Martin-Discovering the last lost world - ONE DISCOVERING...

Unformatted text preview: ONE DISCOVERING THE LAST LOST WORLD To produce a mighty book you must choose a mighty theme . . . to include the whole circle of the sciences, and all the generations of whales and men and mastodons. Herman Melville, Moby-Dick Career paths are notoriously unpredictable, and I never imagined that mine would lead me to focus on prehistoric megafaunal extinction. Be- ginning in I 948 at Rancho del Cielo, a cloud forest just within the Tropic of Cancer in eastern Mexico, I collected birds for ornithologist George M. Sutton and other vertebrates for the University of Michigan’s Museum of Zoology. The best part was the forest of tall, dense trees rising to 100 feet: sweet gum, many species of oak unknown north of the border, Mag- nolia, Podocarpus, redbud, a few scattered palms, and a wealth of tank bromeliads, a home for frogs and lungless salamanders. Had I not suf- fered a handicap from a bout with polio in I 9 50, I might never have turned my attention to peat, rich in fossil pollen, being studied by botanists in the postglacial lakes around Ann Arbor. Then, in I955, I learned from ecologist Ed Deevey at Yale University how to extract, iden- tify, and count fossil pollen. From these counts one could learn what hap~ pened to plants after the glaciers melted away. 7 For Ed, the biogeography of the Pleistocene (the last ice age, 1.8 mil- lion years ago to 10,000 years ago), with all its glacial and interglacial changes in climate, was the key to understanding modern plant and an- imal distributions. Ed took cores of organic sediment from lake beds and counted samples of the fossil pollen, spores, and copepods (minute aquatic crustaceans) they contained. He could date these remains by DISCOVERING THE LAST LOST WORLD / S Willard F. Libby’s then-new radiocarbon method; scientists could now refer to “Libby time” (roughly the last 40,000 years, the period for which radiocarbon dating is most effective). Magically, the fossil pollen record in sediments cored from New England lakes told of the comings and goings of treeless tundra and of spruce, fir, jack pine, and other trees as the climate warmed, the glaciers melted and on occasion readvanced, and eventually the ice—margin boreal vegetation yielded to today’s de- ciduous forest. It even gave clues to the fate of animal species during this period. For example, fossil pollen counts plotted in percentages as a diagram associated with bones of mastodons indicated that they van- ished around the time that, according to the fossil pollen counts, spruce gave way to pine. Some paleontologists thought that the change in tree cover from boreal conifers to temperate hardwoods might help explain mastodon extinction. Then they dropped the extinction question and returned to their primary interests, vertebrate anatomy, evolution, and geochronology. Nobody bothered to study the extinctions. Somehow the Pleistocene megafauna, big as it was, remained out of sight and out of mind. In the winter of I9 56, my wife and I found ourselves raising our chil- dren in a tenant farmhouse we rented from Anatole Cecyre, a French Canadian dairy farmer outside Chateauguay, Quebec. I commuted to a postdoctoral fellowship at the Université de Montréal, working on a pollen record of late-glacial climatic change and offering a seminar on Quaternary biology held jointly with McGill University. Identifying and counting pollen grains can become monotonous. In a break from the microscope one day, I skimmed through George Gaylord Simpson’s monumental Classification of Mammals, a long list of genera organized taxonomically. Malcolm McKenna and Susan Bell of the Amer- ican Museum of Natural History have recently revised this classic tome. They recognize 5,1 58 mammalian genera, of which 4,075, about 80 per- cent, are extinct (McKenna and Bell I 9 97). The large number of mam- mal extinctions is to be expected, because the list embraces the end of the Mesozoic (the era that began about 250 million years ago) and the entire Cenozoic (the era that began about 65 million years ago and con- tinues to today). As a diversion that snowy subzero weekend, a diversion that fit right into the seminar on Quaternary biology, I began to plot all the late-Qua- ternary megafaunal extinctions listed by Simpson against those that had taken place earlier in the Cenozoic. After two days Iwas stunned by what I found. In the Miocene (starting 24 million years ago) and the Pliocene 6 / DISCOVERING THE LAST LOST WORLD (starting five million years ago), many mammals of all sizes turned over (i.e., evolved and went extinct). But at the end of the Quaternary, the pattern of extinctions in North America became very strange. It was the large terrestrial mammals that disappeared, and those for which radio~ carbon dates were available did so suddenly.“‘ Large marine mammals, on the other hand—the whales, dolphins, and pinnipeds—had been hard hit by extinctions in the lVIiocene and Pliocene but survived the Quater- nary virtually intact. So did most of the continental small mammals (the shrews, moles, rats, and mice). The major event in the Quaternary was the extinction of the large ter- restrial mammals. Extinction also doomed their endemic species of par- asites and commensals. (Commensals are species that benefit from ac— companying other species without necessarily harming them.) For example, near—time extinction of internal parasites of ground sloths (Schmidt, Dus— zynski, and Martin I 9 92) or reduction in the number of species of cow— birds, magpie—type corvids, and dung beetles can be accepted as sec— ondary, given the apparent dependence of these species on large mammals (Steadman and Martin I984). Along with mammoths and ground sloths, the late—Quaternary extinctions also involved avian scavengers (such as condors), commensals (such as the Thick-knees, or Stone Curlews, of the Old World and tropical America), or guardians of the big mammals that eat their external parasites, such as tickbirds, which fly away if alarmed, alarming their host (Steadman and Martin I984). In the last Io,ooo years, after most of the extinctions of the big mam— mals on continents, many small mammals, birds, reptiles, and land snails vanished. Small animal extinction and dwarfing, as well as extinction of some large mammals and birds, happened on oceanic islands, such as those in the West Indies and New Zealand, and on those in the Mediter— ranean. Thousands of small islands, especially in the remote Pacific, saw extinctions of small animals, especially birds and endemic species of land snails. The deep water surrounding these islands precluded any connec- tion to each other or to the mainland, even when the sea level dropped "‘Jobn Alroy (I999, 2001) has looked at the fossil record in much greater detail. His analysis shows that late-Pleistocene extinctions in North America are quantitatively un— like any of the changes seen earlier in the Cenozoic. Only in the late Pleistocene is heavy extinction focused so strongly on large mammals. Throughout this book, I call vertebrates large if they weigh over 4 5 kilograms (100 pounds), the size of a small adult human or an adult pronghorn. Some scholars define large mammals as those weighing over I kilogram (Alroy I999, 7.001). Just where the bound— ary is located does not alter the overall pattern: in near time on the continents, far more large mammals Went extinct than small ones. DISCOVERING THE LAST LOST WORLD / 7 Period of Major Extinction __: >50,000 yrs. B.P. Z 50—10,000 yrs. B.P. : 5,000—200 yrs. B.P. Figure 1. Map showing sequence of extinctions. Arrows indicate direction of human dispersals; numbers indicate order of human settlement. Adapted from Martin I970, © American Institute of Biological Sciences. by 400 feet or more, as it last did during the height of continental glacia- tion around I8,ooo years ago. In contrast, islands on the continental shelf, including Britain, Sri Lanka, Java, Sumatra, and Trinidad, were connected to continents when the sea level dropped."’ Their faunas were much less vulnerable to prehistoric loss. Often much smaller than their continental relatives, the animals that evolved on oceanic islands included the dwarf mammoths of Santa Rosa and San Miguel islands off the California coast; in the Mediterranean, the dwarf elephants of Crete and the dwarf hippo of Cyprus; in the West Indies, the dwarf ground sloths of Cuba and Hispaniola (Haiti and the Dominican Republic). The tropical islands of the remote Pacific, some of them quite small, hosted z,ooo taxa of flightless rails. Apparently de— pending on when humans (often accompanied by Pacific rats) first ar- rived, many of the island endemics suffered prehistoric extinction. Pacific ”*Islands surrounded by shallow water of the continental shelves, less than I20 meters (400 feet) deep, would not emerge long enough to evolve highly endemic species. These shelf islands disappear as rising interglacial sea levels shrink and eventually drown them. Islands artificially formed by irnpoundment of rivers experience comparable extinctions of larger animals and artificial increases in smaller ones. In contrast, deep-Water islands could be colonized only by species surviving water transport or by ancient detachment from a continent such as Gondwanaland. They support faunas that are impoverished but rich in endemics, such as Jamaica’s extinct giant rodents, Madagascar’s extinct giant lemurs, and Sulawesi’s extinct dwarf elephants. Phanerozoic Eon (543 mya to the present) Cenozoic Era \ (65 mya to the present) Mesozoic Era (248 to 65 mya) Paleozoic Era (543 to 248 mya) Quaternary (1-8 mya to the present) Tertiary (65 to 1.8 mya) Cretaceous (144 to 65 mya)T Jurassic (206 to 14-4 mya) Triassic (248 to 206 mya) Permian (290 to 248 mya)T Carboniferous (354 to 290 mya) Devonian (417 to 354 mya)1' Silurian (443 to 417 mya) Ordovician (505 to 4-43 mya)T Cambrian (570 to 505 mya) Holocene (10,000 years ago to the present) Pleistocene (1.8 mya to 10,000 years ago) Pliocene (5.3 to 1.8 mya) Miocene (23.8 to 5.3 mya) Oligocene (33.7 to 23.8 mya) Eocene (54.8 to 33.7 mya) Paleocene (65 to 54.8 mya) Pennsylvanian (323 to 290 mya) Mississippian (354 to 323 mya) North American Land Mammal Ages Near Time (the last 50,000 years) Rancholabrean (250,000 years ago to the present) lrvingtonian (1.8 mya to 250,000 years) Blancan (4.7to 1.8 mya) Figure 2. The geologic time scale. Note that Near Time comprises a tiny and recent portion of the Earth’s history, underscoring how close to our own time was the world populated by an array of large mammals. “mya” = million years ago; NA = North America; T = mass extinction. Adapted from McKenna and Bell I997 (© Columbia University Press); Museum of Paleontology, University of California, Berkeley; Merriam-Webster’s Collegiate Dictionary, 11th ed. DISCOVERING THE LAST LOST WORLD ./ 9 rats (Rattus exulens) may have been the agents of over a hundred ex- tinctions of ground-nesting birds in New Zealand and of endemic land snails on thousands of Pacific islands. Significantly, no avian extinctions have been detected in the Galapa- gos, where fossil cave faunas sampled by David Steadman (Steadman and Zousmer I 988) on various islands are made up entirely of living species, including species of Darwin’s Finches. Extinction of the endemic Gala— pagos rat, Megaoryzomys, coincides with historic human contact. On a Visit to the island of Santa Fe, Dave and his field assistants searched for Pleistocene sediments and fossils without success. They found a rich fos- sil record in caves on Floreana, but it was apparently not old enough to include extinct species. While not the first to be intrigued by these patterns, I was among the first to compare near—time extinctions between continents and continental extinctions with those on oceanic islands. Radiocarbon dating made such comparisons possible. Moreover, vertebrate paleontologists have greatly improved and refined the fossil records of mammals, large and small. After a false start early in the twentieth century by paleontologists such as O. P. Hay, who thought that numerous extinctions of mammals took place early in the Quaternary, paleontologists and geologists came to rec- ognize that megafaunal extinctions in America happened mainly around the end of the last glacial episode, at the end of the Quaternary. This was about 13,000 years ago or less. The extinctions were not only of species and genera, but also ofhigher taxonomic categories, such as families and occasionally an order. The evolution of a family of mammals normally takes tens of millions of years. Some paleontologists argued fatalistically that the late-Quaternary extinctions were inevitable; their time had come. But so many, so suddenly, in so many corners of the world, and involv— ing established lineages of large mammals on continents? Something strange had happened. What was it? “Blighted” is the right word for the animal kingdoms of America and Australia after the near—time extinctions ran their course. Globally, ex- tinction was the fate of about half of the genera of large terrestrial mam— mals known to have existed on the continents at the time. They were soon followed by thousands of species or taxa (taxonomic categories, in this case distinct populations) of island birds and land snails. Prehistoric extinctions swept the remote corners of the Pacific, including Hawaii, the Marquesas, Rapanui (Easter Island), and New Zealand. In near time, North America lost more genera than it had in the preced- ing 1.8 million years. Table I lists the living and extinct large mammals TABLE 1 Late Quaternary Extinct and Living Species of Large (>4 5 Kilograms) Land Mammals, Western North America and Northern Mexico Classification Xenarthra TGlyptot/oerium floridanum TParamylodon harlani TMegalonyx jeffersonii TNot/arot/aeriops shastensis Common name glyptodont big-tongued ground sloth Jefferson’s ground sloth Shasta ground sloth Carnivora Canis *dirus Canis lupus Ursus americanus Ursus arctos TArctodus simus TSmilodon fatalis Panthera leo ="atrox Panthera onca TMiracinonyx trumani Puma concolor dire wolf gray wolf black bear " brown (grizzly) bear giant short-faced bear saber—toothed cat American lion jaguar American cheetah mountain lion Proboscidea TMammut americanum TMammut/aus columbi TMammut/ous exilis TMammut/aus primigenius Perissodactyla Equus *conz/ersidens Equus *occidentalis Equus "‘spp. Tapirus ="californicus Artiodactyla American mastodon Columbian mammoth dwarf mammoth woolly mammoth Mexican horse western horse extinct horses or asses tapir TCamelops hesternus THemiauckenia macrocepkala "‘Mylo/ayus nasutus "Platygonus compressus Odocoileus hemionus Odocoileus virginianus *Naz/ahoceros fricki Rangifer tarandus Alces alces Cervus elapkus western camel long—legged llama long—nosed peccary flat—headed peccary mule deer white-tailed deer mountain deer woodland caribou moose, moose deer wapiti, elk DISCOVERING THE LAST LOST WORLD / 11 TABLE 1 continued Classification Common name Antilocapra americana pronghorn Oreamnos “‘laarringtoni Harrington’s mountain goat Oreamnos americanus mountain goat Ovis canadensis bighorn *Eucerat/oerium collinum shrub ox ="Bootloerium bombifrons bonnet-headed musk ox Bison bison bison Bison ”'spp. extinct bison SOURCE: After Martin and Szuter r 999. Courtesy Blackwell Publishing. TExu'nct genus. ”“Extinct species. The more common taxa have terminal radiocarbon dates around 13,000 calendar years ago (Stuart I99I). known in western North America and northern Mexico in near time. Table 2 lists the large mammals of North America north of Mexico over the last two million years and shows the concentration of extinctions in the late Quaternary, with its distinctive Rancholabrean fauna. Table 3 lists all living and extinct large land mammals of near time found through- out the world. It is well worth examining in more detail the kinds of animals that went extinct in the late Quaternary and the geographic regions that were affected. The following sketch treats some of the more common large animals eliminated by extinctions in the last 50,000 years. (For more de- tails and illustrations see E. Anderson 1984; Hulbert 2.001; Kurtén I988; Kurtén and Anderson 1980; Lange 2002; MacPhee 1999; Martin and Klein I9 84; Murray 1991; Steadman n.d.; and Sutcliffe I985.) AFRICA AND EURASIA: THE CONTROLS The large-mammal faunas of Africa and Eurasia escaped severe extinc~ tions over the last two to five million years. Africa suffered no obvious pulse or burst of extinctions of megafauna to match those in near time on other landmasses; in the end it lost less than IO percent of its mega- fauna. The losses in northern Eurasia were not aggregated or spontaneous, as in America and Australia, but more measured, extending over 70,000 years (Stuart I 9 9 9). They included straight-tusked elephants and woolly mammoths, naked and woolly rhinos, hippos, giant deer, and cave bears (Ursus spelaeus). Cave lions and spotted hyenas, close relatives of the TABLE 2 Large (>4 5 Kilograms) Pliocene, Pleistocene, and Holocene Mammals of North America North of Mexico Blancan Irvingtonian Rancholabrean Holocene 1, 2 3 4 5 Early Middle Late Early Late Stage duration l (millions ofyears) 1.0 0.5 0.5 0.2 i 0.9 0.4 0.2 l 0.2 0.1 0.01 XENARTHRA Dasypus ’i'sp., giant armadillo ‘ 0 0 o o o o JrHolmesina, northern pampathere 0 0 o . o o o . TPachyarmat/Jerium, glyptodont o TGIyptot/aerium, glyptodont 0 0 0 o o o o TEremotherium, giant ground sloth I 0 0 o o o TNot/arotheriops, Shasta ground sloth I 0 0 0 o o TMegalonyx spp., Jefferson’s ground 0 0 0 o o o o l o o sloth TParamylodon, big—tongued . . . . . . ‘ . . ground sloth CARNIVORA TBoropbagus sp.> plundering dog ' ° ° ° 0 Canis ”‘spp., dire wolf, others 0 0 o o o o o TABLE 2 continued JrProtocyon, Troxell’s dog ° l Ursus spp., bears ° ° ‘ ° ° ° l ° ° ° "‘Tremarctos, Florida cave bear ° ‘ ° ° ‘ ° ° ° J{Arctodas spp., short—faced bears 0 0 0 0 TCbas‘maport/aetes, hunting hyena ° ‘ ‘ ° ° TMegantereon, western . . . . dirk—toothed cat JrSmilodon, saber—toothed cat ° ° ‘ ° ‘ TIsc/ayrosmilus, Idaho ‘ saber—toothed cat JrHomotheriam, scimitar cat 1 ° ° ‘ I ° ‘ TDinofelis, false saber-toothed cat ‘ ‘ ° l l *‘Pantkera s . American lion jaguar pp ’ ’ x o o o o o TMiracinonyx, American cheetah ‘ ‘ ‘ ° ‘ ° Puma, cougar, panther ° ‘ ‘ ‘ ' ‘ RODENTIA TProcastoroz'des, large beaver ° ° ° 0 o o TCastoroides, giant beaver ° TABLE 2 continued Blancan Irvingtonian Rancholabrean Holocene 1, 2 3 4 5 Early Middle Late 1 Early Late l—_—‘ Stage duration (millions ofyears) 1.0 0.5 0.5 0.2 0.9 0.4 0.2 0.2 0.1 0.01 'I'Neoc/aoems, Pinckney’s capybara ] 0 o o o o "‘Hydroclaoerns sp., . . . Holmes’s capybara PROBOSCIDEA TMammut, American mastodon ' ° ' ° 0 o o o o JrSztegomastodon, stegomastodon ' ’ ’ ° 'I'Rbyncotlverium, rhynchothere ‘ ' ‘ ° TCnUieronius, gornphothere 0 0 o o o o . TMammnt/ms spp., mammoths 1— o o o . . SIRENIA THydrodamalis, Steller’s sea cow ' ° 0 o o o o . . o Tricbec/nts, manatee ' 0 o o o o o o o o TABLE 2 continued PERISSODACTYLA JrCormo/aipparion, extinct equid ’ ‘ ] TNannipus, gazelle—horse ’ ‘ ‘ ' JrPlesippus, extinct equid ' ' ‘ ‘ *‘Equns spp., horses *Tapims spp., tapirs ARTIODACTYLA TMegalotylopns, large camelid ° ° ‘ ‘l TBlancocamelus, giraffe—camel ' TTitanotylopt/is, giant camelid 5 ‘ ‘ 0 o 'l'Camelops spp., camels 0 o o o o . 'I'Hemiauc/aenia, llama ° ' 0 0 0 o o o . TPalaeolania, stout—legged llama o o . TMylo/vyus, long—nosed peccary ° 0 0 o- o o o TPldtygonus, flat-headed peccary 0 0 0 o . o o o . 'l'Bretzia, false elk 0 o o o o . o o . o o Odocoileus spp., deer ' TTorontocems, extinct large cervid TABLE 2 continued TSoergelia, Soergel’s ox Blancan Irvingtonian Rancholabrean Holocene 1, 2 3 4 5 Early Middle Late l Early Late Stage duration ' (millions ofyears) 1.0 0.5 0.5 0.2 0.9 0.4 0.2 0.2 0.1 0.01 TchUa/aoceros, mountain deer ' l ‘ Rangifer, car...
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