A New History of Life (36 page)

There are other clues than dinosaur bones to the nature of life on 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 CO
₂
, the well-known greenhouse effect. And as we saw in chapter 3, levels of oxygen and atmospheric CO
₂
are roughly inverse: when oxygen is high, CO
₂
is low, and the converse. Many periods in the past with low oxygen had high CO
₂
, and thus were hot. In a low-O
₂
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 review
⁸
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.

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 CO
₂
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 C
₁₃
to C
₁₂
, 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.

One would think that the footprints found in strata deposited after the T-J mass extinction would be fewer in number (number of animals around), fewer in kind (a lesser species diversity), and smaller in size, since one lesson that we do know from the asteroid-caused K-T extinction is that it was disproportionably lethal to larger-sized animals. While no dinosaur or any of the many kinds of reptiles and mammal-like reptiles matched size with the biggest dinosaurs going extinct at the end of the Cretaceous, many were equal in size to dinosaurs that did go extinct as a result of the K-T asteroid. Thus, fewer, fewer kinds of, and smaller-sized footprints would be expected in earliest Jurassic rocks, if the Triassic’s end was caused by an impact. Yet just the opposite was observed in all three of these evidence lines: there were more footprints, of more different kinds, and many were larger, much larger,
than the largest of the Triassic footprints. It was this evidence as much as the iridium finding that convinced
Science
that this research article was important enough to publish in their journal.

Just as in the case of Luann Becker’s work of a year prior to Olsen’s publication, the
Science
paper by Olsen et al.
¹⁰
was scrutinized in painstaking detail. Two experts on interpreting impact deposits, Frank Kyte of UCLA and David Kring of Arizona, were both of the opinion that the iridium finding was certainly indicative of an impact about that time; both also pointed out that the amount of iridium reported from the various sites of the Olsen group was at least an order of magnitude less than that found at virtually every K-T boundary site. Something fell to Earth, all right, but it was small—probably too small to cause the amount of extinction at the end of the Triassic. Thus, while evidence for impact at the end of the Triassic was much more believable than at the end of the Permian, it was still hard to believe based on this new evidence that the Triassic extinction was a K-T-like impact extinction.

There is indeed a large crater in Quebec. It is one of the biggest craters visible on the planet, named Manicouagan Crater—with a diameter of about 100 km (in comparison, the Chicxulub crater is 180–200 km in diameter). It had long been thought to be of the right age, too—somewhere near 210 million years in age, which was about the age of the Triassic-Jurassic boundary. The radioactive decay measures indicated that the Triassic came to an end about 199 million years ago. In 2005 this date was slightly changed to 201 million years ago. And not only did the T-J get younger, but the age of the Manicouagan crater got older. Better dating placed its age at 214 million years ago.

Our own work on the Queen Charlotte Islands was designed to look at the T-J extinction, but also to search for any possible fossil die-off prior to it—in rocks we could age as being around 214 million years in age. The “kill curve” estimates of the late twentieth century predicted that any impact event leaving a crater the size of Manicouagan would easily kill off between a quarter and third of all species on Earth—and we found nothing! Perhaps we have overestimated the lethality of asteroid impacts?

By the early years of the new century, geochemist Robert Berner of Yale University had greatly increased the resolution of his complicated computer models that estimated the amount of oxygen and carbon dioxide for any 10-million-year interval of the past 560 million years. His results showed a startling match between the times of lowest oxygen levels or the most rapidly dropping oxygen levels and mass extinction events.