have undergone significant pruning, with most paleontologists recognizing 4-5 species across North America: M. meridionalis (Southern Mammoth), M. columbi (Columbian Mammoth), M. primigenius (Woolly Mammoth), and M. exilis (Channel Island Pygmy Mammoth). A fifth species, M. jeffersonii (Jeffersonian Mammoth) was considered an intermediate form showing characteristics of both Columbian and Woolly mammoths.

The story went something like this…Around 1.5 million years ago, the Southern Mammoth (M. meridionalis) emigrated to North America, settling along the west coast. Shortly after, the Eurasian Steppe Mammoth (M. trogontherii) joined its trunked brethren. Both were found in early deposits in the Anza Borrego Desert of southern California (and potentially the Great Plains and Florida). The Southern Mammoth died out or was swallowed up by the more successful Columbian forms, which radiated throughout most of North America. The BIG mammoths that fill western museums, like Archie at the University of Nebraska and the Angus Mammoth at Denver were initially considered to be too big to be run-of-the-mill Columbian mammoths and were anointed “Imperial” mammoths. Woollies migrated down the front of the continental ice sheets late in the game, during the Wisconsin glaciation sometime in the last 100 thousand years. Jeffersonian mammoths were the love-children of Woolly and Columbian mammoths. And the Island Pygmies were early Columbian mammoths that swam the channel or wandered across a land bridge.

This was a great story. It had action and explanation. And it was the framework that most museums used to explain their monstrous Mammuthus mounts (or miniscule mounts, in the case of the Pygmy Mammoth on display at the Santa Barbara Museum of Natural History). But two papers in the last 6 months have shown that the reality is actually much more complicated…and interesting.

The first paper was a study by Adrian Lister and Andrei Sher. There are few scientists who have seen as many mammoths as Lister (who literally wrote the book on the subject). In a project that spanned decades, Lister and Sher visited many North American collections housing early mammoths. From California to Florida and everywhere in between. They concluded that the earliest mammoths on the continent were, in fact, not M. meridionalis. Rather they were an odd assortment of poorly prepared/reconstructed material, individuals with heavily worn teeth, or simply Columbian mammoths from an early context. The clincher was that they had an excellent sample of Old World Southern Mammoths that didn’t overlap with any of the North American specimens. Any-of-them…

The second paper (Poinar et al. in press) came out this week. A few years ago, we hosted a sharp graduate student from McMaster University (Ontario) who was interested in our midwestern mammoths, Jake Enk. I bought him lunch. We talked at length about messed up mammoth taxonomy. Normal stuff. Ultimately, Jake sampled ~30 teeth for genetic studies (like this one), then moved on to major collections of Mammuthus in Nebraska, Denver, UC-Berkeley, and Santa Barbara. Given the success rate of previous aDNA studies, we expected that one or two of these specimens might actually give us some decent data. To my surprise, Jake was able to extract complete mitochondrial genomes from 67 mammoths from south of the Laurentide ice. An even bigger surprise, was that they were all chips off of the same block. They weren’t even different species.

Well…this was a surprise/not surprise. We had been looking at this issue through the morphology of midwestern mammoth teeth and found that there was a significant amount of overlap between different “species” and shared our data with Jake. The conventional wisdom that Columbian mammoth teeth were distinct from woolly and Jeffersonian mammoth teeth just wasn’t holding up. You could see multiple morphs within a small geographic area–and we had the dates to prove that we weren’t seeing the influx of “new populations” through time. Things seemed to get really complicated in ecotonal areas, like Iowa. During the Last Glacial Maximum Iowa was a transitional landscape between the more open steppic grasslands to the west (“Columbian” mammoth territory) and the forest steppe (think Taiga) of the east (“Jeffersonian” and “Woolly” Mammoth territory). We hit collections at the University of Iowa, Iowa State Historical Society, Putnam Museum (Davenport) and the Sanford Museum (Cherokee) hard, hoping to figure out where one species left off and the other began. We ended up scratching our heads over animals that had jaws that looked like Woolly mammoths, but teeth that were Columbian…or jaws that were Jeffersonian on one side, but Woolly on the other. We found localities like the mammoth bonebed in Mahaska County, that had one jaw that looked like a Columbian mammoth, but two more that were dead ringers for big Woollies. These were exactly the morphological patterns that we might expect if a) Mammoths were a single biological population capable of inter-breeding and producing viable offspring, and 2) the midwestern mammoths were in the middle of the mess, showing characters of both populations.

So do these different “species” of mammoths mean anything? Why bother measuring teeth if all mammoths are the same? After the initial shock wore off I had plenty of time to think about this. As a morphologist, the idea that we are dealing with a single, morphologically variable population is actually…well…kind of liberating. Now we can explore how certain characters may have been selected for in different environments. We can think about functional morphology, broad-scale impacts of landscape/diet on body-size, or the morphological effects of introgressing populations. Before…the pygmy mammoths of California’s Channel Islands were an unrelated off-shoot of my midwestern behemouths, perhaps responding to nutritional stress and landscape changes in very different ways than their mainland cousins. Now, they are just another mammoth population that is using the same set of morphological and genetic tools to deal with the situation at hand. And we can learn from that.

Mammoths are fun to think about, even when we don’t know all of the answers. These papers (and a third that Jeff Saunders and I are hoping to finish up this week) illustrate the importance of retaining natural history collections in museums. A decade ago, there would have been no chance of getting this degree of genetic recovery out of fossil mammoths south of the ice. Even for “traditional” studies of morphology the only way to get sample sizes large enough to say meaningful things about the biogeography of a creature is to rely on the materials collected and accumulated through many generations.