Killer Canadian coelacanth discovered

ORIGINAL: CosmosMagazine

The fast-swimming coelacanth Rebellatrix chasing smaller species of fishes in the Early Triassic ocean west of Pangaea. Credit: Artwork by Michael Skrepnick.

LONDON: Canadian scientists have discovered a fossilised ‘rebel’ coelacanth fish with an unusual body shape that suggests it was a fast-swimming predator.

The finding suggests that contrary to popular belief, coelacanths, the so-called ‘living fossils’, have undergone significant morphological change during their history.

“Rebellatrix divaricerca, most importantly, shatters the commonly held notion that coelacanths were an evolutionarily stagnant group,” said Andrew Wendruff from University of Alberta, in Edmonton, one of the researchers who discovered the fossil and lead author of the paper published in the Journal of Vertebrate Palaeontology.

Dramatically different to other coelacanth

The largest specimen of Rebellatrix, though partial, shows that it grew upwards of 1 meter in length. Credit: Photograph and drawing by A. Wendruff

“Rebellatrix is dramatically different from any coelacanth previously known, and thus had undergone significant evolutionary change in its ancestry,” he said.

Until the 1930s coelacanths were thought to have been extinct since the Late Cretaceous period, around 70 million years ago, when dinosaurs were still alive on Earth.

However, in 1938 a fisherman in Africa caught a living coelacanth. The most striking thing about the newly discovered fish was that their body plan differed very little from ancient fossil specimens resulting in the fish being referred to as ‘living fossils’.

Rebel without a cause

The modern coelacanth, Latimeria chalumnae, and most of the fossil specimens discovered to date have broad, flexible tails suitable for moving at slow speeds and lying in wait for prey.

In contrast, “the forked tail of Rebellatrix indicated that it was a fast-moving, aggressive predator,” said Wendruff.

“Rebellatrix was able to search actively for the fishes that it preyed upon and catch them at high speed,” added Mark Wilson, co-author of the study and fellow University of Alberta researcher.

Wendruff and Wilson found the R. divaricerca fossil in the Lower Triassic Sulphur Mountain Formation near Tumbler Ridge in the Rocky Mountains in British Columbia, Canada.

It is thought to be from the Early Triassic period, approximately 250 million years ago. The closest coelacanth fossil in age is known as Alenypterus. It dates from the Mississippian period and is at least 70 million years older. It also has a very different body plan that is more similar to that of living coelacanths than that of Rebellatrix.

Prior to findings such as Rebellatrix, scientists believed that coelacanths have changed little over time, but this now seems unlikely.

“The similar body forms are not a result of close relationships,” said Wendruff. “Nature is constantly recycling ideas, body forms and structures. This is referred to as convergent evolution. Many times there is one particular form that is most efficient for a particular function.”

When asked about why Rebellatrix evolved such a different body shape to other members of the coelacanth family, Wendruff suggested that as the mass extinction event that happened just before the beginning of the Triassic period wiped out 90% of marine life Rebellatrix could have evolved to fill a newly vacant predator ‘niche’ in the marine ecosystem.

Forked tail before it's time

“Large predators seem to be particularly vulnerable during such intervals, so this extinction might have 'opened the door' for coelacanths to explore new functional roles,” said palaeobiologist and coelacanth expert Matt Friedman from the University of Oxford in the UK.

The newly evolved body form was not successful, however, because “while other fork-tailed fishes appear later in the fossil record, Rebellatrix and its descendants are noticeably absent. This leads us to believe that Rebellatrix was a dead end in the evolution of cruising predation,” said Wendruff.

Friedman said: “this discovery adds to a growing view that coelacanth evolution was more nuanced than textbook portrayals would suggest.”

He added: “It is the latest in a series of finds over the past decade or so that have shown coelacanths didn't just have a static evolutionary history when it came to body form. Rebellatrix, with its unusual tail fin and body proportions, can be added to other anatomical 'experiments' in coelacanth history, including eel- and leaf-shaped forms.”

Wendruff believes that other unique forms of coelacanth have yet to be found and that “in the future they will have many more surprises in store.”