by Peter Ward
The diapsids evolved shapes allowing movement. They were fleet carnivores. One of the other reptile groups, the anapsids, took another direction. No one would accuse a turtle of being fleet afoot, and that is what the anapsids became: turtles, and before that, huge slow-lumbering monsters known as pareiasaurs, one of the largest of all skeletonized reptiles known from the late Permian world.
Based on their earliest members, however, it would have been hard to predict that the anapsids would become so slow and lumbering and hiding inside armor. They were initially smaller, faster, and very successful during the Late Pennsylvanian, but less so into the Permian. As the glaciers receded from the long ice age spanning the first half of the Permian period, they evolved into giant forms, including cotylosaurs and the even larger pareiasaurs. These were armored giants, surely slow moving, and herbivores that lived right until the end of the Permian. It is very likely that the gigantic size of the earlier Permian anapsids was allowed by high oxygen.
The last major reptilian group was the synapsids, and these were our own ancestors. If diapsids did little during the Pennsylvanian through the early Permian oxygen high, the same cannot be said of the third group of amniotes from this time, the synapsids, or mammal-like reptiles. Like the diapsids, the most primitive are known from Pennsylvanian rocks, and also like the diapsids of this time, these ancestors of the mammals had a small, lizard-like shape and mode of life in all probability. It is assumed that like the diapsids (and the amphibians that they came from), these early synapsids were cold-blooded. They, in turn, gave rise to two great stocks: the pelycosaurs, like early Permian Dimetrodon, and their successors, the therapsids, the lineage giving rise to the mammals. It is this latter group that is also called the mammal-like reptiles.
Unlike the diapsids, the synapsids diversified during the oxygen high and at the peak of oxygen became the largest of all land vertebrates. In the latter part of the Pennsylvanian, the pelycosaurs probably looked and acted like large monitor lizards, or even the iguanas of today, with splayed limbs. By the end of the Pennsylvanian some attained the size of the Komodo dragon of today, and they may have been fearsome predators. By the beginning of the Permian period, some 300 million years ago, they made up at least 70 percent of the land vertebrate fauna. And they diversified in terms of feeding as well. Three groups were found: fish eaters, meat eaters, and the first large herbivores.
Both predators and prey could attain a size of close to fifteen feet in length, and some, such as Dimetrodon, had a large sail on the back that would have made them appear even larger. They also either partially or totally solved the reptilian problem of not being able to breathe while running by changing their stance. The synapsids show an evolutionary trend of moving their legs into a position so that they were increasingly under the trunk of the body, rather than splayed out to the side, as in modern lizards. This created a more upright posture, and removed or at least greatly decreased the lung compression that accompanies the sinuous gait of lizards and salamanders. While there was still some splay of the limbs to the sides of the trunk, it was certainly less than in the first tetrapods. With the evolution of the therapsids in the Middle Permian, the stance became even more upright.
The sail present on both carnivores and herbivores of the Late Pennsylvanian and early Permian is a vital clue to the metabolism of the pelycosaurs; it was a device used to rapidly heat up the animal in the morning hours. By positioning the sail so as to catch the morning sun, both predators and prey could rapidly warm their large bodies, allowing rapid movement. The animal first attaining warm internal temperature would have been the winner in the game of predation or escape, and hence natural selection would have worked on this. But the larger clue from this is that during the oxygen high, the ancestors of the mammals had not yet evolved endothermia, or “warm-bloodedness.” So when did this trait first appear? That revolutionary breakthrough must have happened among the successors to the pelycosaurs, the therapsids. We must note as well that this period, while a time of oxygen high, was a period of low temperatures. There was a great glaciation known from this interval, and a sizable portion of the polar regions of both hemispheres would have been covered in ice, both continental and sea ice.
While much of our understanding of pelycosaurs’ evolution comes from fossils found in North America, younger beds in this region have few vertebrate fossils. The transition to the therapsids is best seen in Europe and Russia, but even here the transition is poorly known because of few fossiliferous deposits of the critical age. This gap in our knowledge of the synapsid fossil record extends from perhaps 285 million years ago to around 270 million years ago. Two main regions tell us about the history of this group: the Russian area around the Ural Mountains, and the Karoo region of South Africa. The record in the Karoo begins with glacial deposits perhaps as much as 270 million years in age, and then there is a continuous record right into the Jurassic, giving an unparalleled understanding of this lineage of animals.
The therapsids split into two groups: a predominantly carnivorous group and an herbivorous group. By about 260 million years ago the ice was gone in South Africa, but we can assume that the relatively high latitude of this part of the supercontinent Pangaea (about 60 degrees south latitude) remained cool. It was still a time of high oxygen, certainly higher than now, but that was changing. As the Permian period progressed, oxygen levels were dropping. Seemingly two great radiations of forms occurred, among both carnivores and herbivores. From perhaps 270 to 260 million years ago the dominant land animals were the dinocephalians, and these great bulky beasts reached astounding size: not dinosaur sized, but certainly approaching any land mammal today save, perhaps, elephants, and some of the largest of the dinocephalians certainly must have weighed as much as elephants. Moschops, for instance, a common and well-known genus from South Africa, was five meters high, with an enormous head and front legs longer than the back. It was hunted by a group of similarly sized carnivores.
The dinocephalians and their carnivores were hit by a great extinction, still very poorly understood, that occurred some 260 million years ago. There is still little range data for both the dinocephalians and their immediate successors in terrestrial dominance, the earliest dicynodonts and their predators. Until new fossils from South Africa and Russia are obtained, this uncertainty will remain. Sadly, there are few fossils of this age and fewer paleontologists studying them, so we may not know for generations, assuming that future generations continue to hunt fossils.
Gorgonopsian skull from Late Permian deposits, South Africa. (Photo by Peter Ward.)
The dicynodonts were the dominant herbivores of the time from 260 to 250 million years ago. They were almost eliminated from the planet in the Permian extinction, which we will describe in more detail in the next chapter. They were hunted by three groups of carnivores: the gorgonopsians, which died out at the end of the Permian, the slightly more diverse therocephalians, and the cynodonts, which ultimately evolved into mammals during the Triassic.
ANIMAL SIZE AND OXYGEN LEVELS
The rise of atmospheric oxygen to unprecedented values of over 30 percent was accompanied by the evolution of insects of unprecedented size. The giant dragonflies and others of the late Carboniferous through the early Permian were the largest insects in Earth history. Perhaps it is just coincidence, but most specialists agree that the high oxygen would have enabled insects to grow larger, since the insect respiratory system requires diffusion of oxygen through tubes into the interior of the body, and in times of higher oxygen, more of this vital gas could penetrate into ever larger-bodied insects. So if insects got larger as oxygen rose, what about vertebrates? New data indicates that this is true as well.
In 2006, paleontologist Michel Laurin measured fossil skull lengths and body lengths of various reptiles ranging from the Carboniferous through the Permian, from about 320 million years ago until about 250 million years ago. Both of the size descriptors closely tracked oxygen levels. As O2 levels rose in the Late Carboniferous,
so too did the size of the reptiles increase, and, as O2 began to drop in the mid Permian, size began to trend downward. As we will return to in the chapter on Cenozoic mammals, study on (much) later mammals, by Paul Falkowski and his colleagues, demonstrated a very similar phenomenon during the Early Cenozoic, when oxygen levels have been modeled to have risen significantly, while at the same time, the mean size of mammal species also increased.
This trend of changing size also occurred among the mammal-like reptiles as the Permian Period came to a close. The largest therapsids of all time, the dinocephalians of the Middle Permian, evolved at the peak of oxygen abundance. As oxygen began to drop in the mid-Permian, successive taxa assigned to various therapsid groups, and most important the dicynodonts, showed a trend toward smaller skull sizes. While some relatively large forms still lived in the latest Permian—the genus Dicynodon and even the carnivorous gorgonopsians come to mind—by this time many of the dicynodonts were smaller. The latest Permian taxa Cistecephalus, Diictodon, and a few others were very small. Research in 2007 showed that the Late Permian through Early Triassic genus Lystrosaurus was smaller in the Triassic than it was in the Permian, and the various cynodonts of the late Permian and early Triassic, as oxygen levels were precipitously falling, were all small in size. There are exceptions—a few giants in the Triassic named Kannemeyeria and Tritylodon are examples—but in general the therapsids of the Triassic are much smaller than those of the Permian. A recent paper by our colleague (and now at the University of Washington) Christian Sidor has confirmed the drop in size. Thus there is a strong correlation between terrestrial animal size and oxygen levels from the latest Permian into the Triassic. In high oxygen, tetrapods grew large, and then they grew smaller as oxygen levels diminished.
Antonio Lazcano, origin of life specialist and humanist, in Galapagos Islands contemplating a “lower” life form. (Photo by Peter Ward.)
THE FIRST AGE OF MAMMALS
Yale University’s great Peabody Museum is home to one of the largest collections of fossils in the world. It also is home to the greatest paleontological paintings ever done.
There are two gigantic murals gracing an immense wall in the Peabody Museum of Natural History on the Yale University campus. For generations of Americans these two murals—The Age of Reptiles, painted over three years (1943–1947), and The Age of Mammals, painted over six years (1961–1967)—have been the iconic views of land life’s journey through time.
The first, The Age of Reptiles, begins in dark swamps and ends with exploding volcanoes towering over T. rex. The second also begins in the jungle, but one with very different and very familiar vegetation. Combined, they tell us that amphibians begat reptiles, which begat mammals. But our view would now require two very different murals to correctly show the vertebrate assemblages in these deep time periods pictorially represented. In fact, we advocate here that there were three separate “ages” of mammals (knowing, of course, that an “age” is nothing more than informal labeling and categorizing, without scientific validity).
The first age of mammals was in the Permian period, the heyday of the therapsids and their ancestral synapsids. Technically they are not yet mammals. But they were close. It was a species-rich as well as numerically abundant assemblage. In South Africa, there were as many as fifty genera at a time (and since a normal genus normally contains several [to many] species, the actual diversity at the species level was higher yet; perhaps 150 species is a conservative estimate).
South Africa today is not so different latitudinally and perhaps even climatically from the South Africa of southern Gondwanaland, some 255 million years ago. Today there are 299 species; we can imagine the African veldt of today, but stocked with Dicynodon instead of the large herbivores, and many kinds of carnivores from the lion-sized gorgonopsians to the weasel-sized theriodonts. Vast herds grazing not on grass but the low, bushy Glossopteris and ferns. Africa of the first age of mammals.
The second age of mammals can be thought of as the time between the late Triassic and the end of the Cretaceous: mammals chained. Held in check by the dinosaur overlords. Living in the ecological cracks: at night, in burrows, in trees. Never bigger than a house cat, and usually far smaller.
Finally, the third age of mammals. Zallinger’s age of mammals. The post-K-T mass extinction outpouring of species filling the families so well known to us today. This is the story most obvious to us: from ratlike survivors of the Chicxulub asteroid’s wrath to the early giants such as titanotheres and uintatheres (rhino-like beasts) to the mammalian panoply we know so familiarly today.
Until about 2000 we knew the first age of mammals largely from the South African Karoo desert. But in the twenty-first century vast new collections have been made from north central Africa by Christian Sidor, and in Russia another gigantic assemblage is now known, thanks to paleontologist Michael Benton’s work. In this second age, the mammals remained very small. It would not be until the Paleogene that mammals would finally gain ascendancy, and like some long-denied heir, would finally get an “age” named after them.
One could almost believe that the whole age of dinosaurs was a big mistake. That but for one huge flood of basalt there might have been a quite different history. Human intelligence 250 million years ago? It did not take long to go from apes to something more advanced not so long ago.
CHAPTER XII
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The Great Dying—Anoxia and Global Stagnation: 252–250 MA
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The Karoo desert of central South Africa can be a bit of a disappointment to its first-time visitors. When the two words “Africa” and “desert” are found in the same phrase, there is often an image of the Sahara desert, Africa’s most famous dry place, or the Kalahari, another vast wasteland with little life because of its shifting sands and harsh conditions of blazing heat by day and freezing temperatures each night. With animals and plants having such a hard time, and existing at such low standing diversity and abundance, it is no wonder that the human populations in the Sahara and Kalahari are limited in size as well. Very little in the way of plant or animal crops can be farmed there.
Unlike these two African deserts, the Great Karoo desert has no shifting sand dunes; it is mainly rock that is often well vegetated, and there seems no place in its vastness where one cannot find sheep dung, evidence for the ubiquity of this introduced species. There are no elephants or giraffes, no hippos or crocodiles or water buffaloes or rhinos; it has animal life, and in places lots of it, but its species are not ones redolent in the memory of Africa. There are also quite a few people, on large ranches. Thus the Karoo is not the place for desert-seeking tourists. What it does have, however, is a hundred-million-year-long accumulation of sedimentary rocks deposited in a time interval from about 270 million years ago to perhaps 175 million years ago.
In the middle of this vast rock heap is the world’s best record of large terrestrial animal life living both before and after that most consequential of all mass extinctions, the Permian-Triassic mass extinction. Generations of paleontologists going back to the middle 1800s have searched the ancient riverbeds and river-valley floors that the Karoo strata were created by and in. Animals often are carried into rivers after death, or are in waterholes where they may have been attacked long ago, leaving bones to fall into mud and become preserved there. This region was the prime record for this period until very recently, with new work in both eastern Russia by our colleague Mike Benton of Bristol and the north central part of Africa, in the country of Niger, where another of our colleagues, Christian Sidor of the University of Washington, have unearthed important new records.1 Yet even these new regions cannot compare the richness and temporal resolution that the Karoo rocks have given us—if “given” should be used at all. In fact, the Karoo has given up its vast store of information about one of the most critical times in life’s history on Earth very grudgingly. It has to be taken, and while the work to do this seems glamorous (who does not dream of being a paleontologist finding a
giant leering skull of some ancient predator, such as T. rex), it is at best difficult on the humans who pursue this passion.
A drive from Cape Town into the center of the Karoo is an all-day affair. But because the rocks are slightly tilted, while the landscape inexorably rises in altitude as one travels north and east into the Karoo, the entire book of strata that the Karoo holds can be read from its ancient mid-Permian-period cover to its Jurassic, dinosaur-bearing last chapter. It is not only time that changes as one goes upward through the many thousands of aggregate feet that is the Karoo sedimentary record. One starts in a time of ice and icebergs and ends in what may have been one of the hottest times in Earth history, as well as passing through an interval tens of millions of years in length when atmospheric oxygen receded to its lowest level since animals first occurred at all, nearly 600 million years ago. Yet if much can be understood by reading this entire record, there is one interval of rock, representing time, that has been more studied than any other.
These are the several hundreds of meters of strata deposited between 252 and 248 million years ago—rocks deposited in the last millennia of the Permian period (and thus the Paleozoic era, which ends with the end of the Permian) and first few millions of years following the vast mass extinction of 252 million years ago.
For decades now, geoscientists have been asking several principal questions of these rocks, and their rare but often exquisitely preserved skulls and body skeletons: First is the question of how long the mass extinction took, from the start of extinction rates first exceeding the normal “background” extinction rate, which has been calculated to have been about one extinction each five years. Second, we want to know if the catastrophic extinction on land took place simultaneously with the Permian marine mass extinction. Third, and perhaps most interesting, is the question of what caused the mass extinction. Finally, it is important to discover how quickly terrestrial ecosystems recovered, because these latter clues might give us useful information for surviving any future Permian-like mass extinction, a prospect far more probable than our species seems to realize.
