Prehistoric reptiles of the Great Karoo

2008-05-28 00:00

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, but not with the same profusion. Vegetation comprises mosses, ferns, glossopterids and gymnosperms which grow in profusion along the river channels, while in the more arid flood plains, 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 on to 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 have produced billions of tons 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, the Lystrosaurus but a latecomer on the stage of reptile evolution.

One of the most amazing things about the rocks of the Karoo Supergroup is 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, when an estimated 96% 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. 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. 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 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. 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 at the small Berg and at the Golden Gate National Park.

• Allan Davie is currently putting together a fortnightly newsletter on all things geological, so if you wish to subscribe to the newsletter, please e-mail Davie at

geologist@netactive.co.za

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