When the genus of a fossil fish is changed most people do not get that excited — after all, Angelacanthus went extinct long before the first dinosaurs walked this Earth.
The interesting story of how and why its name has been changed is the subject of a paper published in the Journal of Vertebrate Paleontology last week by palaeontologist Dr Robert Gess of the Rhodes University geology department and Dr Carole Burrow of the Queensland Museum in Australia.
The story began almost 360-million years ago when a small fish that could have looked much like a modern day angelfish was swimming around in an estuary not far from the South Pole.
It was only about 10cm long and had long thin spines on the forward part of its body and smaller spines further back.
Fossils do not give any indication of the colours of their original owners, but it is possible that Angelacanthus had stripes similar to those of a modern day angelfish.
We do not know how the Angelacanthus died but its body settled into the mud at the bottom of the estuary.
Over millions of years, the mud was transformed into black shale that preserved its body.
The shale deposits were inexorably moved around during that extended period.
They were thrust deep below the surface of the Earth and compressed under the pressure of mountains while tectonic plates carried them further north, far beyond the Antarctic Circle.
They were eventually pushed up, re-emerging on Waterloo Farm just outside a town we now know as Makhanda.
In 1999, Gess was retrieving tonnes of shale from the roadworks on the N2 highway.
As he was walking down the hill, he noticed a huge rock that had fallen halfway down the slope.
I took out my hammer and chisel as I went past and I gave it a bit of a whack
“I took out my hammer and chisel as I went past and I gave it a bit of a whack, and it just exposed a small patch of soft tissue at the time and I actually thought it was probably from a shark,” he said.
He then sat on the hillside and “I carefully flaked off, flake by flake, exposing the whole fish and at the same time, reconstructing the flakes on the other side, simultaneously to get a counter specimen”.
It quickly became clear to Gess that it was not a shark, “but it was a kind of fish I had never seen before. In fact, as I was later to discover, it was a kind of fish that no-one had evidence of”.
He formally described the fish in a paper published in 2001 putting it in the Diplacanthus genus within the extinct acanthodian group of fish.
The newly discovered fish was named Diplacanthus acus.
The fish retained that name until Burrow took on the task of revising the genus Diplacanthus.
An expert on the genus, Burrow came to Makhanda to work with Gess because the revised definition of Diplacanthus now excluded the fish found on Waterloo Farm — Diplacanthus acus.
All the other diplacanthids have bony plates either on the gill cover or on the cheeks of their heads.
Acus has no bony plates in either place and so Gess and Burrow agreed that a new genus was justified.
They decided to call it an Angelacanthus because superficially it looked similar to modern angelfish, but Gess stressed that they were totally unrelated.
He said their similarities were a case of convergent evolution.
“They have converged on a similar body form which would have been good for moving around still water, but cluttered still water — containing waterweeds or plant debris.”
Whatever its name, Gess is still very pleased with Angelacanthus.
He said it was a wonderful example of soft tissue preservation — “... its whole body is perfectly preserved, and it’s preserved as a sort of silvery impression on the rock and it’s unique”.
Talk of the Town
Would you like to comment on this article?
Sign up (it's quick and free) or sign in now.
Please read our Comment Policy before commenting.