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How a newly-discovered mastodon jaw became a mammoth mystery

Dr Chris Widga and his team thought the remains they were excavating were ‘just another mastodon’. But when the jaw appeared, it was unlike anything the team had ever seen. What exactly could it be?

He’d been offering tantalising hints throughout his presentation: an ulna here; a large femur there; a calculated weight of 16 tons for this animal. But it wasn’t until he showed an image of the excavated jaw that some of us became really excited.

This wasn’t a typical mastodon.

And when someone like Dr Chris Widga – a palaeontologist with extensive proboscidean knowledge – looks at a fossil and isn’t sure what it is, it’s time to pay attention. Watching his presentation at the Valley of the Mastodons workshop, held early this August at the Western Science Center, California, my interest was certainly captured.

The excavated jaw. Photograph: Charlie Warden_ETSU/Charlie Warden/East Tennessee State University Photographic Services

While the woolly mammoth may dominate public imagination when it comes to ancient animals with trunks (or “proboscis”, hence the name “proboscideans”), there were numerous other similar species that evolved both before and alongside it.

Mastodons (the genus Mammut) were relatively shorter and stockier than mammoths. Like mammoths, they had tusks, although theirs were less curvy. It is their teeth, however, that make them easily recognizable. Mastodon teeth actually look like teeth, whereas mammoth teeth resemble the footprints made by astronauts.

A mammoth molar, left, and a footprint from Nasa’s Apollo 11 lunar mission. Photograph: Composite

Until a year ago, Widga worked at the Illinois State Museum where he had access to a number of mastodon sites and collections in the surrounding states. He is now head curator at the Gray Fossil Site, an exciting Miocene site in Gray, Tennessee.

The site itself was discovered in 2000, the happy result of a road project that was halted and moved after fossils were discovered. Thanks to local citizens, politicians and scientists, a museum literally stands atop the site. Because both site and museum are so new, there is no legacy data. In other words, researchers don’t have to devote time cataloguing information from fossils found within the past century; they’ve been able to digitize everything from day one. So far, they’ve catalogued 20,000 specimens, and the list keeps growing.

Ancient rhino, alligator, invertebrates, plants and literally hundreds of tapirs have been found at the Gray Fossil Site. Mastodons, however, are less common thus far. And the excavation of this one particular proboscidean has been a slow process. A small tusk was discovered in 2000, but it wasn’t until 2015 that the rest of the animal began to be recovered.

One of the mystery proboscidean’s feet. Photograph: Charlie Warden/East Tennessee State University Photographic Services

“When I first saw the tooth,” said Widga, “I thought, ‘oh well, it’s just another mastodon.’ We would expect to see mastodons at that age [in the fossil record]. Not a big deal.

“But when they started excavating the jaw and the skull, those were elements that looked very, very different. That’s when we realized this has the potential to be a very important specimen.”

Again, it all goes back to that jaw.

American mastodon (Mammut americanum) jaws are short in length. Some have small, thin chin tusks; others have no chin tusk at all.

The Gray Fossil mastodon, in contrast, is markedly long with a much thicker chin tusk: two elements that do not indicate Mammut.

It was this elongated jaw that prompted them to compare the fossil to another earlier proboscidean species – the gomphothere – which does have elongated jaws and which has been found in sites relatively close to Tennessee.

The American mastodon has a short jaw, as seen in this specimen at the Beneski Museum, Amherst College. Photograph: Jeanne Timmons

Gomphotheres evolved well before mastodons and mammoths, but they also eventually co-existed with them. They include species with four tusks, as well as species with a large flat tusk extending from the bottom lip, leading to the nickname “shovel-tuskers” (if you’ve seen the “Lord of the Rings” trilogy, you may notice that the elephants in that series bear a striking resemblance to gomphotheres).

According to palaeontologist Michael Pasenko, who has spent much of his career studying that family, “[Gomphotheres] were way more diverse in North America than mastodons and mammoths. [They] are the only proboscideans that [we know] made it to [what is now known as] South America. Mammoths and mastodons never made it, but gomphotheres did.”

But the Gray Fossil jaw doesn’t match those of gomphotheres, so it was ruled out.

The next step? Looking further into mastodon ancestry to a species called Zygolophodon. Granted, I’m not an expert, but I’ve spent the past several years learning about proboscideans, and I’d never heard of Zygolophodon until the “Valley of the Mastodons” workshop. I connected with proboscidean paleontologist Rachel Silverstein for help understanding it.

“I had a lot of trouble trying to understand what Zygolophodon WAS, let alone what its lineage was like,” she explained. “North American Zygolophodon teeth and Mammutamericanum (American mastodon) teeth are nearly identical, so being able to separate the genera was important for my master’s thesis.”

“I think the morphologic features of both genera are so similar,” she continued, “that paleontologists often overlook the possibility that what they find/describe could be Zygolophodon rather than Mammut. There’s just so little work done on Zygolophodon that making connections is difficult, especially since much of the large paleontological research on them is decades old.”

Dr. Alton Dooley Jr. of the Western Science Center with a 14m-year-oldZygolophodon fossil from the Alf Museum. Photograph: Jeanne Timmons

And yet the Gray Fossil jaw just doesn’t fit into this category either.

“That’s part of the reason why we’re like, ‘ok it’s not completely like American mastodon, but yet it’s not completely like Zygolophodon,’” Dr. Widga stated. “It’s something in between. It exhibits characters of both of these taxa. We don’t think it’s a hybrid, but the exact relationship between Zygolophodon and Mammut is not well mapped right now.”

It remains a mystery, then, at least while Widga and his team continue to excavate and prepare the bones for further analysis. So what do we know so far?

“This mastodon shares characters with multiple fossil proboscideans. Ultimately, it belongs to the genus Mammut because of its teeth,” said Widga. “Mammut teeth were very conservative through time, in terms of their shape and size. But the rest of its skeleton is different. And given what we know from recent genetic studies of other elephants, there can be a lot of variability in the skeleton, and you can still end up with animals that can interbreed and be considered the same species, genetically.

“I don’t know whether it will be a new species or not,” he concluded, “in part because we also need better definitions of the other species that are out there. What does Pliomastodon look like? What does early Mammut look like? What does Zygolophodon look like in North America? And once we tease that apart, then we’ll know better how the Gray Fossil Site mastodon fits into the bigger picture.”

Jeanne Timmons is a freelance writer who runs the Mostly Mammoths, Mummies and Museums blog. She can be found on Twitter as @mostlymammoths