A New History of Life

by Peter Ward

  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.4 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.

  TRIASSIC REBOUND

  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,5 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.6 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.

  There are other clues than dinosaur bones to the nature of life o
n Earth, and the challenges it faced during the low-oxygen times of the Triassic. Part of the Triassic explosion was a diversification of reptiles returning to the sea. Many separate lineages did this, and the reasons why this happened may be tied up in the problems posed by the hot, low-oxygen Triassic world.

  Oxygen is necessary to run metabolic reactions in animals; it enables the chemical reactions that are life itself. But as in a chemistry experiment, several factors control the reactions themselves. One of the most important is temperature. Metabolic rate is the pace at which energy is used by an organism. It is far higher in endotherms than in ectoderms. But even in the same organism, the metabolic rate is directly and importantly influenced by temperature to a surprising degree. Recent studies have shown that as much as one third to one half of all energy expenditure by an animal is used for simply staying alive through activities such as protein turnover, ion pumping, blood circulation, and breathing. Other required activities, such as movement, reproduction, feeding, and other behavior come on top of this, and the rate that “fuel” is used goes up with rising temperature.7 But as metabolic rate goes up, so too does the need for oxygen, for the chemical reactions of life are oxygen dependent. The key finding is that metabolic rates double to triple with each ten-degree rise in temperature. The consequences of this in a world that has less oxygen availability than now, but warmer average temperatures, would be major.

  There is no direct link between oxygen levels in the atmosphere and temperature. But there is a direct link between temperature and CO2, the well-known greenhouse effect. And as we saw in chapter 3, levels of oxygen and atmospheric CO2 are roughly inverse: when oxygen is high, CO2 is low, and the converse. Many periods in the past with low oxygen had high CO2, and thus were hot. In a low-O2 world that is hot, the animal loses. We have already seen many solutions to dealing with low oxygen. One of them is obviously the simple solution of staying cool. Some solutions to staying cool—or cool enough—are physiological; some are behavioral.

  One of these is all at once morphological, physiological, and behavioral. It is to return to the sea, the cool sea, for even in the hottest world of the past, the ocean would be essentially cooler in terms of physiology. And for this reason, perhaps, many Mesozoic land animals traded feet for flippers or fins and returned to the sea at a prodigious rate.

  As noted earlier in this chapter, in this time of higher global temperature (perhaps 30°F warmer, in fact, on a global average) and only half the atmospheric oxygen found today, an increasing proportion of tetrapod diversity was composed of animals that re-evolved a marine lifestyle. Never before and never since have so many lineages given up the land for the sea. Today we celebrate the many kinds of whale, seal, and penguin families, the three groups coming from land dwellers that now show the greatest marine adaptations. Yet whales and seals combined make up only 2 percent of all mammal genera, and penguins but 1 percent of birds. But the Triassic oceans had many more kinds of such changed creatures, animals adapted to land that had revolved a body plan for life in the sea. In the Triassic there were giant ichthyosaurs, as well as seagoing tetrapods such as placodonts (the latter were like large seals, but unlike seals, had blunt teeth expressly evolved to crack shellfish); in the Jurassic the ichthyosaurs remained and were joined by a host of long- or short-necked plesiosaurs; and in the Cretaceous the ichthyosaurs disappeared to be replaced by large Mosasaurs. But all had a common theme: back to the ocean.

  The existence of so many marine tetrapods was confirmed with the important research of marine reptile expert Nathalie Bardet, who in 1994 published a review8 of all known marine reptile families of the Mesozoic. The surprise was that proportionately there were so many in the Triassic period. But why would so many animals evolve a marine lifestyle?

  The two dominant environmental factors of those days would have been the low oxygen and the high global temperature of our planet. Ray Huey, a reptile specialist at the University of Washington, suggested too that the high heat of the Early Triassic through Jurassic would have been an evolutionary incentive for some number of reptiles to go back into the sea. In fact, in 2006, coauthor Ward showed that there was a very interesting and inverse correlation between Mesozoic oxygen levels and the number of marine reptiles. When oxygen was low, the percentage of marine reptiles was high. But as oxygen rose, the proportion of tetrapod families fully aquatic markedly dropped. This may not be that the absolute number of marine forms decreased as much as it was that the number of terrestrial dinosaurs markedly increased. Yet it marks an unusual and new view of the greenhouse planet that was Mesozoic Earth.

  TRIASSIC-JURASSIC MASS EXTINCTION

  One of the striking new findings of the oxygen-through-time results has been the level of Triassic oxygen. Only several years ago, the minimum oxygen levels of the past 300 million years was rather universally pegged at the Permian-Triassic boundary of 252 million years ago. But that time of oxygen low has been substantially moved, and now may correspond much more closely to the Triassic-Jurassic (T-J) boundary of 200 million years ago than previously thought. Thus, rather than the Triassic being a time of oxygen rise, or even a time with two downturns—one at the end of the Permian, one at the end of the Triassic—we are confronted with the possibility that oxygen was lower in the Late Triassic than in the early part of the period, perhaps as low as 10 percent of the atmosphere at sea level, or about half the modern-day levels. This time corresponds to one of the major changes of the Triassic, the winnowing out of most land vertebrates, with the exception of the first dinosaurs.

  The cause of this mass extinction, like the others, has been long debated. What is clear is that, like the Permian mass extinction, the Triassic-Jurassic mass extinction occurred in a dead heat (literally and figuratively) with the emplacement of one of the largest flood basalt episodes in the history of Earth, one second only to the Siberian Traps event of the Late Permian. Back-to-back mass extinctions, 50 million years apart, both temporally linked to large flood basalts, events well known to rapidly increase carbon dioxide levels in both air and sea to many times the starting values. Some estimates place the peak CO2 levels in the atmosphere as from 2,000 to 3,000 ppm, compared to our own 400 (2014) ppm (but rising fast!).

  The utter destruction of plant life makes a dent in the carbon cycle and changes the relative proportion of carbon 12 to carbon 13. The use of this comparison, the carbon isotope analyses discussed at many other points in this book, seems to be a fixture of the mass extinctions. But it was not until a report by Ward and others in 2001, from T-J interval strata nestled along a shoreline fronting an old-growth, cold-temperature rainforest located on one of British Columbia’s Queen Charlotte Islands,9 that this carbon isotope perturbation was found. Just as with the Devonian and Permian greenhouse extinction before it, the newly found signal is characterized by oscillating changes in the ratio of C13 to C12, brought about by changes in the abundance, kind, and burial history of diverse kinds of life on the planet.

  As for the Devonian and Permian events, this signal seemed to indicate that this extinction as well as the others were caused by something other than impact. The conclusion that the T-J was yet another in the “family” of greenhouse extinctions was briefly challenged by another kind of discovery soon after the first carbon isotope shift was reported on. Paul Olsen of Columbia University and colleagues announced to great press effect that the T-J was caused, in fact, by large-body impact with the Earth. This seemed to provide a nice symmetry—an asteroid ending the age of dinosaurs, and another, 135 million years earlier, seemingly started that same age of dinosaurs. Or so it seemed. Olsen’s evidence of impact had been found at a site in the Newark, New Jersey, region, home to the most diverse assemblage of late Triassic and early Jurassic dinosaur footprints on the planet. It was the association of dinosaurs and mass death that whetted the journalist’s appetite for extensive press coverage.

  Olsen and his colleagues reported an iridium anomaly from continental T-J boundary beds in New Jersey. It was just
such an anomaly that had first alerted the Alvarez team (in 1980) to the possibility of impact at the end of the Cretaceous; iridium had become the gold standard of impact evidence. But here the two studies wildly diverged. Where the Alvarez group followed the physical and geochemical evidence from their Italian boundary section with data confirming mass extinction of small ocean life at the same time as the impact, the Olsen paper for the Triassic event followed their physical and geochemical evidence with just the opposite: they found that rather than eliminating most life in their section, instead the impact seemed to have acted like a biotic fertilizer, leading to both more and bigger life!

  The Olsen group was sampling strata deposited on land (or more correctly, in streams and shallow lakes on land), and the “fossils” they studied were footprints, not the remains of body parts. But in spite of these rather startling differences, the Olsen et al. conclusion was the same: that a great asteroid had hit the Earth (this time about 200 million years ago, the age of the Triassic-Jurassic boundary), and that like the K-T event, the dinosaurs were affected. But the argument was that the impact killed off competitors of the dinosaurs, leading to a rise in diversity and animal size. And unlike the secrecy surrounding Luann Becker’s work and methods dealing with the Permian extinction, Paul Olsen brought all who cared to look to his urban outcrops. Plenty of the many specialists working on mass extinctions at the time made the trip.

  Olsen’s samples had yielded iridium, and unlike the Becker work, various labs confirmed his findings. But a finding of iridium alone may not have propelled this work into Science, the prestigious flagship of scientific publishing. Olsen and his colleagues had pulled another and totally different array of evidence out of their New Jersey rocks. At numerous outcrops equal in age to that yielding the iridium, Olsen and crew had noticed that a significant change was observable among the footprints. The beautiful three-toed footprints, known to residents of this area for more than two centuries, increased in number, size, and diversity of shape.