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Ancient Cat May Reshape Feline Family Tree
12 November 2013 7:15 pm
The world’s first big cats may have arisen millions of years earlier than previously thought. That’s the conclusion researchers are drawing from a newly discovered species of feline, similar to today’s snow leopard, that lived in the ancient Himalayas. Though the creature doesn’t have any living descendants, it may force researchers to rethink the cat family tree.
Much of our knowledge about the origin of ancient cats comes from the DNA of living ones. In an extensive 2006 study, researchers made a rough sketch of the evolutionary history of pantherines, the lineage that includes today’s tigers, lions, leopards, and jaguars. They used overlap between the DNA sequences of modern species to backtrack to when various cat lineages likely diverged. According to this picture, the first pantherine evolved from an unknown ancestor, probably living in Central Asia, 10 million to 11 million years ago. Later research suggested that the big cat lineage didn’t start branching into other species—ancestors of modern tigers, for example—until roughly 2 million years ago.
But paleontologists have been reluctant to accept this DNA-based picture, says Julie Meachen, a vertebrate paleontologist at Des Moines University who specializes in carnivores. “We want to actually see the fossil,” she says. Until now, the oldest pantherine fossils were 3.8-million-year-old teeth and jaw and skull fragments found in East Africa, not Asia.
For 8 years, vertebrate paleontologist Z. Jack Tseng of the American Museum of Natural History in New York City has been part of a team searching for fossils in the cold, dry Tibetan Plateau—a landscape he says is reminiscent of South Dakota’s Badlands. In 2010, the group discovered a fossil-rich spot in an area called Zanda Basin: 120 fragments from more than a dozen mammal species were crammed into about one square meter of ground.
Among the limbs of extinct antelopes, horses, and rhinos, the researchers turned up a few rare fragments—a skull, several jaws, and teeth—that seemed to belong to a species of cat. Based on the area’s geography, they suspected it would be a relative of the uniquely cold-adapted snow leopard. They soon discovered that the fragments came from at least three individuals of a never-before-seen species dating back 4 million to 6 million years—older than the oldest African find.
In a paper published online today in the Proceedings of the Royal Society B, Tseng and his colleagues introduce Panthera blytheae, named for the daughter of avid supporters of the Natural History Museum of Los Angeles County, with which several of the authors are affiliated. The creature, which they believe to be a sister species of the snow leopard, was a dwarf compared with modern lions and tigers. With an estimated weight of about 20 kilograms, it was roughly 10% smaller than the snow leopard. But it appears to share some of that cold-dwelling carnivore’s features, such as a wide forehead, believed to represent an expanded sinus cavity where frigid Himalayan air warmed up with each inhale.
The Himalayan finding is “a nice surprise,” says Andrew Kitchener, a mammalogist at the National Museum of Scotland in Edinburgh whose team uncovered a primitive member of the tiger lineage in China in 2011. “It’s given us a new part of the world to look at for the evolution of the big cat lineage.”
The group constructed a new evolutionary tree by combining physical features of the blytheae bones with features of other fossils, plus DNA data from living species. Its analysis pushes back the emergence of big cats to roughly 16.4 million years ago. This number has a wide margin of error, Tseng cautions. But more importantly, he says, by 6 million years ago (when previous research claimed big cats had not yet diversified), at least three separate lineages likely roamed Asia: one containing P. blytheae and the snow leopard, one containing the clouded leopard, and another leading to the modern tiger. (The ancestors of jaguars and lions probably arose later.) The team suggests that when shifting tectonic plates forced the Himalayas upward, many mammals—including, according to their new tree, the emerging pantherines—diversified in this snowy refuge. Some species then spread out across the continent during the Pleistocene ice age.
The research gives new support to the idea that the first big cats radiated from Central Asia, says William Murphy, a molecular geneticist at Texas A&M University in College Station and an author on the 2006 study. But he is skeptical of the claim that P. blytheae is a sister species of the snow leopard. With only a few pieces of the skeleton, the group determined this relationship using a limited number of subtle features of the teeth, skull, and jaw, he says, which may not be reliable.
“It’s possible that this fossil species might have a deeper ancestry in the Panthera tree,” he says, in which case they weren’t a part of the more recent diversification that Tseng and his colleagues link to the rising Himalayas. Rather than being nestled among the lineages that led to modern-day cats, it may have been an outlier, which happened to evolve snow leopard-esque features to survive at the top of the world. If P. blytheae actually belongs somewhere else on the tree, this would change the estimates for important splits in the big cat lineage. The only way to clear up these relationships: Dig up more complete fossils.