Extinct Scimitar Toothed Cat

An international team of scientists has sequenced and analyzed the entire nuclear genome of the scimitar-toothed cat Homotherium latidens. Their results demonstrate that this extinct species was highly divergent from all living cat species and did not undergo any detectable gene flow with living cat species after their initial diversification 14 million years ago.

A pair of Homotherium latidens chasing an ancient equid. Image credit: University of Copenhagen.

Homotherium was a genus of large-bodied scimitar-toothed cats, morphologically distinct from any living cat species, that went extinct at the end of the Pleistocene.

They possessed large, saber-form serrated canine teeth, powerful forelimbs, a sloping back, and an enlarged optic bulb, all of which were key characteristics for predation on Pleistocene megafauna.

Previous studies of mitochondrial DNA suggested that Homotherium was a highly divergent sister lineage to all living cat species.

“Their genetic makeup hints towards scimitar-toothed cats being highly skilled hunters,” said senior co-author Dr. Michael Westbury, a postdoctoral researcher in the GLOBE Institute at the University of Copenhagen.

“They likely had very good daytime vision and displayed complex social behaviors.”

“They had genetic adaptations for strong bones and cardiovascular and respiratory systems, meaning they were well suited for endurance running.”

“Based on this, we think they hunted in a pack until their prey reached exhaustion with an endurance-based hunting-style during the day light hours.”

Barnett et al. generated a 7x nuclear genome and a 38x exome from Homotherium latidens using shotgun and target-capture sequencing approaches. Image credit: Barnett et al., doi: 10.1016/j.cub.2020.09.051.

To examine the evolutionary history of Homotherium, Dr. Westbury and colleagues extracted DNA from a fossil of Homotherium latidens recovered from Pleistocene permafrost sediments near Dawson City, Yukon Territory, Canada.

The specimen was so old it could not be dated using conventional radio-carbon dating meaning that it was at least 47,500 years old.

The researchers then used a variety of modern genomic sequencing techniques to map the entire nuclear genome of the ancient species.

“We know that genetic diversity correlates to how many of a given species that exists,” Dr. Westbury said.

“Based on this, our best guess is that there were a lot of these big cats around.”

“This also makes perfect sense given that their fossils have been found on every single continent except Australia and Antarctica.”

Depiction of 18 of the 31 genes under positive selection with high values in the genome of Homotherium latidens; hypothetical functions and the adaptive insights that these provide on the species’ behavior, morphology, and functional adaptations are also shown; additional genes not depicted here are likely involved in cellular processes such as apoptosis, protein synthesis, and protein signaling, as well as immunity/cancer, olfaction, and reproduction. Image credit: Barnett et al., doi: 10.1016/j.cub.2020.09.051.

The analysis showed that Homotherium latidens is very distantly related to all modern cats and diverged from them about 22.5 million years ago, close to the Oligocene/Miocene boundary.

The scientists also found evidence of positive selection in several genes involved in vision, cognitive function, and energy consumption, potentially consistent with the diurnal and hunting/social behavior of this extinct lineage.

Finally, the authors uncovered relatively high levels of genetic diversity in the individual they studied, suggesting that it was not only a successful lineage, but also rather abundant relative to living cat species.

“This was an extremely successful family of cats,” said first author Dr. Ross Barnet, also from the GLOBE Institute at the University of Copenhagen.

“They were present on five continents and roamed the earth for millions of years before going extinct.”

“The current geological period is the first time in 40 million years that the Earth has lacked saber-tooth predators. We just missed them.”

The results were published in the journal Current Biology.

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Ross Barnett et al. Genomic Adaptations and Evolutionary History of the Extinct Scimitar-Toothed Cat, Homotherium latidens. Current Biology, published online October 15, 2020; doi: 10.1016/j.cub.2020.09.051