A new species of killer coelacanth that stalked Triassic seas has been identified from museum fossils, researchers say.

The coelacanth (pronounced SEE-la-kanth) is a type of primitive, slow-moving fish that was thought extinct until its rediscovery in 1938. The modern fish is sometimes called a living fossil, because it apparently existed largely unchanged for 320 million years. (See "Pictures: New 'Rebel' Coelacanth Found.")

But the newfound animal, dubbed Rebellatrix, is bizarre compared with previously known coelacanths, living or extinct, said study leader Andrew Wendruff, a biologist at the University of Alberta in Canada.

That's because all other known coelacanth species have broad tails designed to lunge short distances after prey, Wendruff said.

By contrast, the 3-foot-long (0.9-meter-long) Rebellatrix had a massive, muscular tail built for chasing prey at high speeds, much like the tail of a modern-day tuna.

"The stiff, forked tail and the more streamlined body indicate that this fish could achieve and sustain much higher speeds than other coelacanths," study co-author Mark Wilson, also of the University of Alberta, said by email.

"It could cruise around, covering a lot of territory searching for prey, then capture it by swimming at it quickly."

According to study leader Wendruff, the team named the discovery Rebellatrix because, like a true rebel, "it does everything a coelacanth should not do."

(Read about other scientific names with creative twists.)

Rebellatrix Thrived After Mass Extinction

The Rebellatrix fossils were collected in the 1950s and '80s on the rocky slopes of Wapiti Lake Provincial Park in British Columbia, Canada. This area was the western coast of the supercontinent Pangaea at the time the new coelacanth lived.

The specimens were stored at Royal Tyrrell Museum in Alberta and the Peace Region Palaeontology Research Centre in British Columbia.

In 2009 Wendruff examined a Rebellatrix fossil from these collections, and at first "I didn't believe what I was seeing," he said.

It was "only when I found a second, third, and fourth ... that I realized we had something real and something significant."

Based on the recovered specimens, it seems Rebellatrix first appears in the fossil record about 250 million years ago—immediately after the Permian extinction, when 90 percent of life on Earth was snuffed out by an unknown cause.

The massive loss of life "possibly left a gap in this lifestyle" of fast-moving ocean predators, allowing Rebellatrix to fill that role, speculated Wendruff.

"Otherwise, we wouldn't expect the coelacanth to do this."

New Coelacanth a "Spectacular Failure"

In general, the discovery "shows how plastic and flexible evolution can be," said John Long, a coelacanth expert at the Natural History Museum of Los Angeles County in California.

It really shakes things up "that coelacanths can suddenly deviate what they've been doing for 200 million years and occupy a lifestyle that's radically different from other coelacanths." (See more coelacanth pictures.)

Still, the fossil record shows that the slow-moving version of the coelacanth ultimately won out, while the speedy Rebellatrix was replaced by sharks and other cruising predators, study leader Wendruff said.

"I like to say Rebellatrix was a spectacular failure."