Dinosaurs and their backboned ancestors evolved from single unicellular life back when the Earth was young. About 290 million years ago the first backboned creatures able to live entirely on land arose from an amphibian ancestor. They were able to abandon life by the beach or lakeside, and head for life in the hills and deserts. These cold-blooded reptiles had scaly, water-proof skin which kept body fluid levels high, and almost as importantly their eggs were fertilised inside the female’s body. The egg’s tough, outer skin (or shell) allowed them to be laid away from water courses. The scene was set for further colonisation of the land, and this they did with alacrity.
The Karoo Basin of Southern Africa is home to a treasure house of pre-dinosaur reptiles. John Reader wrote this of African geology in his fascinating book Africa –
“Africa is the Earth’s oldest and most enduring landmass. Ninety-sever percent of the continent has been in place and stable for more than 300 million years, most of it for more than 550 million years and some for as much as 3600 million years. It is a story of accretion that records a large and significant fraction of the history of the Earth. Incontestable forces assembled a continent as millions of years flitted by. This is a timescale on which the average human lifespan is close to irrelevant. Consider: 5 million years spans all of human evolution; 670 million years encompasses the evolutionary history of all animal life, 3600 million years goes back to the beginnings of life itself. Africa has seen it all, and preserves the evidence. The mountain-building episodes and deep geological dislocations which distinguish the landscape of other continents are less evident in Africa. Rock formed more than 1000 million years ago still lies in the horizontal plane – undistorted; many ancient sediments are hardly touched by metamorphic processes. No other part of the world reveals so much of the Earth’s structure and history so clearly, from the beginning to the present.”
And if that wasn’t enough to get excited about South Africa’s unique geological record, “Rocks of the Karoo Basin preserve a world class assemblage of fossils which document the early evolution of tortoises, dinosaurs and mammals” says Professor Bruce Rubidge of the Bernard Price Institute of Palaeontology at the University of the Witwatersrand.
“These rocks, deposited over an extensive period from the Late Carboniferous (300 million years ago) to the Early Jurassic (180 million years ago), witnessed environmental change from glaciers, through aeolian desert dunes, and culminating in vast outpouring of volcanic lava. This is the only place in the world where such an extended fossil record of early evolution of “reptilian” life is preserved in a single basin, and chronicles the most distant evolutionary ancestry of mammals in remarkable detail.” (I have added the bold lettering)
So to our South African readers, we live in a palaeontological hotspot, so read on to find out more.
The Palaeozoic Era ended with the end Permian extinction event – a catastrophic dying off of species. It has been estimated that 96 % of all species died in this, the grandfather of all extinction events. But this event raised the curtain on the age of dinosaurs. As the Palaeozoic gave way to the Mesozoic, a number of major groups of reptiles were evolving with names like Anapsids, Synapsids, and Diapsids.
Archosaurs were different to other Diapsids because they had an extra hole, or a bony depression, in each side of the head which might have housed salt glands used for ridding the body of excess salt in arid environments, although no one can be sure because all the soft tissue has rotted away and all we have left are the fossilised bones.
The direct ancestors of the dinosaurs came from the great group of early Archosaurs called Thecodonts (socket toothed). Thecodonts weren’t pretty to look at – big, heavy, four legged reptiles called proterosuchians meaning ‘early crocodiles’ which they indeed resemble, although very large ones. They were carnivores – flesh eaters – and when they first appeared about 225 million years ago were able to out-compete the mammal–like reptiles, driving them to extinction during the Triassic Period.
Many thecodonts had bodies which equipped them for life in the water. They swam like crocodiles by wagging their long, deep tails. Pseudosuchians took up life on the land, walking on all fours like crocodiles, heaving themselves up off the ground and walking like their modern counterparts. They had powerful hind limbs with the thrust coming from their hips, which allowed them to rear up and on occasion run on their hind legs, their tails trailing behind as a long, powerful balancing beam to keep them upright. Their front legs were weaker and shorter than their back. The ability to get up on the hind legs and run had useful evolutionary advantages, and perhaps assisted in them out-competing the mammal-like reptiles.
Over time, a number of pseudosuchians evolved, and by 205 million years ago, had evolved into the first flesh-eating dinosaurs. A fully upright posture helped the smaller species to outrun prey and rivals. This ability to get around at speed made the new dinosaurs more dangerous than their thecodont ancestors and again they out-competed the more primitive predecessors. At the same time herbivores – plant eating dinosaurs – evolved, with teeth designed for eating vegetation. By the end of the Triassic Period, 193 million years ago, flesh eating dinosaurs had replaced the thecodonts, and nearly all the big herbivores were genuine dinosaurs.