A New History of Life (46 page)

The inoceramids provoke the most wonder. They are up to two feet in diameter, looking like shallow but giant plates lying side by side with smaller versions of themselves—different species, in fact. For a hundred meters of stacked strata they are ubiquitous in each bed, and because of the dip of the beds, some of the bedding planes that can be examined are hundreds of square meters in area. Fossils are most easily found on the tops or bottoms of any stratum, rather than the sides, and the best hunting is always on the tops of large bedding planes. At Zumaya there are many of these to be explored and collected, and thus the fossil numbers are very large for any fossiliferous beds of
any age. But then the large clams disappear, and they do so more than a hundred meters stratigraphically below the well-marked ammonite extinction horizon. The ammonites and echinoids continue in abundance right up to the point where they suddenly and dramatically disappear.

The work at the Bay of Biscay coastline, supplemented by work at other late Cretaceous deposits, tells us that the inoceramid bivalves died out gradually around 2 million years before the ammonites suddenly died out. In fact, using statistical methods developed by Charles Marshall of UC Berkeley, Marshall and coauthor Ward showed that at least twenty-two species of ammonites existed in this region right up to the layer that contains the most important evidence of impact: iridium, shocked quartz, glassy spherules (tektites, which are bits of rock blown upward by the titanic impact and then turning to tiny fragments of glass as they fell back to Earth at high speed).

The curious part about the inoceramids’ extinction is not that they died out well before the ammonites, but that their extinction took place at different times in different places. For instance, the last inoceramids in Antarctic Cretaceous rocks are no younger than 72 million years in age, or about 7 million years prior to the ammonite extinction. We now know that these globally distributed bivalves experienced a wave of species death starting first in Antarctic regions and then gradually moving to the northern hemisphere. It was almost like a disease slowly moving northward and killing off the clams in gradual fashion. But this was no disease: it was cold and oxygen.

Near the end of the Cretaceous, an oxygenated kind of thermohaline circulation began to occur in the high southern latitudes, and over about 2 million years this cold, oxygenated bottom water spread into all the seas, moving from south to north. Its presence spelled the end of the clams that we affectionately called inos, and the disappearance of these clams was a signal event in the history of life, for they had been highly successful up to that point for more than 160 million years. But they were adapted to the other kind of ocean, the low-oxygen and warm bottom water variety. Cold and oxygen killed them off.

We can now summarize current understanding of the primary event that appears to have caused the K-T mass extinction. There was but a single comet strike, coming soon (1–3 million years) after two rapid changes in global sea level, themselves sandwiched around a major change in ocean water chemistry.
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The impact created the large (up to 300 km diameter) crater now named Chicxulub, located in the Yucatán Peninsula. Although there is still debate about the actual crater size, there is now no doubt that the structure is a crater. The impact target geology and geography may have maximized subsequent killing mechanisms. This is especially true because the presence of sulfur-rich evaporites in the target area, and sulfur within the incoming comet itself, may have contributed to subsequent lethality. The 65-million-year-old comet strike in an evaporate-rich carbonate platform, itself covered by a shallow sea at an equatorial latitude, seems to have created unbelievably dire consequences: worldwide change in atmospheric gas inventory, accompanied by temperature drop, acid rain (mainly from sulfur derived from evaporites at the impact site), and global wildfires are all proposed killing mechanisms. Most scientists (but not all) also agree that thick, coarsely clastic sedimentary deposits found at many places along the eastern coast of Mexico were formed by impact waves. The prolonged impact winter was thus the most important killing mechanism—and it was brought about by vastly increasing aerosols in the atmosphere over a short period of time.

Another recently published model describing atmospheric changes following impact suggests that greatly increased levels of atmospheric dust generated by the blast may have been lethal as well. The fine dust would be generated by impact into either an oceanic or a continental target area and would produce a long-term (on the order of months) blackout. This reduction in light levels (below that necessary for photosynthesis) would be accompanied by rapid cooling of land areas. This excess dust would also adversely affect the world’s hydrological cycle. Advanced climate modeling has indicated that, following a large impact event, globally averaged precipitation decreases by more
than 90 percent for several months and is still only about half normal by the end of the first year following the impact. The effect on the biota is now well established.
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The pages above make the case that the K-T mass extinction was largely a one-event mass extinction. The Earth was hit, and that hit led to environmental changes sufficient to kill off more than half of all species then on Earth. There remains only one nagging bit of unexplained information. The asteroid hit a planet that was already in the midst of one of the most extraordinary periods of flood basalt volcanism known from any time in Earth history. Called the Deccan Traps, this event caused untold tons of basalt to issue forth onto the Earth’s surface, with an origin from deep within the Earth. Perhaps 84 million years ago, a gigantic mass of molten rock detached from near the mantle-core boundary to begin an upward voyage that would take around 20 million years. On its way up, this great mass of molten rock very likely caused the Earth to undergo episodes of true polar wander, events that happen when there is a mass imbalance such that internal balance dictated by laws governing the conservation of momentum of our spinning planet cause great landmass movements. These rapid movements might have destabilized some environments. For instance, much of western Canada and Alaska seems to have resided at the latitude of Mexico prior to 84 million years ago—but found itself far from Mexico as the Mesozoic ended.

Yet of all of the effects of flood basalts, the most consequential for life, as we have seen in multiple episodes reported earlier in this book, is the great outpouring of carbon dioxide and other greenhouse gases that accompany flood basalt volcanism. The Earth warmed quickly at the poles and other high-latitude regions, but less quickly at the equator. These conditions have led to what we call greenhouse extinctions. A big flood basalt causes the high latitudes to heat, pushing the oceans into stagnancy and then anoxia. Deep ocean waters filled
with poisonous hydrogen sulfide rise to the surface. Things then died, as they did in the Devonian, Permian, and Late Triassic. Yet the dirty little secret is that we students of these mass extinctions have for too long swept this unfortunate evidence under the carpet. Who needs death by stagnation when there is more than enough death to be handed out by an impacting asteroid?

Science gets things right, eventually, if the question is interesting enough. And there are few more interesting questions than those pertaining to why dinosaurs (and so much else) died out 65 million years ago. Why were there no observable effects of the Deccan Traps,
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when all of the other flood basalts did so much damage and caused so much obvious species extinction?

In fact, the Deccan did a lot of damage. Perhaps the best evidence of this, modesty aside, comes from our own work in Antarctica. In 2012, one of our students, Tom Tobin, showed that there indeed was warming of the oceans some hundred thousand years prior to the impact—and that species did die because of this.
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As noted, global warming (which is the result of a flood basalt, ultimately) is larger in extent (temperature change) at high latitudes. The tropics are already about as warm as they can be. As we are seeing in our own world, it is the Arctic and Antarctic that bear the brunt of temperature change—and temperature-change-caused havoc and extinction.

So too with the K-T. Yes, a large asteroid hit us. But for hundreds of thousands of years prior to this, a suddenly warmer world was made stagnant by flood basalt. We can finish this chapter with a hoary boxing analogy. A knockout punch is by definition a single blow. Yet very few knockouts occur from the first blow, no matter how devastating. It is the many rounds of jabs and body blows that set the stage. The Deccan Traps softened the world. The asteroid finished the job.

The earliest-known mammals were tiny, shrew-sized waifs named morganucodontids, living (probably fearfully) among the many larger predators of 210 million years ago in the latest Triassic—and then somehow surviving the great T-J mass extinction. Soon the morganucodontids were joined by other primitive but “true” mammals. All living mammals today, including us, descend from the one line that survived this extinction. This is what the world came to after its long dinosaur era came to a crashing, fiery end: a plague of rats. Or at least rat-sized survivors.
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Paleontologists have long believed that the ancestors of all mammals living today emerged on one of the northern continents as Pangaea slowly split apart throughout the Mesozoic era, and then only slowly migrated south, all the way to Antarctica and Australia, as land connections (or only narrow waterways) developed between the continents. This has been dubbed the Sherwin-Williams model of evolution, a reference to paint dripping over a globe from north to south by a long-lived US paint company. But this idea has to be tossed on the giant mound of discredited hypotheses, with new evidence coming from both fossils and genetics. It now looks like the wave of mammalian modernization went from south to north. Especially telling are the newly collected fossils of advanced mammals far older than any known in the north.

Geneticists have also joined in, once again replicating a familiar pattern of major new understandings coming from both DNA comparisons as well as evo-devo. There has been no end of surprises in the twenty-first century.
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Here are the three most important. First, the major mammalian “groups”—the eighteen living orders, as well as some suborders and even families still found today—actually diversified long before the
extinction of the dinosaurs, which overturns the long-held idea that these groups did not evolve until after the K-T calamity. Fossils suggest that most modern groups appeared around 60 million years ago, after the dinosaurs were gone. Molecular data suggest they actually began diversifying about 100 million years ago.
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Second, most early mammalian evolution and subsequent divergence happened on southern rather than northern continents. Third, many groups thought to be very distant cousins are in fact next of kin. For example, paleontologists have always assumed that bats were in the same superorder as tree shrews, flying lemurs, and primates. But genetic data place bats with pigs, cows, cats, horses, and whales. Whales themselves are now known to have come from piglike ancestors, rather than from the same stock that gave rise to seals.

Much of mammalian success came from anatomical change, including the separation of the jaw and the ear bones, which allowed the skulls of later mammals to expand sideways and backward, a prerequisite for bigger brains. But the most important of all innovations was by the revolution of mammalian teeth. The upper and lower molars of morganucodontid jawbones interlocked, letting them slice their food into pieces.

Today’s mammals are split into two major groups: the ancestral marsupials, which produce extremely small newborns and then keep them in a pouch—and their more diverse and abundant descendants, the placental mammals. New DNA studies now suggest that placental mammals began to diverge from marsupials as early as 175 million years ago.
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Fossils have also chimed in, most spectacularly from China.
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There, a complete new fossil of a protoplacental species found in Liaoning Province supports the DNA inference that placentals began evolving much earlier than previously thought. Named
Eomaia
, the fossil’s age at 125 million years makes it easier for paleontologists to accept the genetic evidence that says the first protoplacentals began to evolve as much as 50 million years earlier yet, back in the Jurassic.
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The oldest group of living placental mammals include elephants, aardvarks, manatees, and hyraxes.
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When the African continent split off from the former supercontinent of Pangaea, it carried these animals
away to evolve on their own for tens of millions of years. The continental dispersion also split South America from Eurasia and North America for millions of years, and South America became home to sloths, armadillos, and anteaters. The northern continents have the youngest placental mammals on Earth, including seals, cows, horses, whales, hedgehogs, rodents, tree shrews, monkeys, and eventually humans.

Yet, if a great deal of mammalian diversification predated the K-T extinction, the most notable change—size increase—happened soon after the fall. Within 270,000 years mammals were diversifying and growing bigger, although the really large mammals did not appear until around 55 million years ago. Then a rapid increase in global temperature was coincident with a widespread growth of forests around the world, even near both poles, and this aspect of the history of plants may have helped stimulate a great increase in mammalian diversity.

That there was a Paleocene at all is entirely due to the K-T mass extinction. That mass extinction was absolutely unequivocal in its cause and effect. And the world afterward was very, very different, on so many levels.