The Sabertooths, Part 5: Smilodon and Homotherium

Here we are in the Pliocene, some two and a half million years before the present.

Plate tectonics has brought North and South America together for the first time, although the exact timing of this junction is debatable, and the first ice age is looming. (Agustí and Antón; Farris and others; Jaramillo and others; Lyle and others)

The most advanced cats ever – Homotherium, Megantereon, and Smilodon – are prowling around, forcing the ancestors of modern cats to keep a low profile, and you and I are at a crossroads.

A series on sabercats must go in one of two different directions at this point.

The usual thing to do next is to discuss the Homotheriini and Smilodontini tribes. We’re going to do this.

But there’s a problem with this two-tribes approach.

Antón adds in yet another tribe – the Metailurini – because they had some sabertooth features. However, another source (Werdelin and others) says that those fossil cats are very hard to herd, i.e., to classify correctly.

I think the Metailurus group fits better into a discussion of some cutting-edge research that defines saber-cats differently and leads to some surprising conclusions (Christiansen), so let’s look at the Metailurini and another toothy cat called Nimravides tomorrow.

The Homotheriini

Quite apart from the whole scimitar- and dirk-tooth thing mentioned earlier this week, Homotheriini and Smilodontini were two very different sabertoothed cat groups.

Generally speaking, Homotherium species tended to be long-legged, like a modern lion. They were also rather lightly built compared to Smilodon. (Turner and Antón; Werdelin and others), although their powerful forearm muscles were also very good at holding down and immobilizing prey. (Werdelin and others)

Xenosmilus was the homotherin exception to this lion-legs rule. Its teeth were rather odd, too.



Meet Xenosmilus hodsonae, which lived in what is now Florida around 1 Ma (million years ago). (Antón)

This cat had the flat, coarsely serrated sabers of a homotherin, but it also had an arc of big incisors and huge scissor-like cheek teeth for slicing meat. (Antón)

Some experts suspect that Xenosmilus used those incisors and lower canines to take out big chunks of flesh instead of what was probably the more typical killing bite that severed a hapless victim’s neck veins and arteries with the saberteeth while clamping down hard on its windpipe. (Naples and others, 2011; Turner and Antón; Wheeler)

However, the strangest thing about this likely relative of Homotherium is that its legs were short and powerful – like those of the Smilodontini. (Antón)

One explanation for this uses an idea called “ecomorphs.”

According to ecomorph theory, certain body types adapt to fit into specific environments. Generally speaking, the cat-like ecomorph goes with a habitat that has both open areas and trees – perfect for a stalk-and-ambush hunter. (Martin, 1989)

Pursuit hunters usually have longer legs than a cat does. The extra length may help these predators conserve energy during the chase. (Andersson and Werdelin)

Since Homotherium generally had long legs, it may have been best adapted to open areas where it could chase prey, perhaps in a group. (Antón)

Xenosmilus, on the other hand, might have evolved after some homotherins moved into a more forested area, where ambush was a better way to hunt. Shorter, more powerful legs, like those of a modern jaguar, would work better here than speedy lion-legs. (Antón; Schellhorn and Sammgaraja)

This is just an hypothesis, since no clearly supporting evidence has been found yet, but it’s an intriguing possibility.

And it’s a reminder of that paleontologists are very aware of something very few of us laypeople ever realize. Most of the important information we use to understand modern cats – things like hunting behavior and socializing – just doesn’t exist in the fossilized remains of extinct cats.

This information gap makes it very difficult for scientists to understand how the sabertoothed cats lived and why they went extinct.


As far as experts know right now, there were only three species of Smilodon, and they all lived in the New World.

The oldest known species is Smilodon gracilis. It was a sturdy sabertooth, the size of a modern jaguar, and resembled Megantereon, although its teeth and skeletal details were more specialized. (Antón)

Gracilis fossils have been found in the eastern and southern US, and as far away as South America. (Wallace and Hulbert)

Next came Smilodon fatalis. This is the movie star; the California state fossil; the Smilodon of Rancho la Brea asphalt pits. (Antón)


La Brea circa 1910. Any animal or human that carelessly splashed into this deceptively quiet water was trapped and sank into the tarry depths – so did scavengers and predators attracted by the victim’s struggles.

S. fatalis was as tall as a lion but much more of a hulk. A typical adult probably weighed up to 600 pounds (280 kilograms) or more. (Antón)

Its prey was big, too. Over half of the plant-eaters studied at La Brea probably weighed more than 660 pounds (300 kg) in life. (Van Valkenburgh and Hertel)


Three cheers for those who painstakingly clean sticky tar off fragile fossils without destroying them!

Hundreds of these saber-cats died at La Brea. Their preserved bones have given investigators DNA. (Antón; Barnett and others)

As well, the Fatalis skeletons provide an intriguing look into this cat’s lifestyle and some of the injuries it was prone to (Antón):

  • Smilodon didn’t break its saberteeth any more often than its other teeth, but all of the large carnivores at La Brea – wolves, lions, and coyotes, as well as cats – had more tooth fractures than modern ones do. (Van Valkenburgh and Hertel)
  • Smilodon repeatedly strained its shoulder muscles, probably while pulling in prey.
  • Chronic sternum injuries suggest that Smilodon smashed into prey chest first when it attacked. Yes, I’d like to see a video of that, too.
  • Pulled spinal muscles show how hard it was for even these big sabertooths to hold onto Ice-Age megafauna.
  • Leg and foot fractures probably happened accidentally or as the victim struggled.
  • Fights. One Smilodon skull has a hole in it the size of a Smilodon sabertooth; another saber-cat has a similar hole in its shoulder blade. (Antón) And one wolf skull has part of a Smilodon saber still embedded in its forehead. (Martin, 1980) We’ll never know if the victorious cat was able to survive with just one working sabertooth.

The hole in that one cat’s skull didn’t heal – it died of its wound. Usually, though, these skeletal signs of damage show some mending – the injured cats lived on for a while. (Antón)

One Smilodon broke its neck . . . and the fracture healed. (Rothschild and Martin)

This was one rugged cat!

Gracilis and Fatalis are the only Smilodontini that have been found in North America thus far. (Antón)

But there is a third species known. It developed in South America, where an ambush-and-slash niche went vacant just before sabertoothed cats got there.

Before this smilodontin species appeared, one sabertoothed predator had South America – essentially a gigantic island before bumping into North and Central America – all to itself.


This is Thylacosmilus atrox. It looks slightly different from the saber-cats because it was a marsupial – a completely different kind of mammal from nimravids, barbourofelids, and cats.

Thylacosmilus developed its impressive saberteeth through convergent evolution.

No one is sure why South America’s marsupial sabertooth went extinct, but the reasons may have involved climate change. (Antón; Turner and Antón)

Both Fatalis and Gracilis, as well as Homotherium and many other North American carnivores and plant-eaters, were able to cross over a land bridge – the Isthmus of Panama – when plate tectonics finally joined North and South America together. (Antón; Prothero, 2006)

With Thylacosmiulus gone, there was plenty of room on the southern continent for Smilodon to evolve, and soon S. populator – one of the biggest sabertoothed mammals ever – appeared. (Antón; Turner and Antón)

That second name doesn’t mean that it was popular. “Populator,” according to sources in Antón, means “he who brings devastation.”

Fair enough for a cat 4 feet high at its incredibly muscular shoulder, good at leaping despite a weight of almost 900 pounds (400 kg), with paws bigger than any living cat’s, and saberteeth protruding half a foot (17 cm) below its jaw. (Antón; Turner and Antón)

If you have Antón’s book, check out Figure 3.67. It’s his illustration of Populator galloping across a field, and here is Antón’s description of how he made that impressive paleoart image.

I know cats move silently, but it’s hard to believe that the ground didn’t rumble and shake when Populator was in a hurry or when it collided with one of the gigantic Pleistocene plant-eaters during an attack.

Smilodon populator and the smaller S. fatalis divided up South America between them during the closing millennia of the Pleistocene, with Populator thriving east of the Andes, from Venezuela to Patagonia, while Fatalis terrorized the lands along the western coast. (Antón; Turner and Antón)

And so the world’s ambush-and-slash niche was occupied until roughly 11,000 years ago, when the last known Homotherium disappeared.

At that point, Smilodon had been extinct for some 2,000 years. (Werdelin and others)

A great quiet did not descend upon the world when the saber-cats went away, because cats with “normal” upper canines were still around and were as vocal as ever.

But since then, Earth has been missing something powerful and terribly beautiful.

To be continued tomorrow.


Featured image: “Homotherium serum model – Cleveland Museum of Natural History 2014-12-26.” T. Evanson. CC BY-SA 2.0.

Xenosmilus, Gainesville-Florida Museum of Natural History. Jared. CC att 2.0 generic.

La Brea asphalt seep, circa 1910. Public domain.

Smilodon skull from La Brea: James St. John. CC BY 2.0.

Thylacosmilus atrox, American Museum of Natural History. Claire Houck. CC Att-SA 2.0 Generic.

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This series on sabertooths was originally posted at my Robin Huntingdon blog about a year ago.

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