For the past two years, palaeontologists in Tennessee have been working with fervour to dig up the skeleton of a particularly impressive mastodon.

Huge even by elephant standards, the prehistoric beast would have literally reshaped the landscape as it roamed the ancient forests of southeastern North America. The newfound tusked titan isn't just big – it's one of the very largest land mammals of all time. It goes without saying that excavating such massive remains is no easy task.

The lower jaw of the mastodon – not yet fully put together – is impressive all by itself. That grey piece to the right is a 3D-printed stand-in for the lower tusk being worked on elsewhere in the lab. Image: David Moscato

An incredible discovery

Like modern-day elephants, this mastodon preferred to live near a large source of water. Five million years ago in eastern Tennessee, it frequented a water-filled sinkhole (a cenote) surrounded by warm, lush forest. Nowadays, the fossil-rich sediments of that lake make up the Gray Fossil Site.

First uncovered in 2000 during road construction, the Gray Site has produced an incredible diversity of fossils, from turtles and snakes to gators and rhinos, and even some extremely unexpected finds like a large omnivorous red panda! But no other creature at this site even comes close to the size of the mastodon.

This pelvis (well, about half of one) was found during the construction that uncovered the Gray Site back in 2000. And now we finally know what kind of giant animal it belonged to! Image: David Moscato

The story of the mastodon's discovery actually goes back to the earliest days of the site. Among the first fossils found by the original construction crew were small bits of ivory and an impressively sized pelvis. These were clear signs of a prehistoric proboscidean – a member of the mammal group that includes elephants and their extinct relatives – but with so little evidence, researchers couldn't identify it at the time. Based on size and geologic age, they guessed the bones might belong to a shovel-tusked gomphothere.

That initial hunch changed in 2015. Evan Doughty, a former graduate student of nearby East Tennessee State University, was getting in his very last day of excavation for the summer when he came across a small piece of ivory – the tip of a tusk. "We basically dropped everything and started following that for the rest of the day," Doughty told me.

The rest of the day turned into the rest of the year. The dig crew, largely made up of volunteers, followed the tusk until they found teeth, and then followed the teeth to discover an entire skull. "We ended up going all the way to mid-December, when we finally extracted it," said Shawn Haugrud, head of site excavation and the fossil laboratory at the site museum.

An animal this big sure leaves an impression! The water-filled hole in the foreground is the spot where the skull came out, and the flag-covered trench extending to the right held the front limbs. In the background, the crew is hoping to find hind legs. Image: David Moscato

It was the animal's dentition that gave away its identity. "We got to the teeth and it was like, that is not a shovel-tusk," Haugrud recounted. The iconic lumpy chompers pointed to one distinctive animal: a mastodon!

To get the skull out of the ground, the crew surrounded it with a plaster jacket, like a giant version of a cast on a broken arm. Once the fossil was safely enclosed, it took an army to pull it up. "To get the skull out that final day, we had me and four other guys in the trench to push it, and then two rope teams with about ten people on each rope," Haugrud explained.

The plaster jacket required to contain the skull is nearly three metres (ten feet) long.

Mastodon teeth are unmistakable. In fact, the word mastodon comes from the peculiar shape of their dentition, from the Greek words for "breast" and "tooth" ... yes, really! Image: David Moscato

By the end of the following year, the crew had recovered the animal's neck and both front legs, and there's already more poking out of the ground. Everyone on site has their fingers crossed that the whole rest of the body is waiting just below the surface.

And this wasn't a lone mastodon. As the diggers have been uncovering this specimen, they've also found extra tusks and teeth, and even a second skull. All in all, they've found pieces of at least three more mastodons that died alongside the big one.

The fossil ulna (lower leg bone) of the mastodon positively dwarfs the same bone from a modern-day African elephant. In fact, it's the largest proboscidean ulna on record! Image: Chris Widga

Just how big is big?

At least fourteen tons. Possibly quite a bit more.

"The [elephants] you're used to seeing in the zoo, they're on the order of seven, eight or nine tons," said Chris Widga, head curator at the Gray Site Museum. He sat down to speak with me after presenting his research on the mastodon at last month's meeting of the Southeastern Association of Vertebrate Paleontology.

When paleontologists want to estimate the size and weight of an extinct creature, they often look to the bones built to support the body: the legs. So far, the only leg bone of this mastodon that is available for research is an ulna, one of the lower bones from the forelimb.

By measuring that bone and comparing it to other elephants, Widga estimates that this animal may have weighed anywhere between fourteen and eighteen tons. When asked how this compares to the largest known proboscideans through time, he said: "We're easily within the top three."

Behold some of the largest land mammals ever to walk the earth! The Gray Fossil Site mastodon is estimated to have been at least 14 tons, and would fit right in with these titans! Size estimates and images (modified) from Larramendi, 2016.

Widga also offered a colourful comparison for the size of the animal: the buses his children take to school. "Every morning, I go to the bus stop twice, and see these Blue Bird-type buses," he said. "We're probably talking about something that is a similar height and a similar weight."

That's an impressive mental image, but Widga explained that size can be tough to gauge when an animal is so much bigger than most of its comparisons. That's why he wants more bones for an even better size estimate. "I want a femur and a humerus," he told me.

Fortunately, his wish is already coming true. While we chatted on the lower floor of the museum, one entire front leg of the animal was sitting up in the lab waiting to be cleaned and repaired for study. With these, we'll soon have an even better idea of the height and weight of this colossal herbivore.

A mastodon in the neighbourhood

Like modern elephants, and the much younger mastodons of the Ice Age, the Gray mastodons would have been keystone herbivores.

"If you've got a 16-ton animal wandering around on the landscape, they like to push over trees, they punch holes in the canopy," Widga said. "You've got a lot of disturbance and trampling around waterholes."

Similar to living forest elephants, these giants would have had a major influence on the shape of the forest around them: every other animal and plant at this five-million-year-old waterhole would have been impacted by their presence.

Already excavated, cleaned and put together: one huge foot. Image: David Moscato

Its ecological role may have been similar to its relatives, but exactly where this animal fits on the elephant family tree is a bit of a mystery so far. It has teeth very similar to classic mastodons of the genus Mammut, but the long shape of its jaws is much more like an older mastodon relative named Zygolophodon.

The Gray Fossil Site is unique: there are no other sites of its age in the entire southern Appalachian region. Like so many of the animals at this locality, the mastodon is something brand new, a creature glimpsed through the one and only window we have into this particular place in time.

Preparing a giant

Digging up a huge skeleton is certainly a laborious project, but it's only the first step. Next up is the even more time-consuming process of cleaning, consolidating and piecing together the bones. This work is in the hands of the workers in the fossil preparation lab.

One of the mastodon's long upper tusks, still being reconstructed in the lab. The gap in the middle represents about a metre of missing material. Image: David Moscato

Haugrud predicts that the skull alone will likely take six to twelve months before it's fully prepped and ready for study and display. When that day finally comes, a wealth of new information is sure to emerge. "The skull is one of the hardest things to do, but it's also the most research-worthy," he said.

What about the rest of the skeleton? If all of it is really down there, how long will it be before the whole thing is ready? "We will, I'm sure, be discussing it after we watch the second 'Avengers: Infinity War' movie [set for release a couple years from now]!" Haugrud laughed.

Digging up a mastodon might be strenuous, tedious and exhausting, but it's also thrilling above all else. And despite the difficulties of the elephantine undertaking, the entire team seems to have a ready smile.

"This has been probably the most exciting dig I've ever done," Ray Mann told me as we stood on the dig site. Mann has an extensive background volunteering at fossil sites, mainly at dinosaur digs out west, and is currently one of the dig team leaders at Gray. "Being probably a brand new species, [the mastodon's] got its challenges because we don't know what to expect, but actually I think that's my favourite part of it."

While only a few names have appeared in this article, make no mistake, they represent a mere fraction of the people power it's taken to get this job done, in the field and in the lab.

"It's not something that you can just dedicate one or two people to. It's something that we're undertaking as an institution," Widga said in his conference presentation, standing before a room full of fellow fossil enthusiasts. "There's an army of volunteers, there's staff, there's students … and we just couldn't do the work we do without them."


Top header image: Dantheman9758/Wikimedia Commons