A New History of Life (37 page)

All three of the mass extinction events with problematical causes showed strata indicating deposition in low-oxygen conditions.
¹¹
Under such circumstances, strata usually turn black (because they contain the mineral pyrite and other sulfur compounds that are said to be reduced in that they were produced by chemical reactions that can occur only in the absence of oxygen). A second clue came from the fact that the rocks of these ages were thinly bedded or even laminated, often showing delicate sedimentary structures with the strata. Because so many animals burrow, most strata deposited in the sea since the Cambrian are what is called bioturbated by the vast number of invertebrates that ingest sediment at the bottom of a body of water in order to strain out any organic material. The presence of the fine bedding could occur only in environments with no or only rare animals. Through these three avenues—modeling, rock mineralogy (dictating color), and sedimentary bedding—it was clear that the Permian, Triassic, and Paleocene extinctions took place in a low-oxygen world.

Other evidence discovered in the late 1990s and early part of the new century showed that while oxygen may have been low, another constituent of the Earth’s atmosphere was at the same time high: carbon dioxide. Like the evidence for low oxygen, the CO
₂
evidence came from Berner’s models as well as from evidence preserved in the rock record, or more accurately in this case, in the fossil record. Unfortunately, there is no way of actually measuring the exact volume of CO
₂
that was present at any time in the past. Carbon dioxide does not color rocks or affect bedding. But some very clever work on fossil leaves resulted in an important breakthrough that allowed a relative
measure of CO
₂
. Using this method, for instance, a paleobotanist could determine whether carbon dioxide levels were rising, falling, or staying the same over million-year intervals, and furthermore the method allowed estimates on how many times higher or lower the levels were from some base-level observation.

The CO
₂
measure turns out to be both clever and simple, as so often it turns out with wonderful breakthroughs. Botanists looking at modern-day plant leaves had done experiments in which they grew plant species in closed systems where the amount of CO
₂
could be raised or lowered relative to the level found in our atmosphere (about 360 ppm when these experiments were first conducted). Plants, it turns out, are highly sensitive to carbon dioxide levels, since even the small amounts of CO
₂
in the atmosphere must serve as the source for their carbon, the major building block of life. They acquire this mainly in their leaves through tiny portals to the outside world called stomata. When grown in high levels of CO
₂
, the plants produced a small number of stomata, as even just a few sufficed in high CO
₂
levels. The investigator then eagerly turned to the fossil record; leaf stomata are readily observable in leaf fossils. The results confirmed Bob Berner’s model results.

At the end of the Permian and during the early Triassic, the fossil leaves showed only a few stomata. Carbon dioxide was spectacularly high at all three times. Moreover, not only was it high, but the rise in CO
₂
happened quickly, on the order of thousands, not millions of years.

These two results give an entirely new view of mass extinctions. Each occurred in a world quickly warmed by the short-term rise in carbon dioxide (and perhaps methane as well, based on yet another line of evidence). And in addition to being hot, it was a place also low in oxygen. High-temperature, low-oxygen conditions coincided with major mass extinction. While modern-day greenhouses are not places of low oxygen (just the opposite by photosynthesis), they are places that heat very quickly due to the greenhouse properties of the glass panes covering the whole structure. Sunlight comes through the windowpanes, but when sunlight is radiated back in the form of light
waves and heat, the glass panes trap the energy, which then warms the air, much like carbon dioxide, methane, and water vapor molecules do.

Heat is dangerous to any animal. The highest temperature that any animal can withstand is not even halfway to the temperature that boils water. At 40°C most animals die off, and the last holdouts die at 45°C. As is all too tragically known from the many sad cases of kids left in cars on a sunny day, rapid heating can be lethal. And the two aspects of this physiological system—the amount of oxygen available and the amount of heat energy—combine to make things even more lethal: animals need more oxygen as heat increases.

Of the three extinctions, the data for the Triassic-Jurassic CO
₂
rise is particularly stunning. University of Chicago paleobotanist Jenny McElwain, collecting rocks in the dangerous and frigid outcrops amid the ice of Greenland in the last years of the twentieth century, showed without doubt that the end of the Triassic was ushered in by a sudden rise in CO
₂
in an already low-oxygen world.

Increasingly, the Triassic began to look like an event similar to that at the end of the Permian. What it did
not
look like was the K-T extinction event, in which the extinctions were sudden and spread across every animal and plant group. But it was as if none of them “saw” it coming from an ecological or evolutionary sense. At the end of the Triassic, on the other hand, every group except the saurischian dinosaurs were undergoing size reduction (or at best, maintaining roughly equal diversity) in the time intervals leading up to and after the Triassic-Jurassic mass extinction, as if they knew bad times were coming, and small size would be more adaptive.

The groups with the simplest lungs (amphibians and the early-evolved reptiles) fared the worst, and many groups that had been very successful early in the Triassic, such as the phytosaurs, underwent complete extinction. Both amphibians and archosauromorphs probably had very simple lungs inflated by rib musculature only. Mammals and advanced therapsids of this time, probably both having diaphragm-inflated lungs, did better, but crocodiles, presumably with abdominal pumps, did poorly. The success of the saurischians may have been due to a multitude of factors (food acquisition, temperature tolerance,
avoidance of predators, reproductive success), but our conclusion is that this group was unique in possessing a highly septate lung (one with many tiny flaps to increase surface area) that was more efficient than the lungs of any other lineage, and that in the very low oxygen world that occurred both before and after the Triassic-Jurassic mass extinction, this respiratory system conveyed great competitive advantages. Under this scenario, the saurischian dinosaurs took over the Earth at the end of the Triassic and maintained that dominance well into the Jurassic because of superior activity levels.

We now know that alone among the many kinds of reptilian body plans of the mid to late Triassic, the saurischian dinosaurs diversified in the face of either static or, more commonly, falling numbers in the other groups. We also know that oxygen reached its lowest levels of the past 500 million years in the Late Triassic. Something about saurischians enhanced their survival in a low-oxygen world. The ground truth suggests that a long and slow drop in oxygen culminated in the Triassic mass extinction, but that this extinction was really a double event, separated by a range of 3 to 7 million years.

There are few places on land where this time interval with abundant vertebrate fossils can be found. We really do not know the pattern of vertebrate extinction as well as we do the extinction in the sea. We do not know how rapidly the prominent victims of the mass extinction—the phytosaurs, aetosaurs, primitive archosauromorphs, tritylodont therapsids, and other large animals—disappeared. But by the time the gaudy Jurassic ammonites appeared in the seas in abundance, leaving behind an exuberant record of renewal in early Jurassic rocks, the dinosaurs had won the world. What kind of lungs did they have? There is no certainty but one: they had lungs and a respiratory system that could deal with the greatest oxygen crisis the world was to know in the time of animals on Earth.

A new view of things is that saurischian dinosaurs had a lower extinction rate than any other terrestrial vertebrate group because of a competitively superior respiration system—the first air-sac system. The fact that saurischians were actually
expanding
in number across this mass extinction boundary is the most striking aspect of all.

The word “Jurassic” is now irrevocably linked to dinosaurs and dinosaur parks, thanks to the
Jurassic Park
franchise of movies. In fact the real Jurassic was a world looking nothing like the cinematic view seen in those three progressively idiotic movies. Those movies were filled with plants that had not yet evolved in the Jurassic: the angiosperms, or our familiar flowering plants. In fact, it is impossible to even characterize a “Jurassic” world, because it was a world that was utterly changed from its earliest Jurassic appearance (201 million years ago to its latest iteration, of about 135 million years ago). At the beginning, it was a shattered world: a world again coming out of mass extinction; a world without coral reefs; a world where the dinosaurs were still few in number, species, and size; a world of such low oxygen that insects could barely fly, but to no matter, as no vertebrate flier could have caught them anyway. But things were to change in the relatively short time interval (in terms of geological periods, anyway).

By the end of the Jurassic the largest land animals of all time were common: dinosaurs were lords of all creation; tiny primitive birds and tinier primitive mammals hid in the lowest-rent districts in town. At the beginning the seas were so bare that stromatolites had made a comeback, and the larger fish and predators were few indeed.

By its end there was a veritable cornucopia of the most spectacular marine denizens to have ever populated the sea: long-necked reptilian plesiosaurs, dolphin-like ichthyosaurs, and splendid primitive fish—similar to the modern-day gar and sturgeon (both with strange body armor)—schooled among vast coral reefs and an ocean filled to exuberance with all manner of ammonites and their more squid-like relatives, the belemnites. The ammonites came in all varieties, from smooth to ribbed, and varied in shape from planispiral to, at the end of the period, peculiar, gently arcuate cones. The largest of all ammonites
came from Jurassic rocks, the giant from Fernie, British Columbia, that in life was close to eight feet in diameter and surely would have weighed a half ton. Yet an odd thing has happened to the scientists studying this most iconic of prehistoric periods: most are dying off and not being replaced.

It is fair to say that geology as a modern discipline came into being because of the Jurassic. It was Jurassic beds that were first mapped by William “Strata” Smith, in the earliest 1800s, and it was Jurassic strata that demonstrated that fossils could be used to correlate beds in far-off lands. It was ammonites from Jurassic strata that provided Darwin with the then-best-known examples of evolutionary change. (References to this period are found in chapter 1, and as always, the historical works by the British historian of science Martin S. Rudwick are recommended.)

The Jurassic period showed the same pattern of short-term evolutionary explosion common to all of the post-extinction time intervals. They are called recovery periods. Each began with a low diversity of survivors of the mass extinction, but ended only after a 5- to 10-million-year time interval. Yet after this post-extinction hangover, diversity would always then again rise. The new animals and plants were always composed of a largely different assemblage of species. In most cases these species are newly evolved during the recovery interval, but in some cases they are taxa that lived a precarious, low-abundance existence prior to the extinction, but then exploded in numbers and ecological success in the new world. The early Jurassic was no different, and from the seeds of recovery a great new assemblage of marine creatures evolved into existence, composed in large measure of new kinds of mollusks, marine reptiles, and many new kinds of bony fish. But it is not the marine fauna that the Jurassic (or the succeeding Cretaceous period) is known for. No one has made three blockbuster movies featuring marine life with the name Jurassic in the title. The public wanted, and still wants, but one thing.

It is impossible to write a history of life without dwelling at length on dinosaurs. Yet at the outset it seems a losing prospect given the warrant
of this book: that the histories narrated here have an element of the “new.” So much is written about these antediluvian saurians (the Victorians’ view of them) that bringing anything fresh to the table seemed impossible at the outset of this book’s writing. It was thus a very welcome surprise to find that in fact new findings clutter the record of twenty-first-century science. Any nonspecialist summary of dinosaurs normally dwells on three issues: whether they were warm-blooded, how they reproduced and what is known of their behavior of nesting, and how they ultimately died out. But there are other interesting questions as well, including perhaps the most interesting of all: why were there dinosaurs at all, or at least the dinosaurs’ body plan? This, in turn, is related to the question of how they respired. A second question we will look at here is also related to respiration in its own way: what is new about the dinosaur-to-bird story? A great deal, it turns out, most coming from new Chinese discoveries (but new Antarctic discoveries as well, which the authors of this book were witness to). Finally, our new century has given information about two of the most fundamental aspects of dinosaur physiology, a definitive answer to the long-running mystery of whether dinosaurs were warm-blooded, as well as new discoveries about the characteristic growth rate of dinosaurs. And here too one of the fascinating directions coming from these new data bring us right back to the differences between dinosaurs and “true” birds, not just avian dinosaurs, but species with all the traits we now associate with being a bird.