Gigantic, ancient armoured 'B rex' fish discovered

Move over, T rex! Scientists have discovered B rex - a 370 million-year-old species of giant fish with external, bony armoured plates covering their head, shoulders and front fins.

Researchers identified the new fish from fossils first discovered in 2000 in Canada.

The site features fossils from the Devonian Period, a time predating most dinosaurs by hundreds of millions of years.

The new rex - Latin for "king" - belongs in the group Antiarchi, which are extinct fish.Antiarchi, which are extinct fish.

"Bothriolepis rex (Bothriolepis rex (B rex) extends the range of known body sizes for the group Antiarchi," said Jason Downs, research associate at the Academy of Natural Sciences of Drexel University in the US.

"The large body size and the thick, dense armour present a unique opportunity to address questions about the lifestyle of this unusual group of armoured swimmers," said Downs, who is also an assistant professor at Delaware Valley University in the US.

B rex's size eclipses the previous king of the antiarchs, Bothriolepis maxima. Its body length is estimated at 1.7 metres - about 30 per cent longer than B maxima's estimated length.

The large, thick plates on B rex's head were found alongside fossils of other species with thick head plates, suggesting that these were necessary to survive the "stabbing bites of large sarcopterygians," which were predatory fish from the same time period.

Despite that armour, the evidence does not point to B rex sharing T rex's ferocity.

"Bothriolepis is a group of bottom-dwelling armoured aquatic vertebrates," Downs said.

"The flat bottom and the downward-facing mouth suggest feeding on detritus plant or animal material in the mud or sand. It was not equipped for active predation," he said.

The heavy, compact bones could have also solved buoyancy issues for a fish that spent most of its time on the bottom.

One aspect of the fish immediately challenged the researchers' assumptions.

"Skull shape changes with body size in Bothriolepis," Downs said.

"Despite the gigantic size of the B rex, its skull doesn't reflect our expectations for the size. Instead, the skull shape is suggestive of a smaller Bothriolepis," he said.

The finding may alter the way we understand the size-shape relationships in Bothriolepis, according to Downs.

Although popular culture tends to think "bigger is better" when it comes to these prehistoric beasts, it turns out that size may have done B rex in.

"All antiarchs are extinct by the end of the Devonian Period," Downs said.

"We can't know exactly why B rex went extinct, but large-bodied species are often found to be at a higher risk of extinction than small-bodied ones," he said.