Mastodons, the elephant-like Ice Age giants, migrated vast distances in response to shifting climates and were far more genetically diverse than previously known, according to an ancient DNA analysis of the remains of several mastodons, including those that roamed along the Pacific and Atlantic coasts of North America.
In the study published today (Sept. 12) in the journal Science Advances, a team of researchers including Penn State scientists provided the new evidence that significantly revises the prior understanding of the species' evolutionary history.
"The deep history of mastodons in North America is one of climate and landscape change," said Chris Widga, research professor of geosciences at Penn State and co-author on the paper. "Mastodons arrived on the continent from Asia about 17 million years ago, when the West Coast was dynamic with volcanic activity and grasslands had not yet expanded in the middle of the continent. Through time, the Great Plains dried out and became a barrier to mastodon populations, so separate populations evolved in their own niche, adapting to regional changes in climate and vegetation."
The mastodon was among the largest living land animals on Earth during the Ice Age, roaming from Beringia - present-day Alaska and the Yukon - east to Nova Scotia and south to Central Mexico, Widga explained. They primarily lived in swampy settings, eating shrubs and low-hanging tree branches. They occupied a very different habitant from their distant cousins, the iconic woolly mammoths, which roamed on open grasslands and tundra.
"The story of North American mastodons has been surprisingly full of twists and turns," said Wigda, who also directs Penn State's Earth and Mineral Sciences Museum & Art Gallery. "Mastodons are found almost everywhere in North America, in the Yukon and Mexico, Maine and California - and almost all points in between. What we show in this paper is that each population had its own evolutionary history, adapting to their own regional climate."
The research project was the result of a fateful visit to the Tualatin Public Library in Oregon several years ago, Widga said. He was on a research trip, traveling from Portland to Eugene, when he made an impromptu stop at the small library.
"I walked in and there was a fairly complete mastodon skeleton with teeth, mounted just behind the checkout desk," Widga said. "Even better, it looked preserved enough that it might be possible to extract DNA."
He immediately called up his colleagues, who eventually authored this paper, and they began a partnership with the Tualatin Historical Society to sample the skeleton for DNA.
The well-preserved fossilized specimens of teeth, tusks and bone - dating back hundreds of thousands of years - coupled with new scientific techniques, allowed the researchers to reconstruct genomes from ancient, tiny, degraded DNA fragments.
In addition to the unique specimen of Pacific mastodon from Tualatin, the researchers reconstructed the mitochondrial genomes - the genetic code that can trace maternal lines - from several mastodon specimens: five from Nova Scotia and the Eastern Seaboard, one of which may date to approximately 500,000 years ago.
"Ancient DNA is a very important tool to modern paleontology and evolutionary biology," Widga said. "But one of things that set this study apart is that we were able to combine the genetic results with traditional morphological measurements to create a more robust framework for the identification of these different mastodon lineages."
Mastodons were initially split into numerous separate species but later consolidated into a single one: Mammut americanum. More recently, this classification has been revised to potentially include at least two distinct species: the American and the Pacific mastodon, scientific name of M. pacificus, although a debate over the split has persisted, Wigda said.
The genetic analyses confirm the Pacific mastodons belong to a very old, well-established and separate genetic branch, with a range that extended much farther than previously believed - reaching deep into the Pacific Northwest, possibly south to Mexico, and as far north as Alberta.
Interestingly, Wigda noted, Alberta appears to have been a "hot spot," where Pacific and American mastodons congregated, expanded northward and may have interbred.
The East Coast and Northern Ontario specimens revealed two new and distinct genetic groups, known as clades, of mastodons living in the same region but at different times. The eastern species were surprisingly diverse, Wigda said, arriving in distinct waves of migration at least three times - a pattern driven by repeated cycles of climate warming, leading to glacial melting and the opening of new territory for northward expansion. When climate cooled and glaciers expanded, mastodons were driven south or went locally extinct.
"The data shifts our view of the region today known as Alberta and the north more generally, from a marginal roaming ground to a repeatedly occupied migratory corridor and significant landscape for mastodons with possible interbreeding," said Hendrik Poinar, senior author on the paper and director of McMaster University's Ancient DNA Centre and the university's Michael G. DeGroote Chair in Genetic Anthropology.
The researchers also pinpointed a mysterious and genetically distinct Mexican mastodon lineage, which they said they believe could be a deeper branch of the western species M. pacificus or possibly an entirely new, third mastodon species.
"It's important to constantly test our assumptions about how the world works," Widga said. "This study highlights that it is worth revisiting some of our firmly held assumptions about how things worked in the past with new techniques and new eyes."
The team's new findings, combined with those reported in a 2020 study conducted by the same group, create a much more complete picture of how mastodons moved and diversified across North America, helping conservationists today prepare for an ever warming artic and northern migrating species, said lead author Emil Karpinski, a former graduate student at the McMaster Ancient DNA Centre, now a research fellow in the Department of Genetics at the Harvard Medical School.
"This study represents several firsts which includes our work on the Pacific mastodon," Karpinski said. "It also poses many new questions. For example, how did these distant species of mastodon interact in Alberta? Did they compete for resources, or did they interbreed as our lab has previously shown for mammoths?"
The other co-authors on the paper are Tim Fedak of the Nova Scotia Museum of Natural History and Andrew Boehm of the University of Oregon Museum of Natural and Cultural History.
