Amateur fossil collectors discovered a completely new type while searching a mining fissure in central Florida.

"The find is the most interesting fossil carnivore discovery since 1970," says Larry Dean Martin, senior curator of palaeontology at Kansas University's Natural History Museum.

Dr Martin examines the three types of sabre-toothed tiger

Dr Martin is an authority on sabre-toothed cats and believes the two fossilised animals represent a completely new species of sabre-toothed tiger.

He said the "xenosmilus" - the proposed name for the new find - will force researchers to rewrite the textbooks on the extinct animal.

Different categories

"It appears to be a new design of sabre-toothed cat," Dr Martin explained. "The diversity of sabre-toothed cats in North America has changed by one-third."

Before the discovery, the cats fell into two categories based on their upper canines:

• Dirk-tooth - two long, narrow upper canine teeth and short legs. This animal was built like a bear
• Scimitar-tooth - two shorter, broader upper canines and long legs. This cat looked more like the modern day cheetah.

The new discovery "blew that notion away," said Dr Martin. The xenosmilus has two short, broad upper canines and short legs.

"When I first saw the picture of the cranium with the two short and broad upper canines, I knew what it was," Martin said. "Then I saw the legs and the two didn't go together."

Social animals

The two collectors found the skeletons while searching for fossils of peccaries - large wild pigs. In one spot, they found dozens of pig remains and also the two big cats.

The location was probably a den for the two animals in which they devoured their game. The tiger fossils are estimated to be more than one million years ago.

Teeth could have been only for display, like antlers

According to Larry Dean Martin, xenosmilus would appear to be a more specialised sabre-toothed cat.

"This raises the questions: did the xenosmilus become extinct at this time, and did the smilodon - a more modern sabre-toothed cat about 20,000 years old - become dominant only after the xenosmilus became extinct?"

Prior to this discovery, the best-known location for sabre-toothed cat fossils were in the fossil tar pits at Rancho La Brea in Los Angeles.

Sabre-toothed tigers are among the most impressive and best known carnivores to have lived during the last Ice Age. They probably lived on grassy plains and in open woodland.

The purpose of their fearsome teeth has been hotly debated. Some have suggested the canines were used for killing prey.