The partially excavated skull (facing to the right) of an Archeocyon, an ancient doglike species that lives in the area that’s now San Diego up to 28 million years ago. Credit: Cypress Hansen/San Diego Natural History Museum
Sometime around 14,000 years ago, the first humans crossed the Bering Strait to North America with canines, domesticated dogs they used for hunting, by their side.
But long before the canines arrived here, there were predatory doglike canid species who hunted the grasslands and forests of the Americas. A rare and nearly complete fossilized skeleton of one of these long-extinct species was recently discovered by paleontologists at the San Diego Natural History Museum.
This fossil belongs to a group of animals called Archeocyons, which means “ancient dog.” It was embedded in two large chunks of sandstone and mudstone unearthed in 2019 from a construction project in the Otay Ranch area of San Diego County. The fossil dates to the late Oligocene epoch and is believed to be 24 million to 28 million years old.
While the fossilized remains are still awaiting further examination and identification by a canid researcher, its discovery has been a boon for the San Diego museum’s scientists, including the curator of paleontology Tom Deméré, post-doctoral researcher Ashley Poust and curatorial assistant Amanda Linn.
Because the existing fossils in the museum’s collection are incomplete and limited in number, the Archeocyons fossil will help the paleo team fill in the blanks on what they know about the ancient dog mammals that lived in the area we now know as San Diego tens of millions of years ago.
Archeocyons fossils have been found in the Pacific Northwest and Great Plains states, but almost never in Southern California, where glaciers and plate tectonics have scattered, destroyed and buried deep underground many fossils from that period of history. The chief reason this Archeocyons fossil was found and made its way to the museum is a California law that requires paleontologists be onsite at major construction projects to spot and protect potential fossils for later study.
Pat Sena, the San Diego Natural History Museum’s paleo monitor, was observing the rocks tailings in the Otay project nearly three years ago when he saw what looked like tiny white fragments of bone protruding from some excavated rock. He marked the rocks with a black Sharpie marker and had them moved to the museum, where scientific work soon ground to a halt for nearly two years because of the pandemic.
Poust said that once the fossil’s cheekbone and teeth emerged from the rock, it became clear that it was an ancient canid species. In March, Poust was one of three international paleontologists who announced their discovery of a new saber-toothed catlike predator, Diegoaelurus, from the Eocene epoch. But where ancient cats had only flesh-tearing teeth, omnivorous canids had both cutting teeth in front to kill and eat small mammals and flatter molar-like teeth in the back of their mouths used to crush plants, seeds and berries. This mix of teeth and the shape of its skull helped Deméré identify the fossil as an Archeocyons.
The new fossil is fully intact except for a portion of its long tail. Some of its bones have been jumbled about, possibly as the result of earth movements after the animal died, but its skull, teeth, spine, legs, ankles and toes are complete, providing a wealth of information on the Archeocyons’ evolutionary changes.
Poust said the length of the fossil’s ankle bones where they would have connected to the Achilles tendons suggests the Archeocyons had adapted to chase its prey long distances across open grasslands. It’s also believed that its strong, muscular tail may have been used for balance while running and making sharp turns. There are also indications from its feet that it possibly could have lived or climbed in trees.
Physically, the Archeocyons was the size of today’s gray fox, with long legs and a small head. It walked on its toes and had nonretractable claws. Its more foxlike body shape was quite different from an extinct species know as Hesperocyons, which were smaller, longer, had shorter legs and resembled modern-day weasels.