Paleontologist Discovers South American Mammal Fossils

With Federico Anaya from Universidad Autónoma Tomás Frías, Croft reported on the new mammal find named Hemihegetotherium trilobus in the Journal of Vertebrate Paleontology article, "A New Middle Miocene Hegetotherid (Notoungulata: Typotheria) and a Phylogeny of the Hegetotheriidae." It is named for the distinctive three lobes on its back lower molar teeth.

The animal belonged to a group of animals called notoungulates—hoofed mammals native only to South America. The group originated in South America soon after the dinosaurs went extinct and evolved to include hundreds of species over a span of more than 50 million years; all of them are now extinct. Although most notoungulates were gone before humans got to South America, some of the earliest humans to journey to that continent may have seen the last living notoungulates.

For most of the time the notoungulates were living in South America, the continent was an island, isolated from both Antarctica and North America. The main groups of mammals living there were marsupials (like opossums), rodents, monkeys, armadillos, sloths, and various hoofed mammals.

The notoungulates were the most diverse group of hoofed animals in South America before it was reconnected to North America through the Panama land bridge about two to three million years ago. This reconnection began an exchange of mammals between the two continents and dramatically changed the types of mammals found in South America today.

The fossil specimens were collected from the Quebrada Honda and Rio Rosario areas of Bolivia, near the border with Argentina. They were found in an area much like the land formations that comprise the Badlands National Park in South Dakota where the earth is soil poor and comprised of silty clays, sand and gravel. The area, located at an altitude of approximately 11,500 feet, was susceptible to weather conditions that eroded the rocks and exposed the fossils in the lower portions of the formations at the two sites.

It is thought the animals probably lived in open areas but may have burrowed into holes like rabbits. The tall teeth suggest it probably fed on abrasive foods like grasses and other plants close to the ground.

This new animal belongs to a particular family of notoungulates known as hegetotheres. Hegetotheres are relatively rare fossils in the northern part of South America; the Quebrada Honda site is the only fifth place in Bolivia they have been found and the seventh site in the northern half of the continent.

"As the first member of its family known from this particular time interval, it helps us fill in gaps in the history of this group that existed in South America for 30 million years," said Croft.

Although new fossil species are usually found by excavation, during a 1999 paleontology conference in Bolivia, Croft found the fossil remains among other notoungulate specimens at the Museo Nacional de Historia Natural (National Museum of Natural History) in La Paz.

He noticed that the notoungulate specimen drawers contained fossils of an animal that had lower molar teeth with three distinct lobes and were rootless, which meant, like today's rabbits and other rodents, that the teeth continued to grow throughout the animal's life. Most other notoungulates have only two lobes on their molars, said Croft.

He later noticed other differences that supported the identification of the fossils as a new species. When pieced together, the animal would be about the size of a 25-pound beagle and might have looked a bit like a cross between a dog and a rabbit.

He added that this find illustrates the importance of studying museum collections: "Specimens of this animal had been known for nearly 30 years, but had never been recognized as pertaining to a new species."

Some of the specimens included complete skulls of about six inches in length, with jaws and parts of the skeletons.

He also compared the new species with specimens in the paleontology collections at the American Museum of Natural History, the Florida Museum of Natural History, the Museo de La Plata (Argentina), and The Field Museum in Chicago where he spent much of his time during his doctoral studies in organismal biology and anatomy at the University of Chicago.

"Notoungulates were a very successful group of plant-eating mammals for a long time. They existed for some 55 million years, only going extinct about 10,000 years ago," said Croft.

Croft, a paleontologist who joined the faculty in the Case School of Medicine in 2003, specializes in the evolution of South American mammals and annually makes field trips to the continent to study fossils in Chile, Ecuador and/or Bolivia.

He plans to return to the Quebrada Honda site and do fieldwork next year to learn more about this newly discovered mammal and other new species that may occur there.

Research for this paper was supported by the National Science Foundation, the National Geographic Society and the Case School of Medicine.

In addition to this fossil, and while at The Field Museum in Chicago, Croft worked on an education project connected with Sue, one of the most complete fossils of Tyrannosaurus rex ever discovered; he also helped with the development of the new "Evolving Planet" permanent exhibit at the museum. He was nominated for an Emmy Award in Chicago in the category of children's programming for "The Sue Files."