Associate prof stalks clues about mammalian evolution

If you can’t find something you’ve dropped on the floor – say, the back of an earring or a thumbtack – Rachel Dunn, Ph.D., is your go-to person. Years of scouting for fossils have given her eagle eyes in spotting even the tiniest specimens.

This summer, she used a Franklin Research Grant from the American Philosophical Society to apply that superpower at Sand Wash Basin, a 154,940-acre site managed by the Bureau of Land Management in northwest Colorado near the Wyoming border. Her mission: to find fossils from 47 million to 42 million years ago.

Talk about tiny: This primate jaw piece was among the fossils Dr. Dunn discovered at Sand Wash Basin this year.

“That was a time when the mammal fauna of North America was becoming more modern,” says the associate professor of anatomy at DMU. “Prior to that time, the climate of North America was more tropical, but when the climate started to change, the animals had to either adapt to a dryer climate, move or go extinct.”

Dr. Dunn first explored Sand Wash Basin using a DMU research grant in 2015, shortly after she joined the University, and has returned three times since then. This summer, her goal was to find new localities on the site.

“Some places you can go back to year after year to find fossils, but Sand Wash is not like that. The soil there doesn’t erode that fast,” she explains.

Fossils can shed light on the evolution of animals and plants, including how changes in climate and the environment shaped the process. In the period that Dr. Dunn is interested in, the earth was experiencing a global cooling trend, and many – more than one-third – of the major families of mammals that exist today evolved during that time.

Dr. Dunn points out a carnivore jaw “in situ.”

“When an environment changes, what animals go extinct first? Which go second? Third? How long does it take for them to go extinct? Or did they migrate somewhere else?” she says. “When the climate changes, who catches up, and who doesn’t? How do their skeletons respond to that change?”

One technique she’s used at Sand Wash Basin to find small fossils is simply to “walk and look,” using an icepick to flip over potential “keepers.” Once she finds an interesting fossil, she marks the spot and then returns to wash its matrix – basically, soil from the section – using screens to sort out objects as small as one-quarter millimeter. She notes that during her trip there in 2019, she collected 498.3 pounds of matrix that she scrubbed down to 17 pounds. This summer, she estimates she screen-washed 1,000 pounds of matrix that she reduced to fit in a large box.

“As a paleontologist, I’m trying to find samples of all types of animals,” she says. “If I find a tooth, I’m pretty good at determining the family and often can get it down to the genus and even species. If I find ankle bones, which are incredibly diagnostic, I can usually get it down to the family.”

Dr. Dunn screen-washes matrix in a Sand Wash Basin river.

Fossils can also reveal some of the animal’s behavior, such as whether it was a digger or a runner, she says. She’s currently sorting through material she brought back to DMU, which she hopes to complete by next summer, and working on publishing an account of the species that were present in the basin. When she’s finished analyzing the fossils, she will send them to the Denver Museum of Nature and Science, her “designated depository” required by the Bureau of Land Management for specimens taken from BLM land.

The whole process is painstaking, but Dr. Dunn thoroughly enjoys the fieldwork. She found a small skull the first year she went to Sand Wash Basin – “I felt I’d found gold,” she says; this year, a small rodent jaw and a canine tooth were among her discoveries.

“Some places you can walk along, and there are fossils everyone. Sand Wash Basin is not like that,” she says. “But when you find something good, it’s very rewarding.”

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