360-million-year-old fossil fish

Ben Maclennan, SAPA

The 360-million-year-old fossil of a species of fish with elegant needle-like spines - previously unknown to science - has been made in a deposit on the outskirts of Grahamstown, South Africa.

The find was made by paleontologist Robert Gess, who has been working on the site - exposed by a 1985 cutting to re-route the N2 highway around the town - for more than 10 years.

The deposit, a thick band of shale, marks the location of a brackish prehistoric estuarine lagoon, and has yielded a wealth of plant material, fish and fragments of a metre-long water scorpion.

These creatures were swimming in the warm, placid waters of the lagoon 120 million years before the first dinosaurs made their appearance on earth.

The shale is extremely fragile, and Gess's main tool has been a pen knife, with which he systematically prises layers apart, centimetre by centimetre, or even millimetre by millimetre.

The specimens themselves are often so soft that even using a paintbrush to clean them can cause damage. 

He discovered the fish in 1999, but scientific convention prevented him from formally announcing his find until a descriptive paper was published in a scientific journal. 

The fish, named Diplacanthus acus, is only 10 cm long but the distinctive narrow spines on its back and stomach make it almost 15cm from top to bottom.

"It's a particularly pretty thing," he says. "It looks like an aquarium fish. It would have had quite big, graceful fins."

The new specimen is probably the most complete of all the individual fish recovered from the site, with the bulk of its flesh and fin outlines preserved.

"But this fish, like mist of its species, consists of just a thin silvery film on the black shale.

"You could literally rub it off the rock with your thumb if you wanted to," says Gess.

"When I first split the layer in which I found it, I got just a small area where I could see a little of the body and the base of a spine. I cleaned it out, and was so impressed that I just wanted to get it into a safe place as soon as possible for fear that a freak accident might rob me of it. I was extremely excited...you're opening a page in a book that no one's ever opened before."

Gess says many of the fish already found at the site appear to have been in a state of decay when they were first buried in the mud. However, this specimen is unusually well preserved, indicating it was likely buried soon after it died.

He says the Grahamstown lagoon is the only decent site of its age and ecosystem in Southern Africa, and it is likely that all the fish found there and most of the plants will prove new to science.

Some of the other fish from the site, including previously unknown species with interlocking armoured plates, were described in scientific papers in the mid-90s. Others await more complete finds before they can be adequately reconstructed. Gess says the fish and the site are of major importance. 

"Other parts of the world have sites with similar ecosystems. But we had a virtual black hole on the map here. Before the discovery of the site, for that time period and that ecosystem, South Africa was an unknown land."

During the late Devonian era when the lagoon and its inhabitants flourished, the first land vertebrate must have been about to make its debut in a very similar environment to the lagoon, and flowering plants were still a thing of the future. Partly with funding from his employers Geodatec, Gess is continuing work on the specimens, which will eventually be housed in Grahamstown's Albany Museum where the new fish is currently on display.

He also has about 30 cubic meters of shale blocks, carefully moved from harm's way when the road cutting was enlarged in 1999, under shelter in his back garden.

And when he has a spare moment, he goes out with his pen knife and splits a few more layers- centimetre by centimetre, or even millimetre by millimetre.

March 2002