Some 50,000 to 10,000 years ago as ice sheets melted and the planet warmed, around 100 species of gigantic animals started to disappear without a trace.
Paleontologists have sought to understand exactly how these animals died off, including iconic predators like the saber-tooth cat and the dire wolf. Some hypotheses suggest stiff competition for limited food aggravated by the arrival of humans and gray wolves. But new evidence suggests a bone disease that can debilitate modern cats and dogs, and even some of their humans, may have also played a role.
In a paper published Wednesday in the journal PLoS One, researchers report that as the climate shifted, the bones of saber-tooth cats and dire wolves became riddled with defects associated with osteochondrosis dissecans, or OCD, a severe developmental disease where holes form in bone caused by developing tissue that never hardened. In a live animal, the hole is filled with a cartilage flap that can lead to painful inflammation. It is commonly referred to as osteochondritis dissecans.
These findings reveal a fossilized snapshot of how the physiologies of prominent Pleistocene epoch predators most likely faltered under environmental pressures, said Mairin Balisi, curator of the Raymond M. Alf Museum of Paleontology in Claremont, Calif. and an author of the paper.
OCD is a common orthopedic disease affecting the joints of rapidly growing dogs. While it’s less common among cats, cases have been reported among snow leopards, which could mean OCD is underreported in wild animals, said Dr. Hugo Schmökel, a veterinary orthopedic surgeon based in Strömsholm, Sweden, and an author of the paper.
Dr. Schmökel visited the La Brea Tar Pits in Los Angeles in 2022 to study whether saber-tooth cats and dire wolves suffered from cruciate ligament disease. Instead, something else caught his eye: divots of varying sizes furrowing these ancient carnivores’ knee and shoulder joints.
While paleontologists had noticed these defects, “no one had realized that maybe these were premortem damages to the bone and not post-mortem,” Dr. Schmökel said.
With the help of Dr. Balisi, then a postdoctoral fellow at the La Brea Tar Pits, and Aisling Farrell, a senior collections manager, Dr. Schmökel inspected more than 1,000 saber-tooth cat and dire wolf limb bones.
The team discovered that around six percent of the limb bones of young adult and juvenile saber-tooth cats, specifically knee joints, had divots measuring less than seven millimeters.
Nearly three percent of young adult and juvenile dire wolves also had defects in the knee joint that tended to be larger, measuring more than 12 millimeters. Small shoulder joint defects were more common in the wolves, the same as in dogs, totaling almost five percent. A few adult limbs, but no juvenile limbs, showed signs of osteoarthritis, a degenerative joint disease that can result from OCD.
Prevalence of the disease among the animals appeared to be more than among modern animals and humans, Dr. Schmökel said.
Just from bones, it’s unclear why OCD struck the way it did. Nor can the researchers say for sure how it affected the animals’ quality of life or mobility. In modern domestic animals, the disease can cause varying levels of pain and lameness. In early life, these bone defects can heal on their own; it may not have been much of an impairment, at least for some individuals. The animals’ social behavior also may have mitigated the worst of the disease, said Larisa DeSantis, a paleontologist at Vanderbilt University who was not involved in the study.
In an email, she said other specimens from the La Brea Tar Pits had signs of “hip dysplasia and severe arthritis, revealing the ability of these ice age predators to live for an extended period of time with such injuries.”
But to the researchers, the higher prevalence of OCD offers grounds for speculation that there was an inbreeding problem among saber-tooth cats and dire wolves as a result of dwindling, isolated populations. Dr. Schmökel points to modern-day animals like Isle Royale wolves and Florida panthers that have experienced the same.
While Dr. DeSantis is skeptical OCD was solely involved in the extinction of these apex predators, Dr. Balisi says the findings are a prompt for further research.
Signs of the disease, Dr. Balisi said, “could be a morphological manifestation of something deeper that we can’t get to just yet, but I think it’s only a matter of time.”