‘Couldn’t stop smiling’: Rare fossils found along shallow Lake Powell shoreline

Andrew Milner and the group of paleontologists scouring Lake Powell’s shoreline this spring didn’t expect to find anything more than dinosaur tracks.

Milner especially didn’t expect to find something on his walk to the scientists’ makeshift latrine, assembled in a rocky alcove near their campsite.

But there, he saw it, embedded beneath a sandstone ledge — a footlong stone block, covered in what looked like brown stains. Milner, paleontologist and curator at the St. George Dinosaur Discovery Site at Johnson Farm, initially thought they were plant impressions, the fossilized remnants long since washed away.

A closer look revealed something even more extraordinary: evidence of bones, he said.

More digging into that ridge uncovered the skeletons of at least 30 tritylodonts — mole-like, basset hound-sized mammaliaformes, or early mammal relatives — including several intact skulls, all from the early Jurassic Period.

“When we broke that open and the skeleton popped out, I actually fell over with excitement,” Milner said. “We were going crazy. We just couldn’t stop smiling the entire time.”

What are tritylodonts?

The discovery marked some of the youngest tritylodonts ever found in the U.S., and some of the only fossils ever found in this rock formation, molded from from the leftovers of the vast desert that stretched across much of what is now called the Colorado Plateau, back when Earth’s landmass was a single continent known as Pangea.

According to the National Park Service, these tritylodont bones could help understand how the mammal relatives, and others, survived the mass extinction at the end of the Triassic Period and diversified through the Jurassic before going extinct themselves in the early Cretaceous — the beginning of the end of dinosaurs’ reign, which ushered in the rapid diversification of mammals and life as we know it.

Tritylodonts represent the transition between the early mammal precursors and mammals, said Hans Sues, curator of vertebrate paleontology at the Smithsonian National Museum of Natural History, in an email.

“In fact,” Sues wrote, “the earliest discovery (in the 1880s) of a tritylodont, a snout with teeth from South Africa, was first thought to belong to a mammal.”

However, further research showed that these creatures lacked mammals’ characteristic jaw joint between the mandible and temporal bones, part of the skull that contains the middle and inner ear, Sues said.

These animals were herbivores that lived in social groups and burrowed into the ground for shelter. They likely looked similar to rodents, Sues said, because of their large upper and lower incisors. Their name translates to “three cusped teeth,” which originated from the animal’s cheek teeth, made for shredding fibrous plants, Sues said.

Milner said it’s possible that the remains he found showed the tritylodonts in a burrow, but that’s just a theory. It’s also possible these particular animals were trapped in quicksand.

“There’s still a lot more to learn...This is extremely preliminary,” Milner said. “We’ve only just really started breaking the surface.”

An important discovery — with more to come?

“I think I’ve just found one of the biggest finds of my life,” Milner recalled telling the crew that arrived to help excavate the site.

“I’ve found a lot of cool stuff in my life,” he continued, “but this one seemed different.”

Indeed, the National Park Service called it “one of the more important fossil vertebrate discoveries in the United States this year. "

The most common findings in this formation are dinosaur tracks, left as large carnivorous theropods or massive long-necked sauropods treked across ancient oasis or wet sand. Such tracks have become Milner’s specialty, as bone sites in this formation are so few, he said, “you can basically count them on two hands.”

Paleontologists added to that count this spring, eventually uncovering 100 pounds of stones whose fossil innards will be scanned, analyzed and further excavated for years to come, Milner said. They’ll give insight into the evolution of mammals, and serve as a starting point for what will likely be many more excavations in unexplored areas around Glen Canyon.

These bones, or what remains of them, have been sitting along that shoreline for about 180 million years. But Milner said the last time scientists got a proper look at them was around 1967 — four years after the Glen Canyon Dam was completed and Colorado River water began to submerge its namesake canyon.

Extreme drought and aridification have pushed Lake Powell to its brink, with some scientists suggesting it be discarded in favor of filling downstream Lake Mead instead. While last winter’s historic snowpack brought some relief, it’s unlikely Lake Powell will ever fill above 50% capacity again if water consumption continues at current rates.

This spring, before the winter snowmelt raised lake levels, Milner and the other paleontologists “just happened to be the right place at the right time, on the right beach.”

“And luckily, that one rock was there so I could spot it,” Milner said.

That bone bed site, he said, is currently underwater again. But if water levels keep declining and scientists can get back to it, there’s mile and miles of rock to pick through, and, assuredly, more bones waiting to be discovered.

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