clues
The Karoo is the richest repository of therapsid fossils, the group that gave rise to early mammals, writes Ellen Bartlett
To the average motorist passing through it, generally at an unconscionably high rate of speed, the Karoo is that barren bit of infinity that must be crossed to get to Cape Town or Johannesburg: vast and desolate, subject to extremes of heat and cold, to drought and devastating flood.
Passers-by would probably be surprised to know that it was once a more temperate place, a fertile low-lying plain, surrounding a wide inland sea. And, as is true of any place today where warm seas meet sandy beaches, the Karoo also was a hotbed of procreative activity.
From the early Permian through the Jurassic periods, it was the scene of the evolutionary drama that brought about, among other things, the order of mammals, our earliest warm-blooded ancestors.
There are mementos everywhere, if anyone ever bothered to slow down and look around, lying about the dry river beds, embedded in broken shale on the red rock koppies, scattered over the sheep farms. The Karoo contains the longest uninterrupted reptile fossil record in the world, chronicling more than 100-million years of evolution.
The Karoo is also the richest known repository of fossils of the large and diversified group that preceded and gave rise to the mammals – the therapsids, also known as mammal-like reptiles.
Palaeontologist Robert Broom, who is perhaps better known for his work on the fossils of prehumans, wrote as early as 1932 that “the mammal-like reptiles from South Africa may be safely regarded as the most important fossil animals ever discovered, and their importance lies chiefly in the fact that there is little or no doubt that among them we have the ancestors of mammals, and the remote ancestors of man.”
It is possible in South Africa, says Bruce Rubidge, director of the Bernard Price Institute of Palaeontology at the University of the Witwatersrand, “to physically trace the ancestry of mammals” back to where they began.
“We have got the whole story… We have got the whole jigsaw in this country,” Rubidge says.
“Other countries have bits and pieces of the story, and they are very interesting and they certainly help fill in our jigsaw, but they have got to come here and put their pieces into our jigsaw.”
It is an old complaint among scientists here that, popularly speaking, the therapsids have been unfairly confined to the closet of prehistory, upstaged by their distant relations, the dinosaurs. For all that they have captured the imagination of children, not to speak of Hollywood, they say, dinosaurs were merely a detour from the main evolutionary path, a dead end one at that.
Therapsids are not only our direct, if almost unimaginably distant, ancestors, they are also just as weird looking as dinosaurs – maybe not gargantuan like the latter day Jurassic and Cretaceous creatures, but more than enough to fire the imagination.
It’s a matter of marketing. “We have to make them come alive, somehow,” Rubidge says.
The term mammal-like reptile is more descriptive than scientifically accepted. Taxonomists prefer the term therapsid, arguing that therapsids are no longer reptiles, nor yet mammals, but somewhere between.
The first to break away from the reptiles were, in fact, the Pelycosaurs, the best known of which is the sail-backed herbivore called the Dimetrodon. Pelycosaurs are most numerous in North America, though one has been found locally near Prince Albert, christened Elliotsmithia.
Therapsids succeeded the pelycosaurs, the first emerging sometime between 280-million and 256-million years ago, a period of activity for which there is unfortunately scant fossil evidence. “That’s the fossil gap,” Rubidge says, “and we’ve got to fill it.”
The development of therapsids is recorded in the South African fossil record over a period of about 30-million years, from the primitive Eodicynodon of 256-million years ago to the earliest mammal, the shrew-like Megazostrodon, 225-million years ago.
The record is continuous until about 180- million years ago – and contains among other creatures the first ancestral dinosaurs, Euskelosaurus and Massospondylus – when the Karoo erupted in a period of violent volcanic activity, ending in fire what had begun in ice.
The therapsids were a diverse lot, as many and varied as mammals are today. The vast majority of the earliest therapsids were not destined to go far, most were extinct by the end of the Permian period. They included the dinocephalians – the name means “terrible heads” – and the ubiquitous beak-nosed dicynodonts; the dominant herbivores of the period. The dominant early carnivores were the sabre-toothed gorgonopsians.
It was the gorgonopsians that are believed to have given rise to the therocephalia, the group that with its descendants, the cynodonts, held the line leading to mammals.
Though the Karoo has the most complete therapsid fossil record, the oldest fossils have always been found in Russia, supporting theories that the Karoo was populated as the result of a north-south migration.
Over the past decade, Rubidge and others have discovered even deeper rock layers containing some of the most primitive mammal-like reptiles yet, more primitive than in Russia, supporting theories of what Rubidge terms “global faunal interchange”.
Neither Russia nor South Africa existed then, of course. The land mass of the earth in the early Permian period was dominated by the super continent Pangaea. What is now South Africa was then a wide basin in the south, flanked by the Argentinian pampas and what is now Dronning Maud Land in Antarctica.
The earth was emerging from its longest ever ice age, a 25-million-year-long cold snap that lasted from 305-million to 280-million years ago. As the mass of ice retreated south, the rivers flowed north, from high snow-capped mountains into the basin, forming an inland sea. It was from the interface of land and water, that first the reptiles, and then their increasingly mammalian descendents, emerged and evolved.
It is in the remains of those ancient deltaic deposits that their fossils are found, survivors of millions of years of geological change, volcanic upheaval, slow burial under silt, sand and rock, and equally slow exhumation by the forces of erosion.
Erosion also exposed the fossils, though unfortunately the rock layers that once neatly contained them are broken and scattered, as if an ancient giant had put his foot upon a layer cake and sent the crumbs flying over the desert’s dry expanse.
Among the difficult tasks facing palaeontologists and geologists has been that of identifying the layers, placing them in order, determining which fossils belong where, and when.
Fossils also have their limits, they tell only the story of what happened to the animals’ so-called hard parts, their teeth and bones. At the same time, South Africa’s fossil record is so complete palaeontologists have been able to put together a remarkably detailed chronology of change.
It is possible, for example, to follow the progress of skulls, as they evolve from the loosely assembled ones characteristic of reptiles, to the closely knit ones of mammals.
Even the most primitive therapsids already had the large temporal opening behind the eye socket, to aid in the attachment of jaw muscles for chewing and biting. Their teeth were no longer uniform as reptilian teeth are, but had begun to develop into functionally distinct incisors, canines, post-canines.
Later therapsids developed other distinguishing characteristics. The loose- knit bones of the lower jaw fused gradually into a solid jawbone, the jaw remnants migrating back to form the tiny bones of the inner ear. The development of a bony palate allowed more modern therapsids the distinct advantage of being able to breathe and chew their food at the same time, and the ability to breathe continuously also supported an increased metabolism
There remain many unanswered questions. At what point did they acquire the ability to maintain a constant body temperature, which we call warm-bloodedness?
When did they start bearing live young? Did all mammals evolve from a single successful therapsid line, or did mammals descend from multiple lineages? What explains the existence today of the egg-laying mammals, and the marsupials?
“We know there are some modern mammals today that are adapted to lay eggs,” Rubidge says. “Did the duckbill platypus return to laying eggs? Or did they have a separate origin? Say we found evidence that one of these (therapsids) gave birth to its young alive, then it would suggest that platypuses had a separate origin.”
The Karoo fossils have contributed to an understanding not only of the minutiae of mammalian evolution, but also of the larger geological forces that shaped the earth.
It was the Karoo fossils that lent key support to the theory of continental drift. Fossil fragments had been found in the Antarctic, and the belief was they could be related to fossils found elsewhere. South African palaeontologist James Kitching was recruited to go fossil hunting in Antarctica.
He spent six months there, and found a treasure trove of fossil therapsids with counterparts in South Africa, in South America, in India, providing the first conclusive corroborative evidence that the continents had once been joined.
But palaeontology is more detail than drama, a labour of love rather than instant reward. Perhaps it is best illustrated by the massive skeleton, lying in pieces, largely embedded in rock, on a table in the middle of the preparation room in the basement of the Bernard Price Institute. It is a tapinocaninus, a primitive dinocephalian, and it was found more than 12 years ago in the oldest layer of fossil-bearing Karoo rock.
Preparators, research assistants and others have been working on it, slowly extracting it from its rock matrix, ever since. Whenever someone has a bit of free time, he or she chips away at it. No one is assigned to it full-time. No one could, Rubidge says, adding with a rueful smile, “You’d kill a person if you made him work on it for 12 years.”