How St. George paleontologists unravel secrets of dinosaur motion via tracks, skids, ‘butt’ prints

ST. GEORGE — What were dinosaurs doing when they were alive? How did they move? Paleontologists use tracks, skids, claw marks and even tuchus prints to uncover secrets long-kept by Southern Utah’s ancient inhabitants.

While a dinosaur is buoyed in the water, its toes or claws scrape the muddy bottom, leaving marks that can tell paleontologists about their movements, St. George, Utah, May 2, 2023 | Photo by Alysha Lundgren, St. George News

Many of the exhibits at the St. George Dinosaur Discovery Site at Johnson Farm are found on location, as the museum was built over a track site that extends across the street, Paleontologist and Curator Andrew Milner told St. George News. Additionally, they’ve been working with land stewards to collect specimens from private and public land, many of which are sent to other institutions because the discovery site is not yet an official repository.

“The dream is to build a natural-history museum right here on site,” he said. “That’s our goal.”

Milner discovered the first swim track in 2001, “before the museum was even a thought,” and around that time, the property across the street was being developed.

“They pulled one of these big blocks up and I could see the marks going across it,” he said. “And I got the volunteers to get in the hole and start cleaning it out. And sure enough, we found all these scrape marks.”

Dinosaur tracks as seen at the St. George Dinosaur Discovery Site at Johnson Farm, St. George, Utah, May 2, 2023 | Photo by Alysha Lundgren, St. George News

The museum, which opened in 2005, is the oldest Jurassic dinosaur site in Utah and has welcomed over 500,000 people, according to its website. The tracks have been discovered in layers of sediment from Lake Whitmore, a freshwater lake present approximately 200 million years ago, where many ancient inhabitants left their marks.

While a dinosaur is buoyed in the water, its toes or claws scrape the muddy bottom, leaving marks that can tell paleontologists about their movements, including the direction they were traveling, Milner said.

For instance, the claw tip is projected backward, showing researchers which direction the animal was moving — the scrapes are in sets of three, with longer center digits. The tracks created by the left foot were longer than those made by the right due to the current pushing against the creature’s body.

A semionotus fish fossil at the St. George Dinosaur Discovery Site at Johnson Farm, St. George, Utah, May 2, 2023 | Photo by Alysha Lundgren, St. George News

“The animal is trying to keep swimming in a specific direction,” Milner said. “So that’s the great thing about tracks — they tell you a lot about animal behavior, whereas bones give you an idea of what the animals looked like. But this is showing you what the animals are doing when they’re alive.”

The tracks show that meat-eating dinosaurs could swim, Milner said.

“For a long time, paleontologists viewed theropod dinosaurs — the meat eaters — as being afraid of water and that the plant eaters with the long necks and things like that would go into the water, but it’s actually more likely the other way around,” he said. “So these theropods are actively going into the lake and probably fishing and we have good evidence of that here at the site.”

Fossilized dinosaur teeth show that theropods likely feasted on semionotus fish, both of which have been found at the site. The fish had interlocking enamel-coated scales that Milner likens to chain mail, which wore down serrations and left wear patterns on the teeth.

A dinosaur “butt” print, St. George, Utah, May 2, 2023 | Photo by Alysha Lundgren, St. George News

Visitors can view portions of the track site from a boardwalk, where one dinosaur appears to have crouched and sat in the mud, leaving behind hand, foot, tail and ischium, or “butt” prints, before shuffling forward and sitting again, leaving overlapping tracks.

Hand and tuchus prints are rare, especially for bipedal theropods, Milner said. The handprints are turned inward, indicating that these dinosaurs sat similarly to large birds.

“They could hold the basketball but not dribble,” Milner said.

Additionally, digit one, typically only visible in tracks left in deep mud, can be seen on the left footprints but not the right, indicating the animal could have sustained an injury to its foot or tail, or may have suffered from another pathological ailment.

“You can see the movement of the foot through the sediments,” he said. “So, you can see how the foot enters or how the toes are striking and moving.”

A dinosaur appears to have slipped in the mud, leaving visible marks on this treated specimen, St. George, Utah, May 2, 2023 | Photo by Alysha Lundgren, St. George News

Dinosaur locomotion can be analyzed using the tracks and the movements of their modern relatives — birds, like ostriches and emus, Deputy Curator Jaleesa Buchwitz said. For instance, the speed at which a theropod ran can be calculated using a formula created based on watching ostriches run, the length of the animal’s legs and the distance between its prints.

In one specimen, a dinosaur left a slip mark, Milner said. The animal was partially or fully submerged when it slipped sideways in the mud, leaving marks where its scales and claws scratched the surface.

The museum also boasts a large collection of dinosaur skin impressions that show their feet were covered in scales, similar to modern birds, Milner said, adding that the scales’ diameter didn’t change with the size of the dinosaur.

Footprints on a smaller, yellow slab appear to be from a mammal-like creature, Buchwitz said. The specimen was found on private land in Ivins in early May and donated to the museum.

These tracks, found in Ivins, appear to be left by a type of creature that was a precursor to modern mammals, St. George, Utah, May 2, 2023 | Photo by Alysha Lundgren, St. George News

Such tracks are not common and the slab is one of the westernmost incidences found, she added.

The museum is also home to plant and animal fossils, some first discovered in Southern Utah and bone replicas, and in-house paleontologists continue to work on new discoveries. The site continues across the street from the museum, where multiple track horizons, bone layers, plant beds and more wait to be excavated, Milner said.

“There’s generations of research to do just at this site alone,” he said. “It’s that important a discovery and it’s right here in St. George.”

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Copyright St. George News, SaintGeorgeUtah.com LLC, 2023, all rights reserved.

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