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.
Did they walk on their toes like today's dogs? Did they burrow in the ground
or live in trees? What food did they prey on and what animals preyed upon
them? How did they relate to extinct doglike species that came before them?
And, potentially, is this an entirely new undiscovered species? This new
fossil is providing SDNHM scientists with a few more pieces of an incomplete
evolutionary puzzle.
"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, 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.
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 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.
While the Archeocyons fossil is still being studied and not on public
display, the museum does have a large exhibit on its first floor that
features fossils and a large mural of animals that lived here in San Diego's
coastal region during ancient times. Poust said one the animals in the mural
painted by artist William Stout, a foxlike creature standing over a freshly
killed rabbit, is close to what the Archeocyons may have looked like.
Once the Archeocyons fossil was partially identified in February, Deméré 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," Deméré said. "Hopefully, someone comes along. A nearly complete
skeleton like this can answer all sorts of questions, depending on who's
interested."