by Curator Dr. Andrew McDonald
The Pleistocene Epoch saw many profound changes in North America. Humans arrived on the continent. Vast territories were covered by ice sheets, and as glaciers advanced and retreated, so too did habitats and the animals living in them. Mastodons swaggered across this ever shifting stage for thousands upon thousands of years, and their fossil bones and teeth are found all across the continent, from Mexico to Michigan to Alaska. Until recently, all those fossils were assigned to a single widespread species, Mammut americanum, the American mastodon.
In 2019, a team of paleontologists led by Western Science Center Executive Director Alton Dooley determined that Pleistocene mastodon fossils from California and Idaho actually belong to a different species, which they named Mammut pacificus, the Pacific mastodon. All the dozens of amazing mastodon fossils found at Diamond Valley Lake, such as Max himself, are Pacific mastodon. The realization of the new species brings up myriad questions. Did the geographic ranges of American and Pacific mastodons ever overlap? Did they interbreed? How did the ranges of the two species change as the Pleistocene environment changed?
We’ve just added an important piece to this puzzle, with a new paper describing specimen MOR 605, the partial skull of a mastodon housed at the Museum of the Rockies (MOR) in Bozeman, Montana. The paper is authored by me, MOR Collections Manager Amy Atwater, Alton Dooley, and Montana State undergraduate student Charlotte Hohman, and was published on November 16 in the open-access journal PeerJ: https://peerj.com/articles/10030/. A digital 3D model of the fossil can be viewed on MorphoSource: https://morphosource.org/Detail/ProjectDetail/Show/project_id/1025
MOR 605 turns out to be the easternmost discovery of Pacific mastodon, extending the species’ range 500 miles farther east from southern Idaho into eastern Montana. Luckily, MOR 605 has two upper molars still in their sockets. One of these teeth is broken, but the other is complete and we were able to measure its length and width. Pacific mastodon have much narrower teeth than American mastodon, and to our surprise, the length to width ratio of MOR 605 fell squarely among specimens of the Pacific mastodon, including the huge sample of fossil teeth at Western Science Center.
Based on how worn down the teeth are and the size of the tusk sockets, MOR 605 represents a male Pacific mastodon that died when it was between 30 and 36 years of age. This individual lived sometime between 639,000 and 160,000 years ago, between two intervals when eastern Montana was covered by glaciers. This opens up some fascinating possibilities for the biological history of Pacific mastodon.
Research by other paleontologists led by our colleague Grant Zazula has shown that American mastodon went extinct in Alaska when their preferred habitat of wet forests gave way to dry grassland as glaciers expanded. If the same habitat preference holds true for Pacific mastodon, it might be that Pacific mastodon inhabited Montana when it was free of glaciers, but were then pushed out when the glaciers once again expanded into the area. The species would have continued to exist in the warmer, wetter climes of California. Ultimately, we need more fossils to track how the range of Pacific mastodon changed over time, and there is much more to discover about North America’s Ice Age titans.