DOUGLAS H. ERWIN, “The Mother of Mass Extinctions”
So what did cause the greatest mass extinction in the past 600 million years, and perhaps the greatest in the history of life? The short answer is that we do not know, or at least I do not know. . . . In the wake of the Alvarez-impact hypothesis many of us seem to prefer a single dramatic cause as an explanation for such events. Our knowledge of recorded history provides precious little support for such a view, and I see little reason, a priori to expect such a neat and tidy resolution to this riddle.
DOUGLAS H. ERWIN, Extinction
The aim of science is to seek the simplest explanation of complex facts. We are apt to fall into the error of thinking that the facts are simple because simplicity is the goal of our quest. The guiding motto in the life of every natural philosopher should be: Seek simplicity and distrust it.
ALFRED NORTH WHITEHEAD, The Concept of Nature
Two film genres from my childhood remind me of Permian times. The first is the giant arthropod thriller. These sorts of movies are not as popular as they once were, but you’ve probably seen one or more. The classic example is the 1955 thriller Them, whose plot involves gigantic ants mutated by atomic radiation. It was the first giant arthropod thriller and, in my opinion, it was one of the best. Many others have appeared over the years. You can take your pick. There was the massive Tarantula terrorizing teenagers at drive-in movies,1 The Deadly Mantis with preying mantids big enough to eat army tanks, and Mothra, the moth creature whose wing beats created winds strong enough to blow over office buildings in Japan. And of course there was The Fly, an all-time favorite about the hapless inventor who has the rotten luck to get insect parts mixed with his own when he enters his innovative transporting device. The Fly is perhaps the only example of the giant arthropod thriller that Hollywood valued enough to honor with a modern remake. Finally, there are the more recent but very popular Alien movies. In case you were too appalled to notice, the alien had all the key features of an arthropod: a hard external skeleton, segmented body, jointed appendages, and metamorphic development with molting, as well as extendable mouthparts resembling those of an immature dragonfly.
The second genre is the murder mystery, which was very popular when I was a kid. There were the classic novels by Agatha Christie and their movie adaptations, like Murder on the Orient Express and Ten Little Indians. Who could forget Alfred Hitchcock’s Psycho and Rear Window? The weekly television show Perry Mason was a favorite, and judging from the popularity of shows like CSI: Miami and Law and Order, it seems that America’s craving for murder mysteries has not diminished.
Fortunately for us, there are no insects as large as Them or The Deadly Mantis, and humans thankfully don’t have internal parasites as enormous as the Alien. But during the Permian, about 299 million years ago, insects did reach gigantic sizes—not as large as any in these movies, but still impressively bigger than any living today. The fact that these massive insects did exist raises interesting questions about why they lived and why even larger insects never evolved. The Permian also presents a number of unsolved murder mysteries. The colossal insects died during that period, disappearing forever, and the Permian saw the rise and fall of the giant fin-backed reptiles and saw the last of the trilobites. But all these creatures were not alone in their demise. The end of the Permian is marked by the greatest mass extinction in the history of life, when most of the species alive up to that point died. This event is the greatest murder mystery of all time.
Observing the Scene of the Crime and Assessing the Time of Death
The transition into the Permian’s early years was gentle. Many of the Carboniferous period’s plants and animals continued to thrive, and giant fin-backed reptiles were prominent until the Middle Permian. Among them was the massive Dimetrodon, which fed mostly on fish and amphibians, but these beasts probably ate insects too, especially when young. Even a large hundred-pound Dimetrodon would have paused for the meal provided by a newly emerged air dragon nymph just crawled up from the water but not yet able to fly. The great fin-backed reptiles dominated the Early Permian shorelines, but in the Middle Permian years they were ecologically displaced by the appearance of new kinds of large vertebrates, including the first warm-blooded animals—the Kazanian protomammals—and the thecodont reptiles, the clan that would later, in the Mesozoic era, lead to the dinosaurs.
Over just a few million years, the protomammals diversified explosively, and what a bunch of noisy brutes they were. They could run, jump, snap, and bite like anybody’s business. They scouted out the hills first with slim, fast, dog-sized predators, but before you can say “Dimetrodon steaks, a dime a pound,” the protomammals diversified into all sorts of terrestrial ecological roles. The dominant large predators, dome-headed Kazanian protomammals, had bony skulls, could roar, and butted heads in fearsome territorial displays. There were also dozens of smaller species that fed on insects; the hot-blooded pursuit of arthropods had begun. So in the upland meadows, while the bugs first tippled on plant saps and the beetles first chewed on logs, it was a brave new world for the vertebrates. Never before had the terrestrial habitats seen such an assortment of these bony creatures. But after ten million years or so, the Kazanians fell by the wayside, and a whole new wave of protomammals terrorized the Permian forests: the dynasty of the Tartarian protomammals. The old dome-heads were replaced by ferocious two-tuskers. Once again a new regime of warm-bloods filled the big animal niches in the Permian meadows and forests, and once again there were lots of small ones that probably ate insects. Then after only a few more million years, the Tartarian protomammals also fell, this time in the days of the end-Permian mass extinction.2 Overall, about 70 percent of the vertebrate families were lost.
Permian times also saw a startling diversification of insect species, one that surpassed the famed Cambrian explosion in species richness, if not in experimentation with anatomical forms. It is the geological interval with the greatest ordinal-level diversity—there were at least twenty-two insect orders, more than even now—and the old-winged insects reached a peak of diversity unlike any before or since.3 The Permian years also saw the first of the orthopteroid insects, the ur-crickets and ur-katydids; the first of the hemipteroid insects, the true bugs with sophisticated siphoning mouthparts; and the very first insects with complete metamorphosis, the beetles, lacewings, scorpionflies, and caddisflies. However, the insects experienced more extinction than at any other time in history. Eleven of the Permian’s twenty-two orders are now extinct: eight apparently disappeared completely at the end of the period, and three others fell into serious decline and were gone by the Early Triassic.
The plants didn’t fare any better. The old Carboniferous and Permian flora faded away. The seed ferns, giant horsetails, and treelike lycopods disappeared, and tall ferns declined greatly in stature and diversity. Plants that had been previously rare moved in to dominate the drier Mesozoic uplands: these included tall conifers, abundant cycads, gingkoes, and new kinds of short ferns in the undergrowth.
The changes in land communities, although dramatic when viewed from our present day, appear to be more gradual when compared to the changes in ocean communities. In fact, the best case for a comparatively sudden Late Permian extinction event involves the ocean reef inhabitants. Our evidence comes from well-preserved Permian and Triassic communities in marine fossil sediments; these fossils show us that more than 90 percent of ocean-dwelling species went extinct, including the last of the trilobites.4 Some marine groups, notably the trilobites, had been declining long before the end of the period, but the end-Permian events were the final nail in their coffin. It took millions of years, well into the mid-Triassic, for ocean reefs to recover their former levels of species richness and ecological complexity. When they did recover, the new Mesozoic reef communities more closely resembled modern marine ecosystems. Gone forever were trilobites, Petoskey corals, and giant sea scorpions, while the diversity of brachiopod lamp shells and crinoids dropped precipitously.
The Permian extinctions are the ultimate cold case: 252 million years elapsed before anyone noticed that something unusual had happened and started an investigation. From our observation point, the differences between Permian and Triassic communities of life look dramatic and sudden, even if they took millions of years to develop, as was once thought. We are still studying how quickly these changes emerged, but the growing consensus is that the end-Permian extinction was not a singular, momentary event but a prolonged process over a longer period of time, perhaps fifty thousand to a hundred thousand years, that especially affected the oceans. Whether the extinctions happened slowly or quickly, the Late Permian certainly saw stunning upheaval, widespread death, and a remarkable transition to new life. The extinctions and changes in plant and animal communities from the Permian into the Triassic years are so extraordinary that geologists naming the earth’s layers decided to draw a special line at this point in time and divide the geological periods into eras of life. All the periods discussed so far—the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian—are combined into a single time known as the Paleozoic era, the “age of old life.” The second era, the Mesozoic, encompasses the Triassic, Jurassic, and Cretaceous periods, the time of the dinosaurs.
The Suspects
Here’s a brief list of some of the various hypotheses for the Permian extinctions. New kinds of animals evolved, which displaced older animals that could not compete as effectively for resources. New plants arose, which were better adapted for drier climates. Plant-feeding insects went extinct along with their Paleozoic hosts, and they could not adapt to feeding on new plants. A stupendous event, perhaps massive volcanic eruptions or an asteroid collision with the earth, killed oceanic plankton and catastrophically eliminated the base of the marine food web.5 Some have suggested that both widespread volcanic eruptions and a massive asteroid impact happened at the same time. Others implicate the changing continents. Plate tectonics altered the continents’ positions, changing sea levels, reducing shoreline habitats, and creating drier climates inland. Continental drift combined land areas, bringing together communities of organisms that could not coexist. Then there are the glaciers. Some suggest that expanding ice sheets caused turbulent mixing of ocean waters, which brought toxins to the surface from the depths. Others implicate changing atmospheric gases, either increasing carbon dioxide or decreasing oxygen levels or both. It has been suggested that the Triassic’s characteristic red rocks indicate an oxygen-depleted atmosphere. Peter Ward, in his 2004 book Gorgon, argues that the massive gorgon protomammals of South Africa died of asphyxiation, while the dinosaur lineages survived by virtue of their more competent lungs. Declining oxygen levels are claimed to have thinned the air, making it more difficult for gigantic insects to fly, and perhaps impossible for them to function at high metabolic rates.
Smithsonian paleontologist Douglas Erwin has written two books and several articles on the subject of the end-Permian extinction. He points out that there are people who think they know what caused the extinctions and that because some of the proposed answers are “mutually contradictory,” they can’t all be correct. Describing it as “a tangled web rather than a single mechanism,” Erwin proposes a Murder on the Orient Express–type hypothesis, which suggests that there is not one simple answer; instead, the best solution might be a combination of several of the proposed causes. He notes that the end-Permian events seem to have taken place in three parts. The first involved the widespread drying of shallow ocean basins along with wide swings in climate conditions. The second phase involved chemical changes in the oceans, as well as widespread volcanic eruptions that contributed to a rapid increase in atmospheric carbon dioxide levels. This accelerated global warming and a sudden drop in oceanic oxygen levels—a condition known as anoxia—which had disastrous consequences for ocean life. The third part involved widespread extinction in coastal and near-shore terrestrial communities.
In his 2006 book, Extinction, Erwin bluntly says, “I do not know.” That’s a pretty amazing statement coming from the one scientist who has studied the end-Permian extinctions more thoroughly than any other living person, and it probably serves to illustrate the problem’s continuing complexity. But fortunately he goes on to give his best educated guess at the possible solution, echoing his previous thoughts: the Siberian “flood basalt” volcanoes may have triggered the mass extinction by releasing massive amounts of sulfuric aerosols and carbon dioxide into the atmosphere, which could have triggered global warming. Global warming would have caused anoxia and the rapid mass extinction of ocean life. This scenario nicely explains the rapid catastrophic loss of life in the salt-water oceans, but the events on land with terrestrial insects, and in fresh water with aquatic insects, remain more mysterious.
FIGURE 6. 1. A beautifully patterned fossil of Dunbaria fasciipennis (order Paleodictyoptera) from Permian rocks of Kansas. This paleopteran (old-winged) insect order was one of the casualties of the Permian extinctions. (Photo by Frank Carpenter. Museum of Comparative Zoology, Harvard University. © President and Fellows of Harvard College.)
Interviewing the Survivors
One of the prominent features of the Late Permian insect extinction is that the old-winged paleopterans were severely affected. Most did not survive. Four orders went entirely extinct: Paleodictyoptera, Megasecoptera, Diaphanopterodea, and Dicliptera. All of these insects had a sucking beak and immature nymphs that were terrestrial. By contrast, the groups of old-wings that did survive the Permian extinction all had freshwater aquatic nymphs with gills that developed in ponds, streams, and marshes. The order Ephemeroptera, the mayflies, were quite diverse in the Permian, with at least five families, and they survived to the present day. There were also at least six families of Permian damselflies (order Odonata), delicate creatures related to modern dragonflies, and they became highly diversified during the Mesozoic dinosaur days. Then there is the order Protodonata, the giant air dragons. The most gigantic species went extinct during the mid-Permian, but other smaller species survived well into the Mesozoic, flying over the first dinosaurs.
These details suggest that although the end-Permian extinction massacred life in the oceans, insects living in freshwater ponds and streams seemed to find sanctuary. This is further supported by the observation that two other more specialized groups of freshwater aquatic insects also survived: the stoneflies (order Plecoptera) and the caddisflies (order Trichoptera). Therefore, any models of the Permian extinction need to account for events that would have affected saltwater but not freshwater systems. Scenarios involving dropping sea levels, loss of coastal marine habitats, sedimentation, rapid turnover of deep-sea toxins, and depletion of oceanic oxygen levels all make sense. On the other hand, scenarios involving comet or asteroid impacts don’t seem to make much sense, since these impacts should have severely affected freshwater habitats. This is particularly true because the aquatic young of mayflies, stoneflies, and caddisflies are notoriously sensitive to environmental change. Their survival clearly indicates that some fresh water habitats were scarcely affected.
If life was somewhat easier for the aquatic insects during the Permian, then a corollary might be that life was more difficult out of the water, in the terrestrial forest biome. The old-wings faced increasing and ever more novel insect competitors. Most notably, the new-winged insects proliferated extensively. The cockroaches, order Blattaria, we know already from the Carboniferous years. They declined somewhat in the Permian, but are still among the more common fossils from the period. The order Protorthoptera, a motley assortment of cricketlike insects that had chewing mouthparts and the new-winged design, were enormously successful, comprising at least forty families, and over the Middle to Late Permian, they had the greatest species diversity and abundance. The order Orthoptera, a brigade of crunching and munching insects, with strong mandibulate jaws, that includes the ancestors of modern crickets and katydids, reached a new frenzy of activity during the Permian years, ascending into the rank
s of the most diverse insect orders.
If you’ve ever had a roach or cricket in your house, you know that they are fairly aggressive and remarkably omnivorous. They will chew on all sorts of organic materials. We tend to think of grasshoppers and katydids as being vegetarians, but many are also actively carnivorous when they can catch other insects, and others are notoriously cannibalistic. So, you can bet that the Permian protocrickets, roaches, and ur-katydids all munched on old-winged insect nymphs whenever they could catch them. And catching them would have been easy enough. Slow-moving paleodictyopteran nymphs, which would climb up on conspicuous plants and insert their beaks into mature sporangia to suck out nutritious spores, would have been easy targets for the predatory orthopteroid insects. The orthopteroids would have also competed for the same foods more effectively. While the old-wing nymphs could only suck spores from maturing sporangia, the orthopteroids could chew up and consume entire sporangia even before they had matured enough for the old-wing nymphs to feed on them. Finally, the nymphs that did mature to adulthood would have had a real disadvantage in an increasingly new-wing–dominated world. Because the old-wing insects could not fold their wings and hide in small spaces or fly as fast as the new-wings, they would have suffered greater mortality simply because they provided a larger, slower target for predators of all kinds.
One other surprising Permian success story is the icebugs. Also known as rock crawlers, these creatures comprise a small extant order called Grylloblattodea, which means “cricket-roach” and refers to the similarities they share with their cousins, the crickets and the roaches. But the icebugs established their own, very unique lifestyle. Sometime during the Permian years, the ancestors of the rock crawlers presumably moved up along the streambeds to higher and higher altitudes, and eventually adapted to life at the uppermost elevations near permanent ice and snow fields. In alpine settings wings become a detriment—flat parts that can gather heat also can lose heat—so the ice bugs, along with other high-elevation insects, adapted by evolving a wingless body form.6 Secondarily wingless insects are commonplace today—think lice, fleas, and worker ants—but the icebugs were among the first insects to give up their wings. To keep warm, they developed dark pigments for absorbing solar radiation, and useful behaviors like hiding under flat rocks that face the sun. They are also slow to develop. An icebug might take several years to mature to adulthood, because it’s cooled down and inactive far more often than it’s active. When the alpine weather is mild and the icebugs do become active, they emerge from their warm hiding places and scavenge for food over the surface of glaciers and snow fields. You might suppose that there is not much to eat on top of glaciers, but flying insects don’t always end up where they want to be. Many are caught in storms, and clouds of insects are often blown great distances, some to high elevations, where they freeze and fall to the ground. Later, in mild weather, grylloblattids scavenge their flash-frozen carcasses.
Planet of the Bugs: Evolution and the Rise of Insects Page 12
