Researchers at the University of Texas in Austin have discovered not one but three new species of ancient (and now extinct) primate, which have now been described in the Journal of Human Evolution. 

The sandstones and claystones of the Friars Formation in San Diego County, California, are a treasure trove for paleontologists and over the years, the San Diego Museum of Natural History has built a substantial collection of fossil primates. Unfortunately, the paleontologist in charge of the mission, Stephen Walsh, was unable to describe all the specimens before his death in 2007.

Now, anthropologists at UT Austin have examined the teeth of the fossils and confirmed that there at least three previously unidentified species in the collection, all of which inhabited the region some 42 to 46 million years ago when southern California was awash with tropical forests. 

"Teeth can tell us a lot about evolutionary history and give us a good handle on the size and diet of an extinct primate," Chris Kirk, an anthropology professor at the University explained in a statement. 

"Enamel is the hardest tissue in the body. And as a result, teeth are more likely to be preserved in the fossil record."

All three species, which together make up the majority of the collection, are part of the omomyoid family of early primates that existed during the Eocene Epoch (55 to 34 million years ago) – a time when snakes the length of two giraffes and armadilloes the size of cars roamed the Earth.

The new additions to the family bring the total number of known omomyines up to 18. So, who are they?

First, there is the Ekwiiyemakius walshi. This little critter is estimated to have weighed just 113 and 125 grams, making it the smallest of the three and similar in size to some modern-day bushbabies. The name is a mish-mash of Steven Walsh and "Ekwiiyemak", which is the Native American Kumeyaay tribe's place name for the site of the San Diego and Sweetwater Rivers headwaters and translates to "behind the clouds" in English.

Next, there is the Gunnelltarsius randalli, which at 275-303 grams is the second largest and roughly equivalent in size to a fat-tailed dwarf lemur. It was named after Gregg Gunnell, an expert on Eocene mammals, and Kesler Randall, the fossil collections manager at the San Diego Museum of Natural History.

Finally, there is the Brontomomys cerutt. This is the largest of the three and is estimated to have weighed between 719 and 796 grams. That is roughly the same size as a sportive lemur and bigger than most species of omomyoid. Hence its name – "Brontē" means thunder in Ancient Greek. It is also named after Richard Cerutti, who was responsible for collecting many of their fossils during his tenure at the San Diego Museum of Natural History.

"The addition of these primates provides for a better understanding of primate richness in the middle Eocene," Amy Atwater, a graduate student at UT Austin involved in the research, explained. She is now working as a paleontology collection manager at the Museum of the Rockies in 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."