STARK COUNTY, N.D. — Paleontologists unearthed five species of ancient marsupials, including one brand new, previously undiscovered species, in the Brule Formation on two ranches near Dickinson.
These marsupials from the oligocene — roughly 30-32 million years ago — are similar to tiny opossums, about the size of a field mouse or a pocket-sized tree possum native to modern South America, North Dakota Paleontology Society senior paleontologist Clint Boyd said.
The new species is distinct from previously discovered species because of the shape of its teeth, which are only about 1-2 millimeters in size. The difference between what they observe of one species to the next can come down to an extra cusp on a tooth.
“Most of this work is based on studying the teeth, the morphology of the teeth of these animals,” Boyd said. “We compare the teeth that we find to already known species in other places. Most of these animals are in the Great Plains regions — Nebraska, South Dakota, Montana. We compare what we’ve got to what's been described previously and see what species have been documented.”
The paleontologists — Boyd, along with fellow NDGS paleontologist Jeff Persons, Smithsonian National Museum of Natural History paleontologist Robert Emry and William Korth of the Rochester Institute of Vertebrate Paleontology — are attempting to pinpoint the end of an era for marsupials in North Dakota. At some point, the population goes from a diverse set of species down to just one species.
Finding five species means they still haven’t pinpointed that timeframe, but Boyd said it is still valuable information that scientists can use in the future.
“Ideally, when we’re sampling through the rocks like that, what you’d like to find is that point where you go from one layer and everything is fine and then you go to the next layer where you’re losing diversity,” Boyd said. “But even if you don’t, you can say in these rocks, you’re not finding that pattern and that tells researchers that if they’ve got younger rocks, they need to go and look for that there. It’s still useful data even if we aren’t the ones that get to put our finger right on when it happened.”
Last year, the same paleontologists released a study on rodents in the Brule Formation that found fewer species of rodents there than in a sampling from Nebraska from a similar timeframe. They were expecting to see a similar case with the marsupials, but they actually found a similar sampling of species as other studies from different places in the same timeframe.
Because the species diversity in rodents was lower than other places while the diversity in marsupials was consistent, that tells the scientists there was something in the specific environment in North Dakota that was harming rodents but not marsupials.
“In other studies that we’ve completed and published, the trends were not what you expect,” Boyd said. “So it’s nice to find that in one subset of animals they are doing what we expect them to do. It doesn’t mean everything we’re doing with these rocks is wrong, but it’s telling us that something like rainfall or vegetation is causing some parts of the system to react differently. Having some of these specimens come back the way you expected is a little bit reassuring.”
Paleontologists focus on small animals like marsupials and rodents because they are much better indicators of what is going on in a specific place than larger animals. A tiny possum has to live in its optimal habitat and can’t travel very far, whereas a larger animal like a deer can travel hundreds of miles a year.
“If we want to know what’s happening with the vegetation and what rainfall is happening at the same spot, it’s important to study these smaller animals,” Boyd said.
Finding these specimens took years of work. Paleontologists have been studying the Brule Formation in North Dakota going all the way back to the 1940s. And the rock specimens from which these marsupials were found were collected as far back as the 1970s.
They start by finding fossils of larger animals and figure out what layer of rock they are coming from and then dig that rock out and take it back to the lab. They then soak the rock in boxes with fine screens that catch all of the fossils from the rock.
“Sometimes we get jaws full of teeth and sometimes it’s just little teeth,” Boyd said. “And we have to collect several hundred of them so we can get some overlap and tell this is the first molar, this is the second molar, et cetera. It’s slow, painstaking work. Say you wash a hundred pounds of rock, you might get a handful of teeth out of that. We’re talking about washing several tons of rock to find those teeth.”