Waterloo gives up more
Shale deposits from Waterloo Farm outside Grahamstown have yielded two more remarkable discoveries revealing what life was like in these parts about 360 million years ago. Local scientist, Dr Rob Gess has discovered a new type of armoured fish Africanaspis edmountaini, as well as more complete remains of a second type of armoured fish known as Africanaspis doryssa.
The A. edmountaini was named after Professor Edgar D. Mountain who was an honorary curator of the Albany Museum in the 1930s.
The most amazing feature of the Waterloo specimens is the presence of extremely rare soft-tissue impressions. Clearly embedded in the black shale rocks on Gess’s desk are impressions of soft tissue from the body, fins, tail and eyes of Africanaspis.
Gess said that soft tissue of fish similar to Africanaspis “is virtually unknown globally” and he described the fossils as being in an exceptional state of preservation.
In almost all cases, scientists only recover hard plates that come from the front parts of armour plated fishes known as placoderms. The latest finds have made it possible to see what the unarmoured tails looked like.
In a paper published in the peer-reviewed journal PLOS One, Dr Rob Gess of the Albany Museum and Rhodes University and Professor Kate Trinajstic of Curtin University in Australia described how they made reconstructions of the fossils to help them analyse these extraordinary fish.
Placoderms were among the first fish with conventional jaws and they dominated the oceans during the Devonian period from about 416 to 360 million years ago.
Their heads and the front parts of their bodies were covered by articulated armoured plates while the rest of their bodies were scaled or naked. They all died out at the endDevonian extinction event.
Africanaspis was first named in 1997 on the basis of isolated plates representing only three of the trunk armour plates of a single species ( A. doryssa). The new material includes the first head shields of A. doryssa in addition to the full trunk armour of both species, as well as soft anatomy..
Specimens represent fish of various sizes. Although adult A. doryssa were between 20 and 30 centimetres long, one small specimen was less than three centimetres long. Its large eyes suggest that it was newly hatched or born. A range of in between sizes indicates that Africanaspis spent their entire lives around the coastal lagoon of what is now Waterloo Farm.
This differs from the lifestyle of coelacanths previously described ( Grocott’s Mail, 25 September 2015) from the same site, which are only known from juveniles, indicating that they used the ancient estuary exclusively as a breeding nursery.
There are other fish species that we only know from adult specimens.
They probably lived in an adjacent environment and only came into the estuary to feed or when their were seasonal shifts in salinity.
Gess says that over the decades he has systematically collected every single fish fossil, excluding scales, that he has found from Waterloo Farm. This allows him to start understanding the entire ecosystem. He has found altogether about 500 specimens of fossil fish in the Waterloo shale.
Many of these specimens are not complete, some only a bone, or a scatter of bones, but they can be attributed to about 20 different species.
The coastal estuary may have successfully preserved so many exceptional fossils because briny water would seasonally have underlain fresher water in the lake.
Convection of lake waters did not penetrate these bottom waters that became oxygen depleted.
This lack of oxygen stopped the decay of animal or plant remains that sank beneath the brine and essentially were pickled. When fine clay buried fish and plant remains under these conditions, before the seasons changed, they could be preserved in the finest detail.
And 360 million years later, Dr Rob Gess prised them out of the black shale.