Tracing the Roots of Smilodon

Smilodon was the ultimate sabercat. About the size of a tiger, but with burlier arms and elongated fangs that have made the genus famous, the Ice Age carnivore stalked camels, bison, and other herbivorous prey across North and South America until about 10,000 years ago. But where did this fierce felid come from in the first place? Another sabercat discovered in Florida may help fill out the backstory of Smilodon.

Dubbed Rhizosmilodon fiteae, the fossil cat is described by paleontologists Steve Wallace and Richard Hulbert, Jr. in a PLoS One study published last month. The debut of the jaguar-sized sabercat relied on new finds adding context to old ones.

The fragmentary remains of Rhizosmilodon, Wallace and Hulbert report, come from the 5 million year old fossil deposits of south-central Florida. From the time of the site’s discovery in 1989 until now, the fauna seemed to include two sabercats – the larger Machairodus coloradensis, and a smaller form originally referred to the genus Megantereon. But based on the characteristics of newly-excavated material, Wallace and Hulbert propose, that smaller cat is near the base of the lineage from which both Megantereon and Smilodon split.

For now, all paleontologists know of Rhizosmilodon are the lower jaws. The original specimen is a mandible broken at both ends, and one of the newer specimens is a small piece containing the cat’s slicing cheek teeth. But the most representative piece is a nearly-complete mandible that stretches from the point where the lower jaw bones met each other back to just beyond the first molar. (A pair of upper arm bones were found at the site, too, but whether or not these really belong to Rhizosmilodon relies on the future discovery of articulated material.)

Based on the features of the jaw and preserved teeth, Rhizosmilodon appears to have been near the root of the lineage to which both Smilodon and Megantereon belonged. (Hence the “rhizo” prefix.) This suggests two possible patterns for the origins of Smilodon and Megantereon, Wallace and Hulbert suggest.

The occurrence of Rhizosmilodon in Florida 5 million years ago might mean that both Smilodon and Megantereon soon split from a common ancestor in North America, with Megantereon proliferating throughout the Old World while Smilodon pioneered the New. Then again, Rhizosmilodon appears to be more like Smilodon than Megantereon in some ways, meaning that there might be an even older, as-yet-undiscovered common ancestor between the Smilodon and Megantereon lineage that probably existed in the Old World. In this case, the ancestors of both lineages would have invaded North America at different times.

Whichever scenario is closer to what actually happened, the presence of Rhizosmilodon in the 5 million year old fossil beds of Florida extends the range of North America’s most charismatic carnivores further back in time than paleontologists previously knew. And the find makes it all the more perplexing that Smilodon is no longer with us. The sabercat’s lineage prowled North America for at least five million years, persisting through climate change as well as ecological and evolutionary shifts among prey. The world lost Smilodon only yesterday, leaving us to draw the cat’s story from bones.

Reference:

Wallace, S., Hulbert, R. 2013. A New Machairodont from the Palmetto Fauna (Early Pliocene) of Florida, with Comments on the Origin of the Smilodontini (Mammalia, Carnivora, Felidae). PLoS ONE 8. 3: e56173. doi:10.1371/journal.pone.0056173