A new paper published today offers surprising insight into the American mastodon and its reactions to a changing environment. This stocky megafauna — whose tusks, trunk and four legs echo today’s elephant — is thought to have lived predominantly within forests and marshy environments throughout ancient North America before its extinction approximately 11,000 years ago.
With technology not available even a decade ago, scientists have studied genetic information preserved within 33 mastodon individuals. They found that, when the Earth warmed in between Ice Ages, creating a corridor of land between ice sheets, American mastodons (Mammut americanum) took advantage of the expanding trees and plants that were able to grow in those climates. They may have migrated south when temperatures dropped, changing the landscape and possible food sources all around them. Amidst this migration, some mastodon populations thrived; others became isolated. All of them were impacted by the effects of weather that sculpted the world around them.
For the collection of international scientists involved, this is the culmination of years of collaboration and incredibly hard work. The team ranges in experience from lengthy careers to those new to their fields, but all of them were aided by the generosity of museums and universities who donated fossil specimens to be studied.
Emil Karpinski, lead author and paleogeneticist at McMaster Ancient DNA Centre at McMaster University, began this project while he was still an undergraduate in 2014. Even within six years, there have been significant changes to our understanding of North American mastodons. When Karpinski began, for example, regional variability in American mastodons had basically been neglected before last year’s discovery of the Pacific mastodon (Mammut pacificus). The timing of that discovery precluded any Pacific mastodon fossils from being included in this research, unfortunately.
Karpinski, like many people who work in paleontology, was eager to discuss and help explain some of the complex work involved. He’s remarkably frank and perpetually playful. In one of many moments of candor followed by laughter, Karpinski admitted in a phone interview with Gizmodo that, at the onset, he didn’t know what a “mastodon” actually was. He constantly referred to mastodon samples as “mammoths.” This, in all fairness, is a common mistake to make. Not every ancient elephant relative was a mammoth, just as not every two-legged carnivorous dinosaur was a T. rex.
Misconceptions don’t stop at the species level. Karpinski mentioned that work in paleogenetics is not like that outlined in Jurassic Park. While many of their specimens came from well-preserved fossils, there are limits to the type of genomes they are able to pull, if any at all. And mitochondrial genomes don’t include phenotypes, the information within the genetic code that might offer insight into whether, for example, mastodons had hair — a recent topic of debate brought up by paleontologist and paleoartist Mark Witton.
“One of the hardest parts when we do ancient DNA is that there’s very little material from the actual animal left in the bone,” Karpinski explained. “When we deal with samples of these animals, we could be talking in some locations less than a per cent of the total DNA. Some of the better material coming out of Siberia, Alaska, Yukon — where it’s in the permafrost the whole time and has not thawed — occasionally you get up to 60-70 per cent.”
He said this to illustrate why pulling 33 mitochondrial genomes from 122 fossils, as done in this study, is considered very successful. To those of us outside of paleogenetics, these numbers might seem fairly low. But to those in the field, this number of mitochondrial genomes would not have been obtainable even a decade ago. To put that feat into perspective, there have been only two complete American mastodon mitochondrial genomes prior to this study.
“It’s the first large-scale genetic study on megafauna browsers in North America. It’s a huge increase in mastodon mitochondrial genomes,” Karpinski said.
And he credits paleontologist Jacob Enk, lead author on an important paper in 2016 that explored North American mammoth genetic diversity, for designing a method that vastly improves how scientists are able to isolate and amplify ancient DNA.
Still, even with an improved method, this is by no means a quick process. Describing the best-case scenario, in which everything works, (“which it never does!”), Karpinski said it can take weeks of work to bring something from a sample to readable DNA. They may have pulled 33 mitochondrial genomes from 122 fossils, but that doesn’t mean the process was repeated only 122 times. It was done approximately 248 times to get those results, which is part of the reason this research is so significant and why co-author Chris Widga consistently praises Karpinski his contributions to the research.
Widga has spent years studying proboscideans, animals whose anatomy generally — but not always — includes a proboscis (or trunk). He is currently the head curator of the Centre for Excellence in Paleontology in Tennessee, but while working at the Illinois State Museum, he and colleague, Jeff Saunders, did a detailed four-year study of Midwestern mastodons and mammoths. When other paleontologists are unsure of a fossil’s morphology, particularly in terms of proboscideans, they turn to Widga.
He was also co-author on the aforementioned paper published in 2016 that found that, even though North American mammoths have been classified as four different species based on morphological differences, at the genetic level, they are very similar populations.
When asked if he was surprised by today’s results, Widga answered, “Floored, actually.”
“What we’re finding is basically the opposite pattern to what we discovered with North American mammoths,” he said in a phone interview with Gizmodo. “Before the discovery of the Pacific mastodon, you would just call everything ‘American mastodon.’ And now, we realise that there is a lot more diversity than expected.”
That diversity is reflected in the five different clades uncovered by the team. In other words, although currently grouped as American mastodons, these animals belong to not one but five distinct genetic groups from Mexico to Canada, each with its own shared common ancestor.
Even more intriguing are three American mastodon fossils found in Alberta, Canada.
“You get animals that are living more closely together tend to be more related,” noted Karpinski. “And then you have Alberta, which is doing something weird.”
Groups of mastodons cluster in the state of Alaska and in the Midwest, for example, but the three in Alberta offer remarkably puzzling genetic information. The closest genetic relative to one of the Alberta fossils is in Mexico. Another is related to a fossil in Alaska, and yet another is related to one in Missouri.
“There’s some things that really pop out that I’m still scratching my head over,” said Widga. “For instance, the Missouri animal. We’ve got radiocarbon dates on it, and it’s 13,200 years old. It shares a site with about 35 other mastodons. And we all expected it to plot very closely with everything else from the Midwest, but it bumps out, and it plots with this animal from Alberta. So there’s a lot of variability, and we still aren’t really sure what that means yet.”
The authors suggest that figuring out how these ancient animals responded to drastic climate change might help us better understand possible reactions by living animals to the global warming experienced today. One of the things they caution is that, while species might move north and thrive in response to climate change, this might also mean important loss of genetic diversity within those populations. How could that loss impact species’ ability to cope with potential future challenges?
There is so much more to learn.
“This is only the first step in understanding the mastodon’s evolutionary history in North America,” Karpinski noted. “This is just a framework under which future projects could be built. But a lot more of this is going to be dependent upon getting more mastodon DNA and better temporal calibration points.”
“This migration pattern can’t just apply to mastodons. These animals didn’t live in isolation, but in complex ecosystems full of other plants and animals,” he added.
Dick Mol, mammoth expert and research associate at the Natural History Museum Rotterdam in the Netherlands, has spent decades working with proboscidean fossils all over the globe.
“The study by Emil Karpinski and colleagues is a milestone in paleontology,” he wrote in an email to Gizmodo. “Their [ancient DNA] research on these mastodon remains have shown that the distribution and development of the Mammut americanum is more complicated than is usually suggested.”
When mastodons roamed North America, they shared this side of the planet with mammals such as giant sloths, camels, bison, saber-toothed cats, dire wolves, short-faced bears, and giant beavers. As Earth changed and shifted around them, how did these and the myriad other animals at that time react?
“My biggest hope is that people will do this [research] with other species,” said Karpinski, who immediately added, “So I don’t have to.” And laughed.