Paleontologists discovered a rare fossil of an ancient dog species

Rare fossil of ancient dog species

The first humans crossed the Bering Strait to North America some 14,000 years ago, accompanied by canines, tamed dogs employed for hunting. 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 ancient dog was recently discovered by paleontologists at the San Diego Natural History Museum.

This fossil is from a group of animals known as Archeocyons, which translates as “ancient dog.” It was discovered in two large chunks of sandstone and mudstone discovered in 2019 during a construction project in San Diego County’s Otay Ranch area. The fossil is thought to be 24 million to 28 million years old and dates from the late Oligocene epoch.

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 Demere, 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.

The complete Archeocyon fossil at the San Diego Natural History Museum

“It’s like you’ve found a tree branch, but you need more branches to figure out what kind of tree it is,” said Linn, who spent nearly 120 hours from December through February partially uncovering the fragile, and in some places, paper-thin skeleton from the rock. “As soon as you uncover the bones, they start to disintegrate … I used a lot of patience, and a lot of glue.”

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, a paleo monitor for the San Diego Natural History Museum, was observing the rocks tailings in the Otay project nearly three years ago when he noticed what appeared to be tiny white fragments of bone protruding from some excavated rock. He drew a black Sharpie marker on the rocks and had them moved to the museum, where scientific work was soon halted for nearly two years due to the pandemic.

On Dec. 2, Linn started work on the two large rocks, using small carving and cutting tools and brushes to gradually pare away the layers of stone.

“Every time I uncovered a new bone, the picture got clearer,” Linn said. “I’d say, ‘Oh look, here’s where this part matches up with this bone, here’s where the spine extends to the legs, here’s where the rest of the ribs are.’ “

Poust said that once the ancient dog 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 Demere 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.

The partially excavated skull of an Archeocyon, an ancient doglike species that lived in what is now San Diego up to 28 million years ago. Photo: San Diego Natural History Museum

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.

While the Archeocyons fossil is currently being investigated and is not on display, the museum does have a major exhibit on the first floor with fossils and a large mural of prehistoric animals that lived along San Diego’s coast. According to Poust, one of the animals depicted in the mural by artist William Stout, a foxlike creature standing over a freshly slain rabbit, is similar to the Archeocyons.

Once the Archeocyons fossil was partially identified in February, Demere had Linn stop work on the fossil, leaving it partially embedded in the rock. He didn’t want to risk any damage to the intact skull until it can be further studied by a world-renowned carnivore researcher like Xiaoming Wang of the Natural History Museum of Los Angeles County.

“Nothing makes a curator happier than having visiting researchers to the collection,” Demere said. “Hopefully, someone comes along. A nearly complete skeleton like this can answer all sorts of questions, depending on who’s interested.”

Source: The San Diego Union-Tribune