A New History of Life (35 page)

The Permian extinction was clearly one of the most fundamentally catastrophic of all events—if, that is, one was a multicellular plant or animal. From a microbe’s point of view—especially one of the sulfur-loving, oxygen-hating microbes that made up the majority of all life on Earth from its very inception right up to the first evolution of animals—that event was like a return to paradise. Seen from our vantage point so long after, the Permian extinction was a repeat of what happened at the end of the Devonian, itself the first of what we now call greenhouse extinctions. Many more were destined to come at the end of the Triassic, multiple times in the Jurassic and Cretaceous, and ending with the last-known greenhouse extinction at the end of the Paleocene epoch, some 60 million years ago. But none were ever to be so great as the Permian event, or to unleash a more diverse assemblage of animals in the aftermath of extinction.

The Permian extinction gave the world many new creatures, but for us, two entirely new lineages, both thriving and evolving by the end Triassic period. In no small way the Permian extinction brought to life mammals, and brought about the means that would create our long-term nemesis, the dinosaurs. Yet while being among the most important of all land animals (few animal groups are awarded an “age of …” before their names), the Triassic dinosaurs and mammals were late arrivals in the Triassic explosion, and remained both relatively small of stature (especially the mammals, which rarely exceed rat size) and small in both absolute abundance and species diversity. The age of dinosaurs was not to start until the successive Jurassic period, while the still-running age of mammals had to await the Cenozoic era.

Long before the late (Triassic) arrival on the evolutionary stage of dinosaurs and mammals, the other animals and plants of the Triassic period make up a most interesting assemblage of organismal characters, cast with new versions of already long-running taxa mixed with entirely new entrants, new designs arising yet radically different from the actual survivors of the Paleozoic era. It is this mix that makes the Triassic appear to be a veritable crossroads in time. In some ways
it was not unlike the Cambrian explosion—a slew of newly invented body plans filling up an empty world, just as the first animals rapidly evolved into the cornucopia of body plans that filled the seas after the extinction of the first animals, the Ediacarans. And like the great Cambrian explosion, many of the body plans of novelty turned out to be but short-term experiments, to be pushed into extinction by the competition and/or predation of better-designed organisms. There is no time period other than the Cambrian and Triassic in which such a diversity of new forms appeared. Two reasons seem paramount: The Permian extinction emptied the world to such a degree that virtually any new design would work, for a while at least. But there is a second, new view of the Triassic that may be just as (or more) important than this.

Just coming out of the most devastating of all mass extinctions, this early Triassic world was very, very empty of life. At the same time, all modeling suggests that a long interval of the Triassic was a time of oxygen levels lower than those today. Earlier we suggested that times of low oxygen, especially following mass extinction, foster disparity: the diversity of new body plans. These two factors combined to create the largest number of new body plans seen since the Cambrian, and here we propose that it is to that seminal Cambrian time that we most accurately compare the Triassic. We call this time and its biotic consequences the Triassic explosion.

The Triassic was a time of amazing disparity on land and in the sea. In the latter, new stocks of bivalve mollusks took the place of the many extinct brachiopods, while a great diversification of ammonoids and nautiloids refilled the oceans with active predators. Fully a quarter of all the ammonites that ever lived have been found in Triassic rocks, a time interval that is only 10 percent of their total time existence on Earth. The oceans filled with their kind, in shapes and patterns completely new compared to their Paleozoic ancestors, and why not, for, as shown above, this kind of animal was the preeminent, low-oxygen adaptation among all invertebrates. A new kind of coral, the scleractinians, began to build reefs,
³
and many land reptiles returned to the sea. But it is on land that the most sweeping changes in terms of
body plan replacements and body plan experimentation took place. Never before and never since has the world seen such a diverse group of different anatomies on land. Some were familiar Permian types: the therapsids that survived the Permian extinction diversified and competed with archosaurs for dominance of the land early in the Triassic, but this ascendance was short lived. The many kinds of reptiles were locked in a competitive struggle with them, and with each other, for land dominance. From mammal-like reptiles to lizards, earliest mammals to true, the Triassic was a huge experiment in animal design.

On the face of it, the mammals should have come out competitively “ahead” of the pure reptiles. After all, most of the mammal-like reptiles by this time were warm-blooded, probably capable (as now) of far more parental care than the presumably egg-laying dinosaurs; the mammalian teeth, one of the main reasons that mammals eventually did dominate the world, in their endlessly malleable tooth morphologies allowed all kinds of food acquisition, from small seeds to grass to meat of many kinds. Yet they did not win. Their extinction closed out the first age of mammals and gave rise to the second—composed of a very different group of mammals.

One of the major changes that has and continues to allow entirely new kinds of study of all groups of extinct animals is the great revolution in communication, morphological characterization and image analyses, and profound literature search skills that the computer revolution has allowed. Now large databases can be produced and then searched and analyzed in lightning blazes of microprocessor skill.

No longer does each fossil have to be laboriously measured by hand with micrometers, and no longer is it a single investigator traveling from museum to museum to do the work. Almost every new study that brings change to our history of life comes from large teams of investigators, ultimately inputting huge numbers of numbers to be crunched. Now the machines do much of this for us. And the results can produce new insights.

One such study by paleontologists Roland Sookias and Ludwig Maximilian, of the University of Munich, looked at the sizes of Triassic vertebrates that lived on land.

In this and subsequent work by this group it was found that only two major body plans emerged in the Early Triassic amid the emptiness left behind by the Permian mass extinction: those with four legs (quadrupeds), and those that used only two (bipeds). As the nearly 50-million-year-long Triassic period progressed into the Jurassic, with its own 50-million-year-long time interval, they found that the saurians diversified into a far larger number of species and shapes (and absolutely bigger in size, one measure of disparity) than did the mammal-like reptiles. While paleontologists long intuited this by perusing collections, here were numbers for the first time to substantiate this.

Their study also confirmed that the saurian grew faster, reaching adulthood and large size faster than did the other group. This “time to breeding” difference might be the most important metric of all. Faster growth and breeding meant that the saurians quickly adapted to the ecological roles of large herbivores and big predators before the smaller, slower-growing therapsids had a chance to evolve into these anatomical forms and ecological niches.

Questions remain. During the Late Triassic, when dinosaurs were well established, it would be expected that they would have immediately grown large, Jurassic large, and would have been common as well. According to Chicago paleontologist Paul Sereno, who has done more than any other to bring the earliest times of the dinosaur hegemony to light, neither was true. For almost 20 million years, from their first appearance some 221 million years ago until the end of the Triassic, about 201 million years ago, dinosaurs and therapsids alike remained both relatively rare and small in size.
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There may have been more of them than the therapsids during this time, but the overall picture is that neither group was doing very well. Our own take on this is that nothing on land was doing very well at all, and that, in fact, it was perhaps far more advantageous for the four-legged land animals to return to the sea, which they did in higher numbers during the Triassic than at any other time in Earth history.

The conventional answer for the reason for the Triassic explosion is that the Permian extinction removed so many of the dominant land animals that it opened the way for more innovation than any
nonextinction time, or perhaps any other mass extinction time as well. Perhaps, as well, it was simply that many terrestrial animal body plans finally came to an evolutionary point of really working efficiently. Even as late as the end of the Permian and into the Triassic, groups as evolutionarily mature as the mammal-like reptiles (the groups dicynodonts and cynodonts, by this time) were still trying to attain the most efficient kind of upright posture, rather than the less efficient, splayed-leg orientation of the land reptiles, with all of the ramifications and penalties that this entailed.

Body plans were being evolutionarily modified by intense selective pressures, and dominant among these was the need to access sufficient oxygen to feed, breed, and compete in a low-oxygen world. There is an old adage about nothing sharpening the mind faster than imminent death. The same might be said about evolutionary forces when faced with the most pressing of all selective pressures, which was attaining the oxygen necessary for the high levels of animal activity that had been evolutionarily attained in the high-oxygen world of the Permian, when nothing was easier to extract from the atmosphere. The two-thirds drop in atmospheric oxygen certainly lit the fuse to an evolutionary bomb, which exploded in the Triassic. Thus the diversity of Triassic animal plans is analogous to the diversity of marine body plans that resulted from the Cambrian explosion. As we have earlier recounted, the Cambrian explosion followed a mass extinction (of the Ediacaran fauna), and it was a time of lower oxygen than today. The latter stimulated much new design.

The officially designated early Triassic time interval was from 250 to about 245 million years ago, and during this time there is little in the way of recovery from the mass extinction. The oxygen story for the Triassic is stunning. Oxygen dropped to minimal levels of between 10 and 15 percent, and then stayed there for at least 5 million years, from 245 to 240 million years ago. There is also a very curious record of large-scale oscillation in carbon isotopes from this time, indicating
that the very carbon cycle was being perturbed in what looks like either a succession of methane gas entering the oceans and atmosphere or a succession of small-scale extinctions taking place. Again, the similarity to the early Cambrian is striking.

All evidence certainly paints a picture of a stark and environmentally challenging world for animal life. Microbes may have thrived, especially those that fixed sulfur, but animals had a long period of difficult times. However, difficult times are what best drive the engines of evolution and innovation, and from this trough in oxygen on planet Earth emerged new kinds of animals, most sporting respiratory systems better able to cope with the extended oxygen crisis. On land two new groups were to emerge from the wreckage: mammals and dinosaurs. The former would become understudies while the latter would take over the world.

As we saw in the last chapter, the Permian extinction annihilated almost all land life. The therapsids were hard hit. Much less is known about the archosauromorphs (reptiles with a somewhat crocodile-like anatomy), for at the end of the Permian they are a rare and little-seen group in areas such as the Karoo or Russia that have yielded rich deposits with abundant dicynodont (mammal-like reptile) faunas. In the Karoo desert, at least, very few well-preserved archosauromorphs have come from uppermost Permian study sections worked on by coauthors Ward and Kirschvink in the company of South Africa’s Roger Smith.

If we are still poorly informed about their Permian ancestry, there is no ambiguity about the success of the earliest Triassic archosauromorphs. In the Karoo, in only a few meters of the strata that seem to mark the transition from Permian to Triassic there are relatively common remains of a fairly large reptile known as
Proterosuchus
(also known as
Chasmatosaurus
). This was definitely a land animal with a very impressive set of sharply pointed teeth. It was also definitely a predator, but like those of a crocodile, its legs were splayed to the sides (if somewhat more upright than the crocodilian condition). But this condition was to rapidly change in the archosauromorphs to a more upright orientation as the Triassic progressed, and more gracile
and rapid predators soon replaced the early archosauromorphs such as
Proterosuchus
.

While the need for speed was surely a driver toward this better locomotor posture, just as important may have been the need to be able to breathe while walking. Like a lizard,
Proterosuchus
may still have had a back and forth sway to its body as it walked, and as we have seen previously, this sort of locomotion causes compression on the lung area due to what is known as Carrier’s constraint,
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the concept that quadrupeds with splayed-out legs cannot breathe while they run, because their sinuous side-to-side swaying of the body impinges on the lungs and rib cage, inhibiting inspiration. For this reason lizards and salamanders cannot breathe while walking, and
Proterosuchus
may have had something of this effect, although not as pronounced as in modern-day salamanders or lizards.

A solution is to put the legs beneath, but this is only a partial solution.
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To truly be free of the constraint that breathing put on posture, extensive modification to the respiratory system as well as the locomotor system had to be made. The lineage that led to dinosaurs and birds found an effective and novel adaptation to overcome this breathing problem: bipedalism. By removing the quadruped stance, they were freed of the constraints of motion and lung function. The ancestors of the mammals also made new innovations, including a secondary palate (which allows simultaneous eating and breathing) as well as a complete upright (but still quadruped) stance. But this was still not satisfactory, and a new kind of breathing system evolved. A powerful set of muscles known as the diaphragm allowed a much more forceful system for inspiring and then exhaling air.