/ 26 August 2005

Fossil fish puts SA on the map

It is really a bit of minnow, this fossil fish of which a team of British and South African scientists have now found a dozen or so examples, embedded in 450-million year old Ordovician shale in a roadside quarry in the Cederberg. Still, this fish, and some other ancient beasties found with it, have put the Cederberg ”Soom” fossil site firmly on the map.

The soom (Afrikaans for seam) shale bed where the fish were found has, over the past 15 or so years, established itself as one of a small number of the world’s Lagerstätten, sites where early fossils are found in an unusual state of preservation.

The fossils of the Cambrian and the Ordovician periods — dating from about 580-million to 450-million years ago — appear unimpressive to modern eyes used to dinosaur fossils. The Soomichthys (Soom-fish, as it has provisionally been named) is no larger than the tiddlers kids catch in rock pools, but this little fish was one of the larger animals around at the time and it tells us important things about the unfolding history of life on Earth.

From impressions in the shale, palaeontologists have identified several of Soomichthys’s determining features: it has well-developed eyes, gill slits, scales, guts, brains and fins and a round, jawless mouth, similar to that of modern hagfish and lampreys. It is older than any other fish at a similar stage of development and is thought to be the ancestor of all modern fish species alive today.

The Cederberg shale also contains numerous tiny objects, which under a magnifying glass look like the back legs of grasshoppers. These curious objects were first thought to be the spiny stems of ancient plants, maybe even the first remains of plants with a more organised physical structure.

Later, after similar objects were found in Scotland, attached to the body impression of a little animal, scientists realised that they were the jaw structures of a creature not unlike the Soom-fish, which they called a conodont, after its cone-shaped teeth.

The Cederberg Soom is a deposit of soft shale 10m to 15m thick, sandwiched between the harder, darker layers of Table Mountain sandstone. It is remarkable for harbouring fossils that tell us about the great expansion in the variety of life forms on Earth that started long before the Cambrian era, but left hardly any traces.

The Soom was deposited during an ice age at a time when the entire Southern Africa (then still part of the ancient continent Gondwanaland) was covered by glaciers. During their journey to the sea, these glaciers ground the underlying granite rocks to powder.

The glacial era that produced the Soom has been estimated to have lasted for about 20 000 years and the shale appears in distinctive layers varying from one millimetre to 10 millimetres thick. This shale was deposited as a fine-grained kaolin clay in the sea or brackish water where these ancient animals lived. The clay was laid down exceptionally evenly, in water free from turbulence or currents.

These highly unusual circumstances, where a stagnant body of water (as it would exist in a shallow, enclosed bay or lagoon) is fed with extremely fine clay particles in suspension, caused the organisms and plants living and dying there to be entombed where they landed on the sea bed.

Another factor leading to their exceptional state of preservation was the absence of oxygen on the seabed. This was owing to the fact that salt and fresh water do not mix readily in a static body of water, for example, where glacial meltwater trickles gently into the sea.

The result was that the tissues of these soft, boneless animals did not decay, but left an impression in the fine-grained mud that encased them.

The ancient marine landscape of the Soom was populated with a variety of crawling, swimming, burrowing or rooted cold-water animal species.

There were also larger predators or scavengers, because their droppings containing shell fragments and the hard bits of other creatures are common, although the animals themselves remain undiscovered. There are also traces of worms, numerous spicules from unknown species (probably sponges), algal spores and tiny ostracods (shelled arthropods).

It is perhaps fitting that modern humans found these tiny animals, so rich in promise about the future direction of life on Earth, at this spot. For it is near here, a mere 100 000 years or so ago, that the first homo sapiens, ancestors of us all, were born with the same inquiring and inventive mind that we use today in deciphering the importance of ancient creatures to our own descent.