Fossilized teeth of Ice Age predators yield clues to why certain species survived

Visitors to the La Brea Tar Pits in Los Angeles are accustomed to seeing recreated scenes of, say, extinct saber-toothed cats taking down a horse or bison in an open field. But according to the most detailed study yet of Ice Age predators by paleontologists at Vanderbilt University, this would have been an extremely rare occurrence.

They describe a different ecological scenario in their latest paper in Current Biology—one in which saber-toothed cats preferred to hunt under the cover of a forest, while dire wolves preferred to track their prey in open environments. And their findings may offer valuable hints about why some ancient predator species went extinct (dire wolves and saber-toothed cats), while others survived (coyotes).

The so-called Pleistocene Epoch dates from 2.5 million years ago to roughly 10,000 years ago, a time period that includes dramatic shifts in climate and the evolution of humans into the ecological mix. The La Brea Tar Pits is one of the best repositories for preserving the remains of now-extinct ancient species from this era. For tens of thousands of years, animals would become mired in the tar, gradually getting sucked down into the pit before dying of asphyxiation. Over time, their remains became fossilized as the lighter fractions of the petroleum evaporated, leaving the bones trapped in a more solid substance until archaeologists uncovered them thousands of years later. And there are far more carnivore and predator specimens in the pit than herbivores, perhaps because carnivores were drawn to the pits when herbivores were mired in the tar and became mired themselves.

The first recorded sighting of the La Brea Tar Pits was in 1769, when the Spanish governor of Baja California headed an excursion down what is now Wilshire Boulevard in midtown Los Angeles. The expedition observed large marshes of a pitch-like substance, bubbling and boiling. The George C. Page Museum of La Brea Discoveries now stands adjacent to the pits, where scientists continue to excavate new fossilized remains from the tar to this day, aided by volunteers during the summer keen to hunt for fossilized treasure.

Vanderbilt University paleontologist Larisa DeSantis grew up in the Los Angeles area and frequently visited the tar pits in her youth, even taking museum classes on offer. But she didn’t start studying the fossilized creatures in earnest as a professional scientist until after 2009, when she and colleagues studied the micro wear on teeth of modern animals to determine whether the wear patterns could tell them anything about the dietary habits of those animals. Cheetahs, for instance, will avoid bone, while scavenging hyenas will devour it. Lion behavior falls somewhere in between. The dental wear patterns matched that observed behavior.

With that proof of principle, they applied the same approach to studying fossils excavated from the La Brea Tar Pits. For example, a 2012 paper published in PLOS ONE contradicted a popular “tough times” hypothesis suggesting that the La Brea carnivore species that went extinct did so because the changing climate made food scarce, and hence there was more competition for fewer resources. This was based in part on the presence of broken canine teeth in American lions, suggesting the animals had broken their teeth chewing on the bones of scavenged carcasses. But DeSantis and her colleagues found no evidence of this when they examined the teeth of lions and other Pleistocene species under a powerful microscope.

“We found that the American lion was a bit more cheetah-like, and saber-toothed cats were a bit more lion-like, but they weren’t hyena-like [in their dietary behavior],” said DeSantis. “None of the animals were desperately chewing on carcasses.” Subsequent studies looked at Pleistocene cougar teeth for indications of their dietary habits, which did show signs of hyena-like scavenging—possibly one reason cougars survived—as well as the giant short-faced bear, dire wolves, and even herbivores like bison, horses, and camels.

The current study combines that earlier work with a large amount of fresh data—the teeth of some 135 saber-toothed cats, for instance, compared to just 15 in the earlier study—with the objective of gaining a better understanding of how predators and herbivores co-existed during the Pleistocene. She and her cohorts were particularly interested in analyzing the isotopes contained in the samples, since those hold all the information needed about an animal’s diet. “You are what you eat,” she said. “Everything you eat is incorporated into your tissue—your hair, your enamel, your fingernails—but it does so at different fractionations.”

Prior studies of isotopes in the fossilized bones of the La Brea nails indicated a lot of overlap in their dietary preferences—more support for the “tough times” hypothesis. But DeSantis found that wasn’t the case with the isotopes present in the teeth. Cats like the saber-tooth, lion, and cougar hunted in forests, while canine species like the dire wolves hunted bison and horses in open fields. It’s not yet clear why the isotopes should be different in bones and teeth, but DeSantis said it might have something to do with the enamel on the latter. “Enamel is highly inorganic, so it’s less prone to alteration after it’s been deposited [in a tar pit],” she said. That makes enamel the gold standard in terms of preserving isotopes.

So what does all this data tell us about why so many species became extinct by the end of the Pleistocene? “We can’t really say for certain what the cause of the extinction was, but we can say what the consequences of the extinction were,” said DeSantis. The species that survived, like cougars, were those best able to adapt their dietary behaviors, switching from hunting increasingly rare large prey to smaller mammals, and/or scavenging. The saber-toothed cat, for instance, simply couldn’t adapt. “I always joke that I’d like to see a saber-toothed cat take down a rabbit,” said DeSantis. “They were not effective hunters of smaller things, and being so highly specialized was likely to their detriment.”

DeSantis is currently participating in an even bigger study across six different institutions, in hopes of further clarifying the respective impacts of dramatic shifts in climate and the arrival of humans on species extinctions. “The upside of tar seeps is that you have remarkable preservation, especially of carnivores,” said DeSantis. “The downside of La Brea is that it’s a jumble of bone. Unlike most fossil sites where you have deposition just like your laundry hamper—things on the bottom are the oldest, things on the top are the most recent—these things get mixed around.”

The upcoming study will use radiocarbon dating on some 400 bones from different species excavated from different pits. The researchers can then say with confidence that a given animal lived at a precise time period, and they can then cross-reference that with corresponding climate records. “We will be able to more precisely trace the ecology of these animals through time as climates are changing, and then as humans are arriving on the landscape, to better get at the causes of extinction, at least locally at La Brea,” said DeSantis.

DOI: , 2019. 10.1016/j.cub.2019.06.059  (About DOIs).

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