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Ancient rebel rewrites fish history

Fast fish A shark-like predatory fish that partly resembled tuna, terrorised ocean dwellers 240 million years ago, according to new research, shattering the myth that they were an evolutionary dead end.

The appropriately named Rebellatrix, meaning the rebel coelacanth, is described in the latest issue of the Journal of Vertebrate Paleontology .

Coelacanths are iconic fishes today, well known as living fossils. They are thought of as slow movers, so the speediness of Rebellatrix comes as a surprise.

Lead author Andrew Wendruff from the University of Alberta says the fish measured more than a metre long and had a tuna-like forked tail.

"Since the tail of a fish is used for locomotion, much can be deduced about the type of locomotion as well as its lifestyle," says Wendruff. "Fish with forked tails are able to achieve higher speeds and sustain them over a greater period of time. The forked tail of Rebellatrix indicated that it was a fast-moving aggressive predator."

Co-author Mark Wilson, also from the University of Alberta, continued that most fossil coelacanths have broad, flexible tails.

"Those coelacanths were slow moving and usually lay in wait for their prey," says Wilson. "Rebellatrix was able to search actively for the fishes that it preyed upon and catch them at high speed."

Remains of the killer fish were found on rocky slopes in the Hart Ranges of Wapiti Lake Provincial Park, British Columbia, which was once off the western coast of the supercontinent Pangaea. Rebellatrix is so unusual that it has been put in its own family. The fish represents the first major change in body shape for the coelacanth group in more than 70 million years.

Not an evolutionary dead end

After analysing its fossils, Wendruff and his team think the unusual shape comes down to two possibilities. One could be that the fossil record for coelacanths is vastly undiscovered and there are others like this fish yet to be found.

Another possibility is that Rebellatrix represents a bizarre adaptation to life following Earth's greatest mass extinction event, which happened at the end of the Permian 250 million years ago. Coelacanths then may have evolved to fill a vacant niche unoccupied by other predatory fish.

"Rebellatrix, most importantly, shatters the commonly held notion that coelacanths were an evolutionarily stagnant group in that their body shape and lifestyle changed little since the origin of the group," says Wendruff. "Rebellatrix is dramatically different from any coelacanth previously known, and thus had undergone significant evolutionary change in its ancestry."

He further believes that the fish was a "dead end in the evolution of cruising predation" since, once it went extinct, no other coelacanth evolved the tuna-like forked tail and other adaptations for shark-like hunting.

Modern tuna, of course, have that type of tail, but they aren't directly related to Rebellatrix. The researchers just think the similarities, both to tuna and to sharks, are examples of convergent evolution, when nature recycles ideas, body forms and structures in response to the needs of a species, such as those driven by environmental conditions.

John Long of the National History Museum of Los Angeles County is an expert in fossil fishes.

He says, "This is an amazing discovery which overturns the age old image of coelacanths as slow moving fishes and shows the resilience of the group to come back in true fighting form after surviving the world's most devastating mass extinction event."

Coelacanths reached their evolutionary peak back in the age of the dinosaurs. When one was caught off the coast of South Africa in 1938, it sent shock waves throughout the scientific world, since researchers thought these fish were long gone from the planet. They are now represented by two living forms, which sadly may go the way of dinos and Rebellatrix soon.

"Modern coelacanths are very rare, do not survive in captivity, and are endangered where they live," says Wendruff.