Mammalian tracksites dating back to the Paleocene epoch are extremely rare, and researchers have just uncovered the largest one ever seen, pushing back the evolutionary timeline of mammals by millions of years. 

The new tracksite, located in Wyoming's Hanna Basin, is 1,032 meters (3,386 feet) long and appears to indicate the possible presence of hippopotamus-like creatures, and possibly others related to the modern tapir, in a marine environment much earlier than prior research has shown. The discovery is documented in a paper published Thursday in Scientific Reports.

The tracksite, only the fourth ever found to be left by mammals from the Paleocene, revealed itself to one of the study's authors while he was conducting other research at the Hanna Formation, a geological formation inside the basin.

"When the sun hit it at just the right angle, I could actually see depressions in the rock layers, and those depressions were not random," said lead author Anton Wroblewski, an adjunct professor in the geology and geophysics department at the University of Utah. "They were regularly spaced, and they're organized in the trails. And I looked at it, and I said, 'Oh my God, these are footprints!'"

Unlike previously described mammalian tracksites from the Paleocene in Canada and Norway, which only contain a few to a dozen prints, the Hanna site contains thousands spread across five layers of sediment, representing tens of thousands of years of time, rather than a mere moment.  

Wroblewski and his co-author and wife, Bonnie E. Gulas-Wroblewski, were able to discern the features of some of the tracks, finding four with five-toed imprints, and two with four-toed imprints. The four-toed animals that traversed may have been a species of artiodactyls, which are an order of ungulates, or hoofed animals, that includes giraffes, pigs, deer and many other species. The four-toed track-making animals may have included a kind of tapir, as well, a large herbivore that resembled a pig and is found in South and Central America. 

This discovery of four-toed artiodactyl tracks contributes to a paleontological discrepancy between the fossil record and the molecular record of this order. Prior fossil research has suggested that these mammals appeared only in the Eocene epoch, roughly 48.6 million to 37.8 million years ago, but DNA research suggests they evolved as early as the Cretaceous-Paleogene boundary, about 67.7 million years ago. The tracks, determined to be around 58 million years old, provide additional evidence to back the molecular point of view. 

"We know there was something with four toes that matches the hypothesis proposed by the molecular specialists, and it pushes back the evolutionary story based on the paleontology data," Wroblewski said in an interview with The Academic Times. "This is filling an important gap between the fossils and the molecular data."

During the Paleocene epoch, the Hanna Formation would have resembled bayou habitats similar to those on the Texas Gulf Coast: a shallow, muddy body of water teeming with anemones and bivalves, which are aquatic mollusks. 

The tracks, found near what would have been a delta, seem to have been made by four-toed artiodactyls as well as members of a five-toed group called pantodonts, specifically from the coryphodon genus. Coryphodons resembled hippopotami, with their short limbs, large, round bodies, and skulls akin to a hippo's.

These species may have been using this aquatic habitat for a variety of reasons, such as protection against predators and insect bites, or adding sodium to their diet. They may also have simply been cooling off, as the microenvironment where the tracks were made is similar to where hippos cool off today, according to the study. 

The Hanna tracksite also changes paleontological understanding of the period of time it took for mammals to grow larger and show more diverse behaviors enough to start exploring other environments. According to Wroblewski, when the dinosaurs went extinct around 66 million years ago, mammals were relatively small, about the size of dogs or raccoons. They avoided marine environments inhabited by large, dangerous sea creatures such as mosasaurs and plesiosaurs.

The extinction of those sea creatures made marine environments more appealing, but prior research in Amazonia, based on skeletal remains, held that mammals did not go into the ocean until the Eocene, about 9 million years later than these tracks from the Paleocene were created.  

"What we now have is direct physical proof of animals walking repeatedly in a marine setting, so it pushes back the entry of big mammals into the seaway by 9 million years," Wroblewski said. "That's a significant chunk of time."

According to Wroblewski, the study also highlights the importance of trace fossils.  

"Bones tell you where an animal's remains wound up, but they don't necessarily tell you about where that animal is living or what it was doing," Wroblewski said. "The trace fossils are direct evidence showing this animal was here, and it was doing this stuff in this environment. They fill an important behavioral gap that you cannot get from bones."

While Wroblewski can identify the depositional environment of the Hanna Formation 58 million years ago, the complete environmental setting is less clear. To better understand the habitat of the creatures that placed these tracks, Wroblweski intends to collaborate with other researchers to help create a more vivid picture of the Hanna Formation during the Paleocene epoch. 

Plant fossils can indicate plant diversity and average annual temperature, Wroblewski said, while body fossils of mollusks and other aquatic animals can help pinpoint the salinity of the Hanna Formation's marine habitat. 

Wroblewski intends to return to the Hanna Formation in June 2021 to continue looking for additional tracks, as well as create 3D models of some of them to share with other researchers. 

"If these are out there and I've missed them for years, and other people have missed them for years, who knows what else is out there?" Wroblewski said. "You can't even guess what you're going to find next."

The study, "Earliest evidence of marine habitat use by mammals," published May 13 in Nature Scientific Reports, was authored by Anton F.-J. Wroblewski, University of Utah; and Bonnie E. Gulas-Wroblewski, Texas A&M Natural Resources Institute.