When the curator mentioned a huge saber-toothed tiger skull stored behind the scenes of the National Museum of Natural History in Montevideo, Uruguay, Aldo Manzuetti had to see for himself.

The skull belonged to Smilodon populator. Extinct for about 10,000 years, the heavily muscled species once Hulk-smashed its way through South American fauna in the Pleistocene. To picture a normal individual, start with an African lion. Then double its size and add giant fangs.

But this one wasn’t normal. The skull was 16 inches long, making previous large specimens from the species look small. “I thought I was doing something wrong,” said Mr. Manzuetti, a doctoral student in paleontology at Uruguay’s University of the Republic. He was using the head to infer the likely size of the animal’s body. “I checked the results a lot of times, and only after doing that I realized I hadn’t made any mistakes.”

His analysis showed the skull sat atop a beast that likely tipped the scales at around 960 pounds. The specimen’s existence, he and colleagues reported earlier this month in the journal Alcheringa, suggests that the largest saber-toothed tigers might have been able to take down giant plant-eaters, heavy as pickup trucks, that researchers had thought were untouchable.