by Peter Ward
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.
ONLY IMPACT?
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.5 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.6
BUT WHAT ABOUT THE DECCAN TRAPS FLOOD BASALTS?
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,7 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.8 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.
CHAPTER XVII
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The Long-Delayed Third Age of Mammals: 65–50 MA
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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.1
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.2
Here are the three most important. First, the major mammalian “groups”—the ei
ghteen 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.3
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.4 Fossils have also chimed in, most spectacularly from China.5 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.6
The oldest group of living placental mammals include elephants, aardvarks, manatees, and hyraxes.7 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.
THE TERRESTRIAL WORLD OF THE PALEOCENE
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.
On land, the dinosaurs had ruled for so long that with their passing, a whole set of new ecological relationships among the survivors had to be rather quickly worked out. And with the sudden absence of so many land animals, the evolutionary faucet of new species formation opened with one of the greatest gushers of diversity that the world has ever seen. The mammals, obviously, were the big winners on land, but birds made a comeback as well, and for some time competed with land mammals for various resources.
So it was into this ocean regime that the rock from space dropped. The great climate effects reverberated through the ecosystems for thousands of years, and there was great climatic instability added to the already slightly cooled world, both on land and in the sea. The biotic changes were no less devastating. For one thing, the disappearance of the dinosaurs led to denser forests. Just as modern-day elephants go a long way toward maintaining open spaces in forests by their movement and destructive eating habits, so too the dinosaurs, of far greater size, must have really affected vegetation patterns. But with their sudden disappearance the forests thickened up; it was as if some exacting gardener suddenly walked off the job, letting long-tended and pruned trees run riot.
By the Late Paleocene, more than 7 million years after the catastrophic K-T extinction, global climate had stabilized. The planet had slowly warmed to produce globally warm temperatures. From oxygen isotope evidence we know that equatorial surface waters of the oceans were in excess of 20°C, reaching as much as 26°C in some places, and thus were quite similar to ocean temperatures in similar latitudes today. But the big difference to our world occurred at higher latitudes. In the Arctic and Antarctic the surface of the sea was between 10°C and 12°C, compared to the near-freezing temperatures of our time. Thus, the difference in heat from equator and pole was some 10°C to 15°C, which is about half of what it is today. Nevertheless, in spite of these temperature differences, the oceanic circulation patterns were fairly similar to those of today. Most important, oxygenated water masses that ultimately would end up on the bottom of the ocean formed at high-latitude sites, just as they do today.
After the K-T mass extinction of 65 million years ago, it took some millions of years for the surviving mammals to grow large enough to start affecting plant patterns. There have been many artistic images of tiny, rat-sized mammals crawling from bomb-shelter-like burrows in a world of stinking, rotting dinosaur corpses. For some months, those mammals that could eat carrion would have been in Nirvana. But soon enough there were but bones, and even these rotted away or were buried in fairly short order, forcing all of the mammals to strike out in a newly organizing series of food webs that were unprecedented. It was before the time of grass, so the herbivores of early Paleocene time were leaf or fruit eaters rather than grazers. Seemingly, there were few leaf eaters at all. Most of the teeth of the Paleocene mammals argue for a diet of insects, fruit, or soft shoots rather than tougher leaves; others may have been root or tuber eaters. It was only in the latter half of the epoch that tooth morphology appropriate for eating leaves appeared in any number. But once opened, the evolutionary faucet fairly spewed out new kinds of mammals, ever-larger mammals among the new in a torrent of evolution. Then, only 9 million years after the great K-T mass extinction, once again the biotic world was affected by environmental crisis.
THE PALEOCENE EOCENE THERMAL (PETM)
By the early Cenozoic era, the Earth had suffered through at least nine mass extinctions that we know of: the first was the great oxygenation event and the snowball it triggered, the second more than one billion years later was during the Cryogenian, then, in order, the late Ediacaran, late Cambrian, late Ordovician, late Devonian, late Permian, late Triassic, and late Cretaceous mass extinctions. The causes were amazingly varied: from sudden oxygen to too little of it; from the appearance of predators to the onset of anoxia coupled with hydrogen sulfide emissions to asteroid impact. But at the end of the Paleocene epoch, only 9 million years after the dinosaurs died out, there was to be a new assassin: methane, which precipitated one of the most rapid rises in global temperatures known. It is called the PETM: the Paleocene-Eocene thermal event.
This event was first discovered by oceanographers8 who were not at all looking for any sort of temperature event of late Paleocene age. They were trying to get new data on the K-T mass extinction from deep-sea cores drilled by the US Ocean Drilling Program (ODP). But to drill down into the Cretaceous, the drills first had to pass through Eocene and then Paleocene sediment. Cores were taken from those depths while the drills went ever deeper toward their real quarry.
When these younger cores were eventually examined and measured for the carbon and o
xygen isotope found in the shells of tiny, single-celled protists known as benthic Foraminifera, the registered temperatures, as well as the ration of carbon 12 to 13, looked like they had to be in error: they showed that when a series of cores were compared, those with strata pulled up from ancient, deeper-water parts of the ocean showed warmer paleotemperatures than those from shallower paleolocalities. Even in the frigid Antarctic today, water cools with depth, and back in the surely much warmer Paleocene, deeper water should be obviously colder than shallower. But the numbers here said just the opposite. Warm, deep waters and cool, shallow waters. Over a relatively short period of time the deep ocean had anomalously warmed.
Near the Paleocene-Eocene boundary there is a striking increase in global volcanic ash.9 Like dust, this fine material makes its way to the seafloor from the atmosphere, but is put up there by volcanic eruption, not atmospheric storms. This increase could only be due to a sudden increase in global volcanic activity, about 58 to 56 million years ago. Further work in many places around the globe confirmed these findings as being global phenomena, not anomalous events limited to one ocean basin.