Genetic Clues from Sabercat Bones

Smilodon is the most famous of North America’s long-lost sabercats. But the burly carnivore wasn’t the only long-fanged carnivore prowling the continent during the last Ice Age. The remains of Homotherium serum – a cougar-sized cat with shorter, serrated canines – have been found at sites from Alaska to Texas, including a den near San Antonio that contains the bones of baby sabercats as well as the dismembered remnants of little mammoths that were dragged back to the cave. And the record of Homotherium goes beyond bones. In a Boreas paper published last year, paleontologist Chris Widga and colleagues announced that they were able to retrieve genetic scraps from the recently-extinct felid.

What remained of the carnivore was discovered in Minnesota’s Tyson Spring Cave. Excavators had to create a tunnel to explore the Pleistocene deposit – the front entrance is underwater – and in doing so they found a partial skull of the “elk moose” Cervalces, a deer that sported an impressive set of antlers. Once inside, researchers found a partial skull, left shoulderblade, and right humerus of Homotherium scattered through the cave. These bones represent the first known record of the sabercat in the Great Lakes region.

Genetic evidence supported what the skeletal anatomy indicated. Widga and coauthors were able to obtain a 311 base pair chunk of mitochondrial DNA from the Tyson’s Spring Cave bones, and this genetic fragment was a close match – differing in only two base pairs – from a previously-discovered Homotherium specimen.

In the larger scheme of cat evolution, Homotherium grouped most closely with the famous Smilodon, confirming that sabercats truly were an evolutionary lineage separate from other big cats. (This is why no one says “saber-toothed tiger” anymore. The sabercats split from a common ancestor with tigers and other big cats in the distant past.) This doesn’t mean that Homotherium and Smilodon were evolutionary siblings, though.

Based on anatomical evidence, paleontologists have separated Homotherium and Smilodon out into distinct sabercat lineages. Homotherium was a “scimitar-toothed cat” that belonged to the group Homotherini, while Smilodon was a “dirk-toothed cat” representing the Smilodontini (which sounds like a cocktail with bite). The separate lineages represented two different sabercat styles – Homotherium was a big leggier and had shorter, serrated canines, while Smilodon was a burlier predator with longer canines.  The reason the two came out as close relatives in the genetic analysis is that researchers have yet to extract and analyze DNA from other sabercats for comparison.

Of course, retrieving genetic clues from other sabercats will rely on the discovery of additional bones with preserved DNA. That might not be possible for the earliest sabercats, but Homotherium lived close enough to us in time that there’s hope of obtaining more scraps. The Tyson Spring Cave bones, in particular, are about 27,590-26,200 years old. Minnesota has changed quite a bit since then, though. According to Widga and coauthors, the cave was about 60 kilometers away from the nearest ice sheet in the middle of a nearly “treeless steppe-tundra environment” roamed by elk moose, mammoths, and the muskox-like Bootherium. And on that chilly open landscape, tracking the shaggy megamammals, was the last of America’s scimitar cats.

Reference:

Widga, C. Fulton, T., Martin, L., Shapiro, B. 2012. Homotherium serum and Cervalces from the Great Lakes Region, USA: geochronology, morphology and ancient DNAHomotherium serum and Cervalces from the Great Lakes Region, USA: geochronology, morphology and ancient DNAHomotherium serum and Cervalces from the Great Lakes Region, USA: geochronology, morphology and ancient DNAHomotherium serum and Cervalces from the Great Lakes Region, USA: geochronology, morphology and ancient DNA. BOREAS. 41: 546-556