Reptiles of the Great Karoo

Fifty million years on, and the Karoo Sea is but a hint of its former self. In its place is a giant lake fed by a number of northward flowing rivers draining from the high Cape Mountains which shimmer through the heat haze far to the south. Along the river banks vegetation grows in abundance thanks to a plentiful supply of silt, water and the hot climate. On the flood plains between the rivers vegetation still grows abundantly although not with the same profusion of that along the river banks. Vegetation comprises mosses, ferns, glossopterids and gymnosperms which grow in profusion along the river channels, while in the more arid flood plains, more glossopterids and gymnosperms dominate. For the most part the rivers remain confined to their channels, but during the wetter, winter months when torrential rains lash the high Cape peaks to the south, the rivers rise and overflow onto the low-lying plains in an all encompassing flood, bringing water and nourishment to the arid areas between the rivers. Insects and beetles rattle through the undergrowth while cockroaches scratch in the leaf mould. And where there was little animal life in this ancient world until now, things have changed, for rooting between the plants and wading in the shallows are herds of strange creatures with shovel-like heads and tusks protruding downwards from the rear of their mouths. This is Lystrosaurus, a beast with a long robust body and short legs - a clumsy and inelegant creature. They wade and forage, chomping their way through vast amounts of vegetation while some work their shovel-like heads as spades, burrowing into the river banks to make shelters from a hostile world.

Gondwana has continued its inexorable wanderings from the southern high latitudes to warmer climes. Ongoing sedimentation over millions of years has filled the Karoo Sea to the extent that only a large lake remains, fringed with primitive vegetation. The Cargonian highlands to the north have been eroded down and become covered by younger sediments of the Karoo Basin. The only significant topography now comprises the Cape Mountains far to the south, which due to the ongoing processes of erosion have produced billions of tonnes of sediment which has filled the basin. Fossil trees show growth rings which indicate changing seasons and possible droughts. Desert roses, so typical of arid conditions, are found within the fossil sands of the Beaufort Group. The flood plains are home to a passing parade of reptiles which are the forerunners of the dinosaurs and ultimately mammal life, and Lystrosaurus but a latecomer on the stage of reptile evolution. One of the most amazing things about the rocks of the Karoo Supergroup are their level of preservation. Often rocks of this age get caught up in the tectonic mill, or are subject to erosion which destroys the record of Earth history which is written therein. The Karoo is internationally famous in palaeontological circles for its abundance of pre-dinosaur fossils. They preserve an almost unbroken record of 80 million years of vertebrate evolution and record the progression of life from primitive reptiles to the transitional stage between reptiles and mammals. These rocks also record the largest extinction event which has ever occurred in the history of life. The end Cretaceous extinction which led to the demise of the dinosaurs is more famous, thanks to popular movies and media coverage, but was nothing compared to the end-Permian event which shook the very foundations of life, when an estimated 96 percent of all life was extinguished. This event took place 251 million years ago and no one has yet come up with a watertight theory as to the reasons for this extinction, but changes in Earth’s atmosphere due to volcanic activity, a major meteorite impact, or drop in sea levels due to an ice age which exposed organic sediments leading to sudden oxygen depletion of the atmosphere have been postulated. For whatever reason, life was almost snuffed out. No longer was there any vegetation to slow the transport of sediment and the meandering rivers which fed our great lake were replaced by a myriad of sand-laden drainage channels which spread over the flood plains. Lystrosaurus survived the extinction event and not only are its remains found in the rocks of the Karoo, but also in Antarctica, additional evidence for the great landmass of Gondwana. Ongoing northward drift of Gondwana led to increasingly arid conditions. Local swamps were scattered here and there, but over time a vast desert formed in the centre of Gondwana, far from the oceans and possibly in the rain shadow of the towering Cape Mountains. Massive sand dunes formed which are now preserved as the golden cliffs of the Clarens Formation, so spectacularly displayed as the little Berg and at the Golden Gate National Park.

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The Karoo Sea

This is swampland on a giant scale. Teaming, pushing, living, scrambling forest grows riotously along the muddy banks of lagoons and stinking back swamps. Trees, club mosses, cycads and ferns compete for space, water and light, while those that have seen their days out collapse back exhausted to the forest floor, to be rapidly colonised by fungi and bacteria. Beetles scramble through the leaf litter or fly ungainly through the canopy, while cockroaches scuttle and scratch in the leaf mould. In the creeks and backwaters amphibians populate the mud flats and dark pools, while out on the sunny waterways fish leap and squadrons of giant dragonflies keep station above the languid waters.

Times have clearly moved on from the icy grip of the Dwyka Ice Age. Gondwana has wandered northwards and South Africa now finds itself at approximately 30° south where temperatures are more conducive to life, which has seized the day, colonising the land to form vast subtropical forests along the northern rim of an ancient sea. But now we need to step back a little and look at the geological processes which have led to this change of circumstances. We have looked at the great ice age known as the Dwyka glaciation of 300 million years ago, but now we must take our focus off KwaZulu Natal for a short time and look at what was going on in South Africa as a whole. Many of us will have travelled through the passes of the Cape Mountains and wondered at the spectacular folding which characterise this mountain chain. These mountains began to form approximately 330 million years ago during the late stages of the Dwyka glaciation. The mountains formed the southern boundary of an ancient sea which covered the majority of Southern Africa – bearing in mind of course that we were part of a larger supercontinent. This sea had a low tidal range and is thought to resemble the modern Black Sea - open to the larger ocean, but without the dynamics of a full-blown ocean. To the north, stretching from the Northern Cape through Gauteng to Mpumalanga, were the Cargonian Highlands – a range of hills which defined the northern limits of this sea. Our Dwyka ice sheets have been bumping and grinding their way southward across these northern highlands before being cast adrift as icebergs on the turbulent waters. As the ice scoured the landscape, Gondwana continued to drift northwards, ultimately bringing us to warmer latitudes and creating conditions conducive for the burgeoning of life.

With decreasing latitude, the ice melted, to be replaced by southward flowing rivers which began to drain the Cargonian highlands, depositing vast quantities of sediment along the northern margins of the Karoo Sea to form large, meandering deltas. Like deltas everywhere, they supported the forests and swamp life where today’s narrative began. The trees that dominated these deltas were a Gondwana classic known as Glossopteris - the distribution of this species another nail in the coffin for the naysayers of continental drift and plate tectonics – it is found in South America, South Africa, Australia, India and Antarctica, strong evidence indeed for the supercontinent of Gondwana. Glossopteris, ferns, horsetails and club mosses thrived within these vast deltas, to the extent that thick accumulations of wood and organic matter were unable to rot and were buried and ultimately converted to coal. These deposits form our coal reserves of KwaZulu-Natal and the Highveld.

The southern margins of the Karoo Sea were characterised by deep water and a narrow littoral, set against the towering peaks of the Cape Mountains. Small, fast flowing rivers typical of mountainous regions deposited their load along a narrow littoral on the southern margin of the Karoo Sea. Every so often some of this accumulation of sediments would slump into the depths to form large, fan like deposits on the sea floor called turbidites, and these may still be seen fossilised in the hills of the Eastern Cape. But this is treatise on the local geology, so we will stay focussed on what is in our backyard. The accumulation of sediment in the Karoo Sea continued for in excess of 150 million years, eventually filling the basin. Karoo rocks have the widest distribution across the subcontinent and reflect the changing latitude and depositional environments as the basin began to fill. It is an ongoing tail of evolution and change and we will continue with the Karoo story in the next article. However any journey inland from Pietermaritzburg will traverse sediments once laid down either on an ancient sea floor or on the swampy margins thereof, where verdant forests thrived in a magnificent outburst of life. Karoo rocks extend to the crest of van Reenen’s Pass and beyond – a truly magnificent heap of sediment which reflects in part the geological history of a subcontinent.