Clues to saber-toothed cats’ demise are emerging from fossils entombed in California’s sticky La Brea Tar Pits.
Fossils of the extinct predators preserved in tar reveal a diversity of spinal maladies. These included a high frequency of a normally rare tumor, researchers report July 27 in Frontiers in Veterinary Science. The diseased spines may have been caused by inbreeding during the twilight years of the species, as cat populations waned.
At the La Brea Tar Pits in Los Angeles, crude petroleum in the form of asphalt oozes out of the ground, forming a collection of tar-filled ponds. For millennia, a wide range of species — some of which have since gone extinct — have become ensnared in the sticky mire and met their doom. Alongside the remains of mammoths, dire wolves and bison, the bones of hundreds of saber-toothed cats (Smilodon fatalis) are exquisitely preserved in the asphalt. Researchers have recovered and studied them for decades.
This includes veterinary surgeon Hugo Schmökel at IVC Evidensia Academy in Stockholm, who visited the La Brea collection to study joint diseases in dire wolves. While there, he noticed evidence of disease in Smilodon vertebrae.
“As a spine surgeon for dogs and cats, I was immediately interested and went back twice to La Brea,” Schmökel says.
Schmökel and his colleagues systematically analyzed Smilodon vertebrae in the collection, using a combination of careful observations of the bones’ features and X-ray scans. In all, they examined over 3,000 vertebrae, most of them lumbar vertebrae from the lower back. The team found hundreds of spinal deformities, including conditions where spinal bones were fused or grew with an abnormally split shape.
Three different cats had a particularly rare condition in their lower back vertebrae. Small holes in the vertebrae that allow nerves and blood vessels to pass through were hugely inflated into cavernous hollows. In modern humans and dogs, these hollows in the bone are caused by slow-growing tumors on the spinal nerves.
In humans, the tumors are very rare, with no more than 0.38 cases per 100,000 individuals. The researchers calculate the rate in saber-toothed cats from the La Brea collection to be about 1,000 times higher. That rate may not apply more broadly to saber-toothed cats, though.
These types of tumors tend to cause pain. And the cats’ hunting strategy could have exacerbated that pain. It “would have required bracing on their hind legs, pulling back and then launching themselves at their prey,” says paleontologist Mairin Balisi of the Raymond M. Alf Museum of Paleontology in Claremont, Calif. That prey, often strong, heavy animals like bison, “would not go down peacefully,” Balisi says, leading to grappling that stressed the cats’ lower back. So sick cats may have been more likely to seek easy prey and then get trapped in the tar too.
Schmökel and his colleagues suggest, though, that these tumors and other spinal deformities might have been common because of inbreeding. Similar spinal deformities are found in inbred gray wolf populations today. Skeletons and DNA dated to right before the disappearance of some now-extinct mammals such as mammoths, woolly rhinos and giant deer also showed signs of inbreeding.
Notably, two of the three Smilodon fossils with tumors date back to 12,000 and 13,000 years ago, shortly before the species went extinct at the close of the Pleistocene Epoch. In 2023, Schmökel and other researchers described high rates of bone disease in saber-toothed cat limbs, which may have also been caused by inbreeding.
“This might be how extinction happens,” Balisi says. “You start getting smaller population sizes and more isolated populations, resulting in less genetic diversity and more inbreeding.” Scientists have yet to pull usable DNA from La Brea’s saber-toothed cat fossils, though. “I think,” Balisi says,” that the real smoking gun would be genetic evidence.”
