Sabertoothed carnivores are some of evolution’s greatest hits. Time and again, mammals with long, piercing fangs have evolved and are often epitomized by burly, impressive predators like the sabercat Smilodon. But now paleontologists have uncovered the oldest sabertoothed carnivore known, a very different hunter that belonged to a mysterious and entirely-extinct group of beasts. Among the humid forests of ancient San Diego about 42 million years ago, the earliest-known sabertooth made its debut.
The discovery of Earth’s earliest true sabertooth started in a museum collection. San Diego Natural History Museum paleontologist Ashley Poust was looking through the institution’s fossil library when he saw a curious and as-yet-unidentified jaw. The fossil was recovered from a construction site, as many southern California fossils are, and had been labeled as potentially belonging to a new species of nimravid–sabertoothed carnivores that were close relatives of cats. But upon closer inspection, as Poust and colleagues report today in the journal PeerJ, the jaw turned out to belong to a very different species of little-known mammals that evolved knife-like fangs millions of years before cats even existed.Named Diegoaelurus vanvalkenburghae, the bobcat-sized carnivore belonged to an ancient group of beasts called machaeroidines. Fossils of these creatures are so rare that it’s difficult to understand how they relate to other mammals, but they were part of the early burst of carnivorous beasts that evolved in the wake of the mass extinction that ended the Age of Dinosaurs. At about 42 million years old, Diegoaelurus was among the last of its family but also specialized in a different way than other machaeroidines. Diegoaelurus is the oldest animal, Poust says, that has the whole set of sabertoothed characteristics. Paleontologists don’t just define sabertooths by their flat scimitar fangs, but by other adaptations like the ability to shear meat with their cheek teeth, a groove in the lower jaw for their canine teeth to fit, and other feeding adaptations.
Describing Diegoaelurus is challenging because there’s nothing quite like the animal alive today. “I try to start with ‘cat-like’ and walk it back,” Poust says. While about the size of a bobcat, Diegoaelurus didn’t have the domed head–related to a bigger brain–that cats do, and the face of the Eocene predator would have been longer. Overall, Poust notes, Diegoaelurus was probably closer in appearance to a civet or a fossa. These are cat-like carnivores with lanky proportions and long faces. So a sabertoothed civet is getting a little closer to what Diegoaelurus looked like despite belonging to an entirely different lineage of beasts.
“Diegoaelurus is very different from its closest relative,” says Midwestern University paleontologist Beth Townsend, who was not involved in the new study. The new fossil species is related to a larger animal from Utah named Apataelurus kayi that was not sabertoothed, hinting that Diegoaelurus became much more carnivorous than even it’s already meat-focused relatives. The presence of such an animal raises questions about precisely what this beast was eating. Saberteeth are often interpreted as specializations to take on large prey, by delivering gouging bites to the throat or belly, but the most common herbivores 42 million years ago–such as the rhino-like brontotheres–were far too big for Diegoaelurus to subdue. “If Diegoaelurus were to take down a brontothere,” Townsend says, “it would be like a coyote-size animal taking down a rhino.”
The ecological context of the ancient beast might offer some clues. The world Diegoaelurus prowled was very different from the San Diego we know today. The area the coastal city sits on was further south back then, and the Gulf of California did not yet exist. There were no grasslands, either, and the world was a relatively warm place where lush forests filled with ancient palms, ferns and avocadoes. If a time traveler were to go back to these forests, Townsend notes, they might spot early horses, deer, bunnies and relatives of other familiar animals. Then again, she says, “During the Eocene there were also mammals that were just weird.”
Primates also inhabited North America back then, ancient cousins of today’s lemurs and tarsiers. The presence of these primates might have something to do with the evolution of Diegoaelurus. Modern fossa hunt lemurs and other primates on the island of Madagascar and are quite adept in the trees. It’s possible that Diegoaelurus evolved to fill a similar niche, Poust suggests, with adaptation nudging the mammals to a sabertoothed specialization. Regardless, the presence of such a predator indicates that mammal communities were starting to stabilize into a modern-looking set of interactions during the Eocene.
Diegoaelurus was not the only predator in its habitat, and hints that some mammals were evolving specialized diets in order to coexist. “The predatory to prey ratios were beginning to look like what we’d expect to see in a modern, living fauna,” Townsend notes, with a food web ranging from small animals like rodents up through rhino-sized herbivores and their predators. That mammal communities 42 million years ago were already developing niches and relationships resembling those we see today, Townsend says, “is damn cool.”
Naturally, Diegoaelurus was not the last of the sabertoothed carnivores. The animal lived close in time to other mammals that evolved some of the same dental adjustments, including the nimravids. Precisely what made this window in time such a good staging ground for early sabertooths is unknown. Many of these carnivores were small–not like the huge, mammoth-hunting sabertooths of the Ice Age–and belonged to different mammal lineages. Paleontologists don’t yet know why saberteeth evolved among relatively small carnivores during the Eocene, but Poust sees this study as an initial step towards better understanding the broad pattern.
Impressive as their fangs are, sabertooths have always had a somewhat tenuous existence. Even though sabertoothed mammals have evolved multiple times during the past 42 million years, no true sabertooths are left–the last Smilodon went extinct around 8,000 years ago or so. “Once you start becoming a hypercarnivore” and having a diet focused on soft flesh, Poust notes, “it’s hard to back out and switch.” If prey changes or disappears, sabertooths are less able to consume bone or other food sources and are more likely to go extinct while carnivores with more cosmopolitan tastes have better chances for survival.
Diegoaelurus is part of a story that paleontologists are just beginning to perceive, a time when many mammals existed that resembled familiar beasts but belonged to different, extinct lineages. The ancestors of today’s cats and dogs were just beginning to step onto the scene as sabertooths like Diegoaelurus were the prominent predators of their time. Despite the paleontological puzzles that await solutions, though, even just one piece of the past can offer a sense of connection to a time we can never visit. “I got to know this individual creature,” Poust says, “a sense of intimacy with a totally lost past that makes me feel like the whole world is connected.”