Rare Fossil of Ancient Dog Species Discovered by Paleontologists

The partially excavated skull (facing right) of an Archaeocyon, an ancient dog-like species that lived in the area that is now San Diego until 28 million years ago. Credit: Cypress Hansen/San Diego Museum of Natural History

Around 14,000 years ago, the first humans crossed the Bering Strait to North America with canids, domestic dogs they used for hunting, by their side.

But long before dogs arrived here, there were species of dog-like predatory canids that 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 Museum of Natural History.

This fossil belongs to a group of animals called Archaeocyons, which means “ancient dog”. It was embedded in two large chunks of sandstone and mudstone unearthed in 2019 during a construction project in the Otay Ranch area of ​​San Diego County. The fossil dates from the end of the Oligocene and would be between 24 and 28 million years old.

While the fossilized remains still await further examination and identification by a canid researcher, its discovery was a boon to scientists at the San Diego museum, including 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 in limited numbers, the Archeocyons fossil will help the paleo team fill in the gaps about what they know about the ancient dog mammals that lived in the area we now know as the name of San Diego’s tens of millions. years ago.

Did they tiptoe like today’s dogs? Did they burrow into the ground or live in the trees? What food did they eat and what animals fed on them? How were they related to the extinct canine species that preceded them? And, potentially, is it a whole new undiscovered species? This new fossil provides SDNHM scientists with a few more pieces of an incomplete evolutionary puzzle.

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

Archaeocyon fossils have been found in the Pacific Northwest and Great Plains states, but almost never in southern California, where glaciers and plate tectonics have dispersed, destroyed, and buried deep underground many numerous fossils from this period of history. The main reason this Archaocyons fossil was found and brought to the museum is a California law that requires paleontologists to be on site during major construction projects to spot and protect potential fossils for further study.

Pat Sena, the paleo monitor at the San Diego Museum of Natural History, was observing the rock residue from the Otay Project nearly three years ago when he saw what looked like tiny fragments of white bone protruding from an excavated rock. He marked the rocks with a black Sharpie marker and had them moved to the museum, where scientific work was quickly halted for nearly two years due to the pandemic.

On December 2, Linn began work on the two large boulders, using small carving and cutting tools and brushes to gradually remove the layers of stone.

“Each time I discovered a new bone, the picture became 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 once the fossil’s cheekbone and teeth emerged from the rock, it became clear that it was an ancient species of canid. In March, Poust was one of three international paleontologists who announced the discovery of a new saber-toothed feline predator, Diegoaelurus, from Eocene times. But where ancient cats had only flesh-ripping teeth, omnivorous canids had both sharp front teeth for killing and eating small mammals and flatter molar-like teeth on the back. back of the mouth used to crush plants, seeds and berries. This mixture of teeth and the shape of its skull helped Deméré identify the fossil as an Archaeocyon.

The new fossil is entirely intact except for part of its long tail. Some of its bones have been mixed in, possibly from earthly movements after the animal’s death, but its skull, teeth, spine, legs, ankles and toes are complete, providing a mine information on the evolutionary changes of Archaeocyons.

Poust said the length of the fossil’s ankle bones where they would have connected to the Achilles tendons suggests Archaeocyons were adapted to hunt prey over long distances across open grasslands. It is also believed that its strong and muscular tail may have been used for balance when running and making tight turns. There are also indications from his feet that he might have lived or climbed trees.

Physically, Archaeocyons were the size of today’s gray fox, with long legs and a small head. It walked on its toes and had non-retractable claws. Its more fox-like body shape was quite different from that of an extinct species known as Hesperocyons, which were smaller, longer, had shorter legs, and resembled modern-day weasels.

While the Archeocyons fossil is still under study and not on public display, the museum has a large exhibit on the first floor that features fossils and a large mural of animals that lived here in coastal San Diego. during ancient times. Poust said one of the animals in the mural painted by artist William Stout, a fox-like creature hovering over a freshly killed rabbit, is close to what Archaeocyons might have looked like. .

Once the Archeocyons fossil was partially identified in February, Deméré asked Linn to stop working on the fossil, leaving it partially embedded in rock. He didn’t want to risk damaging the intact skull until it could be studied further 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 scholars invited to the collection,” Deméré said. “Let’s hope someone comes along. A nearly complete skeleton like this can answer all sorts of questions, depending on who’s interested.”

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