Biological anthropologists from The University of Texas at Austin have identified three new species of fossil primates that have been previously unknown to science. All of these new primates lived in San Diego County at a time when southern California was characterized by lush tropical forests.

Since the 1930s, many primate fossils have been discovered in the sandstones and claystones in the Friars Formation in San Diego County. Paleontologist Stephen Walsh and fieldworkers from the San Diego Museum of Natural History (SDNHM) have built up a large collection of fossil primates from the San Diego area, but Walsh was unable to describe these specimens before he passed away in 2007.

Nearly a decade later, University of Texas, Austin graduate student Amy Atwater and anthropology professor Chris Kirk took up the task to describe and name three previously unknown omomyoid primates that lived some 42 million to 46 million years ago. The researchers named these new species Ekwiiyemakius walshi, Gunnelltarsius randalli and Brontomomys cerutti.

These findings have doubled the number of known primate genera from Friars Formation and also increased the total number of known omomyine primates of that period from 15 to 18. The research team’s descriptions were published recently in the Journal of Human Evolution.

“The addition of these primates provides for a better understanding of primate richness in the middle Eocene,” said Atwater, who is now the paleontology collection manager at the Museum of the Rockies in Bozeman, Montana. “Previous research in the Rocky Mountain basins suggested the primate richness declined during this time period, but we argue that primate richness increased concurrently in other locations.”

Researchers studied the teeth of these primates and concluded that the three new genera, which represent most of the undescribed Friars Formation omomyoid sample at SDNHM, ranging in size from 113 to 796 grams- are most likely related to a group of extinct species comprising the primate subfamily Omomyinae.

“Teeth can tell us a lot about evolutionary history and give us a good handle on the size and diet of an extinct primate,” Kirk added. “Enamel is the hardest tissue in the body. And as a result, teeth are more likely to be preserved in the fossil record.”

Ekwiiyemakius walshi, the smallest of all the three new species, was estimated to weigh between 113 and 125 grams – which is comparable in size to some modern bushbabies. It was named after Walsh, the person who collected and prepared many of the specimens, and also comes from the Native American Kumeyaay tribe’s place name, Ekwiiyemak – which means “behind the clouds”.

Gunnelltarsius randalli was named after Gregg Gunnell, the research team’s late colleague and an expert on Eocene mammals, and also after SDNHM’s fossil collections manager Kesler Randall. It was predicted to weigh somewhat between 275 and 303 grams, which is close to the size of today’s fat-tailed dwarf lemur.

Brontomomys cerutti was estimated to weigh between 719 and 796 grams — similar to the size of a sportive lemur. Since it is quite large in size, its name is derived from the Greek word bront menaing “thunder,” as well as for Richard Cerutti, who is the retired SDNHM paleontologist responsible for collecting many of the Brontomomys specimens.