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© Velizar Simeonovski , Field Museum of Natural HistorySkull photo and artist's reconstructions of (em>Panthera zdanskyi.
A tiger-like skull unearthed from 2.5-million-year-old rock is the oldest known complete specimen related to modern big cats, according to a new study.

Representing a new species, the skull isn't that different from those of modern tigers, suggesting evolution hit on a winning formula early on and stuck with it.

Paleontologists in 2004 discovered the remarkably complete skull in eastern China. Now an international group of researchers has teased out the specimen's age and its place on the feline evolutionary tree.

The head is as big as that of a very large modern jaguar's. But the teeth and other skeletal features make it most similar to the skulls of tigers, the largest living big cats. Siberian, or amur, tigers, for example - the world's largest cats - stretch about 11 feet (3.3 meters) long and weigh in at about 660 pounds (300 kilograms). (Pictures: "Toygers" vs. Tigers.)

"The skull is smashed a bit, but it's still really beautiful. I'd love to see it in person," said Julie Meachen, a carnivore paleontologist at the National Evolutionary Synthesis Center in Durham, North Carolina, who wasn't involved in the study.

Until now, a 1.8-million-year-old set of jawbones from the Panthera palaeosinensis species was the best ancient fossils related to modern big cats.

Cranial Number-Crunching

Skull fragments of big cats dating back to 3.8 million years ago have been found but are too incomplete to provide a useful glimpse into the evolutionary history of tigers, according to the study.

The big cat that owned the recently recovered skull likely roamed China between 2.16 and 2.55 million years ago, according to the study, led by paleontologist Ji H. Mazák of the Shanghai Science and Technology Museum.

At 10 inches (26 centimeters) long, the skull is between the sizes of an average jaguar skull (about 8.7 inches/22 centimeters) and a tiger skull (about 12 inches/30 centimeters).

By plugging measurements and images of the skull into a database of fossilized and modern-day specimens, the study authors were able to place the new species - named Panthera zdanskyi - alongside tigers in the big cat family tree.

The National Evolutionary Synthesis Center's Meachen said the skull's similarity to those of living tigers and jaguars is more striking than the differences.

"[Big cats] were great at what they did right away in their evolution, so their [anatomy] hasn't changed much ... ," she said. "They were - and still are - really good predators, in part because of their extremely successful body plan."