Tiny Fossils Show Tiny Primates in San Diego

There are few primates that are native to North America. But recently, paleontologists announced the discovery of three new, prehistoric species of primates that lived in San Diego about 46 million years ago.

Primates are a class of mammal characterized by their large brains and acute sense of vision. Many have opposable thumbs and a manual dexterity that distinguishes them from other mammals.

The new discoveries are exciting – and they may be only the beginning.

A Treasure Trove of Fossils

Steve Walsh worked at the San Diego Natural History Museum. His colleagues once described him as a “collecting monster in the field.” He was tireless in his efforts to collect and analyze fossils. After his death in 2007, the museum’s cache of fossils awaited analysis by scientists ready to continue with Walsh’s work.

In 2017, two scientists from the University of Texas at Austin, Amy Atwater and Christopher Kirk, took a selection of fossils to try to identify them. They published their findings in the Journal of Human Evolution in 2018.

Their research and analysis revealed three new species of primates, all indigenous to the San Diego area. The presence of primates there is surprising because most primates live in tropical or subtropical climates. San Diego’s climate today is a coastal desert, but there’s evidence to suggest that large rivers once flowed through it to the ocean. The water carried animals with it and deposited their bones in deltas, where they fossilized.

Tree Dwelling Mammals

The three new species were all small. The smallest, Ekwiiyemakius walshi, is named after Walsh and would have weighed in between four and five ounces – about the weight of a baseball.

The largest species, Brontomomys cerutti, would have weighed about two pounds. The third primate species, dubbed Gunnelltarsius randalli, weighed around 10 ounces.

According to Kirk, he and Atwater are able to speculate about what the newly-discovered species would have eaten. They are members of the primate family Omomyidae, which means that they were tree dwellers.

The smallest species would have likely relied on insects for protein, while the larger ones might have eaten leaves as well. He noted that they would have also eaten fruit when it was available since “there are very few primates that won’t eat a piece of fruit if you give them one.”

Thomas Deméré, the curator of paleontology at the museum, said that it was probable that the species would have looked similar to modern-day bush babies, with their large eyes and nimble hands.

Additional Discoveries Await

It is highly likely that there are additional species waiting to be discovered in Walsh’s collection. As he and Atwater worked to identify the three species of primates, they made note of additional fossils that did not match the others.

He said, “I’ve seen enough of the material to know there are definitely additional species.” Kirk and Atwater examined only a small fraction of the museum’s collection, and that’s a strong indication that the discoveries have just begun.

Deméré finds the prospect exciting. He thinks of paleontology as the “archives of the Earth’s history.” Studying fossils like the ones in the museum’s collection gives scientists a chance to get a glimpse of what the world was like millions of years ago.


It will likely take years for the rest of Walsh’s samples to be analyzed and categorized – and the wider scientific community will be part of it. Kirk said that it will “take the efforts of a village” to bring Walsh’s work to fruition.

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