Fossils tucked away in a museum drawer and identified merely as "feline" are actually from a very ancient and enigmatic saber-toothed cat that inhabited North America more than 5 million years ago. Newly identified by a UC Berkeley paleontologist, the nearly complete skull helps clarify how these large-fanged felines evolved over millennia before going extinct about 10,000 years ago.
One clear takeaway is that these cats started out with smaller fangs - the upper canines - but evolved increasingly longer ones that may have led to their ultimate demise. California's state fossil, Smilodon fatalis (originally called S. californicus), was the culmination of that trend. It had some of the largest upper canines of any saber-toothed animal - up to 7 inches long - but was the last saber-toothed animal to survive.
According to Berkeley postdoctoral fellow Narimane Chatar, the cranium, teeth and lower jaw (or mandible) she stumbled upon in the American Museum of Natural History in New York are from the species Adelphailurus kansensis, originally discovered in Kansas and known only from jaw fragments and teeth. Now, with the first complete skull of the cat, she has been able to tentatively place the animal within the family tree of saber-toothed carnivores and contrast it with the most recognizable saber-toothed cat, Smilodon, which ranged throughout the American continent.
"Back in the days when we thought about sabertooths, we thought 'Smilodon' and that's it. We thought that all species that exhibited a somewhat saber-like tooth morphology must have hunted like Smilodon and behaved like Smilodon," Chatar said. "We are now starting to see a great disparity within those animals, and especially in the early diverging taxa, like Adelphailurus kansensis."
Narimane Chatar and American Museum of Natural History
Adapted for slicing flesh and severing arteries, the upper canines differ from those of today's cats in being flattened laterally, like a knife, instead of having a round cross-section like the canines of lions, tigers and house cats. Their premolars are also like knives, suitable for chopping and shredding meat.
But saber-shaped teeth are more fragile than the sturdier teeth of today's felines, as previous research has shown. Chatar herself has tested the sturdiness and killing efficiency of sabertooth upper canines and is currently conducting experiments on carnivore molars in the lab of Jack Tseng, a Berkeley professor of integrative biology. In simulations, 3D-printed saber teeth from various species proved ideal at penetrating a gel with the consistency of flesh but fractured easily against simulated bone. In the former tests, Smilodon came out on top. In the latter, Smilodon fared the worst.
Perhaps as a result of their fragile dentition, when these fierce hyper carnivores lost their preferred food - large plant eaters like bison and camels, which died out after the last Ice Age - they were outcompeted by carnivores with rounder and sturdier teeth, not to mention bone-crunching molars.
"Slicing and crushing are basically the two main things a carnivorous mammal's teeth can do," Chatar said. "But for saber-toothed animals, there's a clear trade off. Those upper canines were extremely efficient but also break very easily."
The new fossil was described in a paper published online June 19 in the Journal of Vertebrate Paleontology.
Were saber-like teeth a dead end?
Despite the ultimate extinction of saber-toothed animals, for several tens of millions of years large upper canines were a thing. They were found not only in two groups of felids (the group that includes living cats) - sabertooths such as Smilodon and scimitar-toothed cats called Homotherini - but also in a group of marsupials called Thylacosmilids in South America and an ancient lineage of cat-like carnivores called Nimravids. Thylacosmilus, which died out several million years ago, had continuously growing saber teeth that rivaled those of Smilodon.
Narimane Chatar/UC Berkeley
"There was a crazy variety of saber-toothed cats," Chatar said. "Taxa like Smilodon and Homotherium and other species with very long upper canines tend to attract all of the attention, but most of the early diverging species in this group actually had shorter upper canines. Unfortunately, we know more about the late-diverging species, like Smilodon and Homotherium, than we know about those early-diverging ones."
Chatar's PhD thesis focused on the evolution of saber-toothed animals. While conducting her research, she visited numerous museums around the world with a portable laser scanner to obtain detailed surface scans of sabertooth fossils. She later combined these scans to reconstruct precise 3D images of each. While at the American Museum, she pulled out every drawer labeled "felids" or "cats" and noticed one cranium from Arizona labeled Pseudaelurus that looked suspiciously unlike modern cats. Pseudaelurus means "false cat" and is a "wastebasket" term used to label any unidentified fossil that looks like a cat, she said.
"I was a bit intrigued because the cranium was quite complete and had a fragmentary mandible with the whole dentition," she said. "And in the drawer I also found upper canines. When I saw that they were laterally compressed, I knew it was not a cat or a tiger."
She scanned it but forgot about it until she was at the Yale Peabody Museum in New Haven to scan a cast of the original fossil of Adelphailurus kansensis. The original, or holotype, has been at the University of Kansas since it was discovered in 1924, though it was only described as a new species in 1934. The cast looked remarkably like the fossils at the American Museum.
Narimane Chatar, J. Gamarra, AMNH
After completing her thesis from the University of Liege in Belgium, Chatar moved to Berkeley and dived into other research: comparing the biting and crunching abilities of many types of carnivores, living and extinct, to determine how these abilities were correlated with tooth and skull shape. Only after that project got fully underway did she take up the "side project" of analyzing the Pseudaelurus skull. In the new paper, co-authored with Tseng, Chatar describes the features of the American Museum fossil that align with the original Adelphailurus fragments found in Kansas and bits of similar fossils later discovered in Nebraska, Texas and Mexico.
One difference is that the snout is quite narrow and long in Adelphailurus, she said, which is different from other saber-toothed cats from the same era. The animal's teeth also have slight serrations on the edge, which are found in some but not all sabertooths and perhaps operated like serrated knives.
The shorter upper canines of the ancestral species of saber-toothed animals, including Adelphailurus, support Chatar's theory that once these hyper carnivores started to evolve longer fangs, they were unable to get rid of them.
Courtesy of Narimane Chatar/UC Berkeley
"We've never found any lineage that started developing long upper canines and then stopped and went back to a less specialized state; once a group starts, they (the fangs) go crazy and then they go extinct," Chatar said. "Saber-toothed carnivores are an example of something called the macroevolutionary ratchet, where basically, you start developing some super specialized morphology and you get very efficient at doing one single thing, but when the environment changes to make that thing harder - harder to find prey and hunt, for example - then you're just more likely to go extinct."
Chatar's chance discovery of the Adelphailurus suggests that other unique and important specimens lie undiscovered in museums.
"It highlights the need to go back to those old collections and open every single drawer and look at those specimens, because there might be amazing fossils like this one just hidden somewhere, labeled cat or Pseudaelurus or something else, that just need to be described," she said.
The work was supported by grants from the U.S. National Science Foundation (NSFDBI-2128146), Belgium's Fund for Scientific Research (FC 36251) and the Belgian American Educational Foundation and by a Marie Skłodowska-Curie Actions Postdoctoral Fellowship.
