Quaternary extinction event

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The latter half of the Late Pleistocene to the beginning of the Holocene (~50,000-10,000 years Before Present) saw extinctions of numerous predominantly megafaunal (large) animal species (the Pleistocene megafauna), which resulted in a collapse in faunal density and diversity across the globe. The extinctions during the Late Pleistocene are differentiated from previous extinctions by the widespread absence of ecological succession to replace these extinct megafaunal species, and the regime shift of previously established faunal relationships and habitats as a consequence. The timing and severity of the extinctions varied by region and are thought to have been driven by varying combinations of human and climatic factors. Human impact on megafauna populations is thought to have been driven by hunting ("overkill") as well as possibly environmental alteration. The relative importance of human vs climatic factors in the extinctions has been the subject of long-running controversy.

Major extinctions occurred in Australia-New Guinea (Sahul) beginning approximately 50,000 years ago and in the Americas about 13,000 years ago, coinciding in time with the early human migrations into these regions. Extinctions in northern Eurasia were staggered over tens of thousands of years between 50,000 and 10,000 years ago, while extinctions in the Americas were virtually simultaneous, spanning only 3000 years at most. Overall, during Late Pleistocene about 65% of all megafaunal species worldwide became extinct, rising to 72% in North America, 83% in South America and 88% in Australia, with Africa, South Asia and Southeast Asia having much lower extinctions than other regions.

Extinctions by biogeographic realm

Summary

Introduction

The Late Pleistocene saw the extinction of many mammals weighing more than . The proportion of megafauna extinctions is progressively larger the further the human migratory distance from Africa, with the highest extinction rates in Australia, and North and South America.

The increased extent of extinction mirrors the migration pattern of modern humans: the further away from Africa, the more recently humans inhabited the area, the less time those environments (including its megafauna) had to become accustomed to humans (and vice versa).

There are two main hypotheses to explain this extinction:
Climate change associated with the advance and retreat of major ice caps or ice sheets causing reduction in favorable habitat.
Human hunting causing attrition of megafauna populations, commonly known as "overkill".
There are some inconsistencies between the current available data and the prehistoric overkill hypothesis. For instance, there are ambiguities around the timing of Australian megafauna extinctions. Evidence supporting the prehistoric overkill hypothesis includes the persistence of megafauna on some islands for millennia past the disappearance of their continental cousins. For instance, ground sloths survived on the Antilles long after North and South American ground sloths were extinct, woolly mammoths died out on remote Wrangel Island 6,000 years after their extinction on the mainland, while Steller's sea cows persisted off the isolated and uninhabited Commander Islands for thousands of years after they had vanished from the continental shores of the north Pacific. The later disappearance of these island species correlates with the later colonization of these islands by humans.

The original debates as to whether human arrival times or climate change constituted the primary cause of megafaunal extinctions necessarily were based on paleontological evidence coupled with geological dating techniques. Recently, genetic analyses of surviving megafaunal populations have contributed new evidence, leading to the conclusion: "The inability of climate to predict the observed population decline of megafauna, especially during the past 75,000 years, implies that human impact became the main driver of megafauna dynamics around this date."

An alternative hypothesis to the theory of human responsibility is climate change associated with the last glacial period. Discredited explanations include the Younger Dryas impact hypothesis and Tollmann's hypothesis that extinctions resulted from bolide impacts.

Recent research indicates that each species responded differently to environmental changes, and no one factor by itself explains the large variety of extinctions. The causes may involve the interplay of climate change, competition between species, unstable population dynamics, and human predation.

Africa
Although Africa was one of the least affected regions, the region still suffered extinctions, particularly around the Late Pleistocene-Holocene transition. These extinctions were likely predominantly climatically driven by changes to grassland habitats.

Ungulates
Even-Toed Ungulates
Suidae (swine)
Metridiochoerus
Kolpochoerus
Bovidae (bovines, antelope)
Giant buffalo (Syncerus antiquus)
Megalotragus
Rusingoryx
Antidorcas australis
Antidorcas bondi
Damaliscus hypsodon
Damaliscus niro
Gazella atlantica
Gazella tingitana
Caprinae
Makapania?
Odd-toed Ungulates
Rhinoceros (Rhinocerotidae).
Stephanorhinus hemitoechus (North Africa)
Wild Equus spp.
Caballine horses
Equus algericus (North Africa)
Subgenus Asinus (asses)
Equus melkiensis (North Africa)
Zebras
Equus capensis
Saharan zebra (Equus mauritanicus)
Proboscidea
Elephantidae (elephants)
Palaeoloxodon iolensis? (other authors suggest that this taxon went extinct at the end of the Middle Pleistocene)

South Asia, Southeast Asia and East Asia

The timing of extinctions on the Indian subcontinent is uncertain due to a lack of reliable dating. Similar issues have been reported for Chinese sites, though there is no evidence for any of the megafaunal taxa having survived into the Holocene in that region. Extinctions in Southeast Asia and South China have been proposed to be the result of environmental shift from open to closed forested habitats.
Ungulates
Even-Toed Ungulates
Several Bovidae spp.
Bos palaesondaicus (ancestor to the banteng)
Bison hanaizumiensis
Cebu tamaraw (Bubalus cebuensis)
Bubalus grovesi
Bubalus wansijocki
Short-horned water buffalo (Bubalus mephistopheles)
Cervidae
Sinomegaceros spp. (including Sinomegaceros yabei in Japan, and Sinomegaceros ordosianus in China).
Hippopotamidae
Hexaprotodon (Indian subcontinent)
Odd-toed Ungulates
Equus spp.
Equus namadicus (Indian subcontinent)
Yunnan horse (Equus yunanensis)
Giant tapir (Tapirus augustus, Southeast Asia and Southern China)
Merck's rhinoceros (Stephanorhinus kirchbergensis Eastern Asia)
Carnivora
Caniformia
Arctoidea
Bears
Ailuropoda baconi (ancestor to the giant panda)
Afrotheria
Afroinsectiphilia
Orycteropodidae/Tubulidentata
Aardvark (Orycteropus afer; extirpated in South Asia circa 13,000 BCE)
Paenungulata
Tethytheria
Proboscideans
Stegodontidae
Stegodon spp. (including Stegodon florensis on Flores, Stegodon orientalis in East and Southeast Asia, and Stegodon sp. in the Indian subcontinent)
Palaeoloxodon spp.
Palaeoloxodon namadicus (Indian subcontinent, possibly also Southeast Asia)
Naumann's elephant (Japan)
Palaeoloxodon huaihoensis (China)
Birds
Japanese flightless duck (Shiriyanetta hasegawai)
Leptoptilos robustus
Primates
Several simian (Simiiformes) spp.
Pongo (orangutans)
Pongo weidenreichi (South China)
Various Homo spp. (archaic humans)
Homo erectus (Java)
Homo floresiensis (Flores)
Homo luzonensis (Luzon, Philippines)
Denisovans (Homo sp.)

Europe and northern Asia

The Palearctic realm spans the entirety of the European continent and stretches into northern Asia, through the Caucasus and central Asia to northern China, Siberia and Beringia. Extinctions were more severe in Northern Eurasia than in Africa or South and Southeast Asia. These extinctions were staggered over tens of thousands of years, spanning from around 50,000 years Before Present (BP) to around 10,000 years BP, with temperate adapted species like the straight-tusked elephant and the narrow-nosed rhinoceros generally going extinct earlier than cold adapted species like the woolly mammoth and woolly rhinoceros. Climate change has been considered a probable major factor in the extinctions, possibly in combination with human hunting.

Ungulates
Even-Toed Hoofed Mammals
Various Bovidae spp.
Steppe bison (Bison priscus)
Baikal yak (Bos baikalensis)
European water buffalo (Bubalus murrensis)
European tahr (Hemitragus cedrensis)
Giant muskox (Praeovibos priscus)
Northern saiga antelope (Saiga borealis)
Twisted-horned antelope (Spirocerus kiakhtensis)
Goat-horned antelope (Parabubalis capricornis)
Various deer (Cervidae) spp.
Broad-fronted moose (Cervalces latifrons)
Giant deer (Megaloceros giganteus)
Praemegaceros savini
Cretan deer (Candiacervus)
Haploidoceros mediterraneus
All native Hippopotamus spp.
Hippopotamus amphibius (European range)
Maltese dwarf hippopotamus (Hippopotamus melitensis)
Cyprus dwarf hippopotamus (Hippopotamus minor)
Sicilian dwarf hippopotamus (Hippopotamus pentlandi)
Camelus knoblochi and other Camelus spp.
Odd-Toed Hoofed Mammals
Various Equus spp. e.g.
Wild horse (Equus ferus ssp.)
Equus cf. gallicus
European wild ass (Equus hydruntinus)
Equus cf. latipes
Equus lenensis
Equus cf. uralensis
All native Rhinoceros (Rhinocerotidae) spp.
Elasmotherium
Woolly rhinoceros (Coelodonta antiquitatis)
Stephanorhinus spp.
Merck's rhinoceros (Stephanorhinus kirchbergensis)
Narrow-nosed rhinoceros (Stephanorhinus hemiotoechus)
Carnivora
Caniformia
Canidae
Caninae
Wolves
Cave wolf (Canis lupus spelaeus)
Dire wolf (Aenocyon dirus)
Dholes
European dhole (Cuon alpinus europaeus)
Sardinian dhole (Cynotherium sardous)
Arctoidea
Various Ursus spp.
Steppe brown bear (Ursus arctos "priscus")
Gamssulzen cave bear (Ursus ingressus)
Pleistocene small cave bear (Ursus rossicus)
Cave bear (Ursus spelaeus)
Giant polar bear (Ursus maritimus tyrannus)
Musteloidea
Mustelidae
Several otter (Lutrinae) spp.
Robust Pleistocene European otter (Cyrnaonyx)
Algarolutra
Sardinian giant otter (Megalenhydris barbaricina)
Sardinian dwarf otter (Sardolutra)
Cretan otter (Lutrogale cretensis)
Feliformia
Various Felidae spp.
Eurasian scimitar cat (Homotherium latidens)
Cave lynx (Lynx pardinus spelaeus)
Issoire lynx (Lynx issiodorensis)
Panthera spp.
Cave lion (Panthera spelaea)
European ice age leopard (Panthera pardus spelaea)
Herpestoidea
Cave hyena (Crocuta crocuta spelaea)
All native Elephant (Elephantidae) spp.
Mammoths
Woolly mammoth (Mammuthus primigenius)
Dwarf Sardinian mammoth (Mammuthus lamarmorai)
Straight-tusked elephant (Palaeoloxodon antiquus)
Dwarf elephant
Palaeoloxodon creutzburgi
Cyprus dwarf elephant (Palaeoloxodon cypriotes)
Palaeoloxodon mnaidriensis
Rodents
Giant Eurasian porcupine (Hystrix refossa)
Leithia spp. (Maltese and Sicilian giant dormouse)
Lagomorpha
Pika (Ochotona) spp. e.g.
Giant pika (Ochotona whartoni)
Eurasian giant beavers (Trogontherium cuiveri)
Birds
Asian ostrich (Struthio asiaticus)
Giant swan (Cygnus falconeri)
Yakutian goose (Anser djuktaiensis)
Various European crane spp. (Genus Grus)
Grus primigenia
Grus melitensis
Cretan owl (Athene cretensis)
Primates
Homo
Denisovans (Homo sp.)
Neanderthals (Homo (sapiens) neanderthalensis; survived until about 40,000 years ago on the Iberian peninsula)
Barbary macaque (Macaca sylvanus) (European range)

Extinctions in North America were concentrated at the end of the Late Pleistocene, around 13,800–11,400 years Before Present, which were coincident with the onset of the Younger Dryas cooling period, as well as the emergence of the hunter-gatherer Clovis culture. The relative importance of human and climactic factors in the North American extinctions has been the subject of significant controversy. Extinctions totalled around 35 genera. The radiocarbon record for North America south of the Alaska-Yukon region has been described as "inadequate" to construct a reliable chronology.

North American extinctions (noted as herbivores (H) or carnivores (C)) included:

Ungulates
Even-Toed Hoofed Mammals
Various Bovidae spp.
Most forms of Pleistocene bison (only Bison bison in North America, and Bison bonasus in Eurasia, survived)
Ancient bison (Bison antiquus) (H)
Long-horned/Giant bison (Bison latifrons) (H)
Steppe bison (Bison priscus) (H)
Bison occidentalis (H)
Several members of Caprinae (the muskox survived)
Giant muskox (Praeovibos priscus) (H)
Shrub-ox (Euceratherium collinum) (H)
Harlan's muskox (Bootherium bombifrons) (H)
Soergel's ox (Soergelia mayfieldi) (H)
Harrington's mountain goat (Oreamnos harringtoni; smaller and more southern distribution than its surviving relative) (H)
Saiga antelope (Saiga tatarica; extirpated) (H)
Deer
Stag-moose (Cervalces scotti) (H)
American mountain deer (Odocoileus lucasi) (H)
Torontoceros hypnogeos (H)
Various Antilocapridae genera (pronghorns survived)
Capromeryx (H)
Stockoceros (H)
Tetrameryx (H)
Pacific pronghorn (Antilocapra pacifica) (H)
Several peccary (Tayassuidae) spp.
Flat-headed peccary (Platygonus) (H)
Long-nosed peccary (Mylohyus) (H)
Collared peccary (Dicotyles tajacu; extirpated, range semi-recolonised) (H) (Muknalia minimus is a junior synonym)
Various members of Camelidae
Western camel (Camelops hesternus) (H)
Stilt legged llamas (Hemiauchenia ssp.) (H)
Stout legged llamas (Palaeolama ssp.) (H)
Odd-Toed Hoofed Mammals
All native forms of Equidae
Caballine true horses (Equus cf. ferus), has historically been assigned to many different species, but the taxonomy of these horses is unclear, and many of these species are likely invalid.
Stilt-legged horse (Haringtonhippus francisci / Equus francisci; (H)
All members of North American tapir (Tapirus; four species)
California tapir (Tapirus californicus) (H)
Merriam's tapir (Tapirus merriami) (H)
Vero tapir (Tapirus veroensis) (H)
Order Notoungulata
Mixotoxodon (H)
Carnivora
Feliformia
Several Felidae spp.
Saber-Tooths
North American saber-toothed cat (Smilodon fatalis) (C)
North American scimitar cat (Homotherium serum) (C)
American cheetah (Miracinonyx; not true cheetah)
Miracinonyx trumani (C)
Cougar (Puma concolor; megafaunal ecomorph extirpated from North America, South American populations recolonised former range) (C)
Jaguarundi (Herpailurus yagouaroundi; extirpated, range semi-recolonised) (C)
Margay (Leopardus weidii; extirpated) (C)
Ocelot (Leopardus pardalis; extirpated, range marginally recolonised) (C)
Eurasian lynx (Lynx lynx; extirpated) (C)
Jaguars
Pleistocene North American jaguar (Panthera onca augusta; range semi-recolonised by other subspecies) (C)
North America Jaguar
Lions
American lion (Panthera atrox; endemic to North America after 340,000 BP) (C)
Eurasian cave lion (Panthera spelaea; present only as far as modern day Yukon) (C)
Caniformia
Canidae
Dire wolf (Aenocyon dirus) (C)
Pleistocene coyote (Canis latrans orcutti) (C)
Megafaunal wolf e.g.
Beringian wolf (Canis lupus ssp.) (C)
Dhole (Cuon alpinus; extirpated) (C)
Protocyon troglodytes (C)
Arctoidea
Musteloidea
Mephitidae
Short-faced skunk (Brachyprotoma obtusata) (C)
Mustelidae
Steppe polecat (Mustela eversmanii; extirpated) (C)
Various bear (Ursidae) spp.
Arctodus simus (C)
Florida spectacled bear (Tremarctos floridanus) (C)
South American short-faced bear (Arctotherium wingei) (C)
Giant polar bear (Ursus maritimus tyrannus; a possible inhabitant) (C)
Afrotheria
Afroinsectiphilia
Orycteropodidae/Tubulidentata
Giant anteater (Myrmecophaga tridactyla; extirpated, range partially recolonised) (C)
Paenungulata
Tethytheria
All native spp. of Proboscidea
Mastodons
American mastodon (Mammut americanum) (H)
Pacific mastodon (Mammut pacificus) (H) (validity uncertain)
Gomphotheriidae spp.
Cuvieronius (H)
Stegomastodon (H)
Mammoth (Mammuthus) spp.
Columbian mammoth (Mammuthus columbi) (H)
Pygmy mammoth (Mammuthus exilis) (H)
Woolly mammoth (Mammuthus primigenius) (H)
Sirenia
Dugongidae
Steller's sea cow (Hydrodamalis gigas; extirpated in North America) (H)
Euarchontoglires
Bats
Stock's vampire bat (Desmodus stocki) (C)
Pristine mustached bat (Pteronotus (Phyllodia) pristinus) (C)
Rodents
Giant beaver (Castoroides) spp.
Castoroides ohioensis (H)
Castoroides leiseyorum (H)
Klein's porcupine (Erethizon kleini) (H)
Giant island deer mouse (Peromyscus nesodytes) (C)
Neochoerus spp. e.g.
Pinckney's capybara (Neochoerus pinckneyi) (H)
Neochoerus aesopi (H)
All giant hutia (Heptaxodontidae) spp.
Blunt-toothed giant hutia (Amblyrhiza inundata; could grow as large as an American black bear) (H)
Plate-toothed giant hutia (Elasmodontomys obliquus) (H)
Twisted-toothed mouse (Quemisia gravis) (H)
Osborn's key mouse (Clidomys osborn's) (H)
Xaymaca fulvopulvis (H)
Lagomorphs
Aztlan rabbit (Aztlanolagus sp.) (H)
Giant pika (Ochotona whartoni) (H)
Xenarthrana
All remaining ground sloth spp.
Eremotherium (megatheriid giant ground sloth) (H)
Nothrotheriops (nothrotheriid ground sloth) (H)
Megalonychid ground sloth spp.
Megalonyx (H)
Nohochichak (H)
Xibalbaonyx (H)
Meizonyx
Megalocnid Greater Antillean dwarf ground sloth spp. (some were probably at least partly arboreal)
Acratocnus (H)
Habanocnus (H)
Megalocnus (H)
Miocnus (H)
Neocnus (H)
Mylodontid ground sloth spp.
Paramylodon (H)
Glossotherium (H)
All members of Glyptodontidae
Glyptotherium (H)
Pachyarmatherium (H)
Beautiful armadillo (Dasypus bellus) (H)
All Pampatheriidae spp.
Holmesina (H)
Pampatherium (H)
Birds
Water Fowl
Ducks
Bermuda flightless duck (Anas pachyscelus) (H)
Californian flightless sea duck (Chendytes lawi) (C)
Mexican stiff-tailed duck (Oxyura zapatima) (H)
Turkey (Meleagris) spp.
Californian turkey (Meleagris californica) (H)
Meleagris crassipes (H)
Various Gruiformes spp.
All cave rail (Nesotrochis) spp. e.g.
Antillean cave rail (Nesotrochis debooyi) (C)
Barbados rail (Incertae sedis) (C)
Cuban flightless crane (Antigone cubensis) (H)
La Brea crane (Grus pagei) (H)
Various flamingo (Phoenicopteridae) spp.
Minute flamingo (Phoenicopterus minutus) (C)
Cope's flamingo (Phoenicopterus copei) (C)
Dow's puffin (Fratercula dowi) (C)
Pleistocene Mexican diver spp.
Plyolimbus baryosteus (C)
Podiceps spp.
Podiceps parvus (C)
Storks
La Brea/Asphalt stork (Ciconia maltha) (C)
Wetmore's stork (Mycteria wetmorei) (C)
Pleistocene Mexican cormorants spp. (genus Phalacrocorax)
Phalacrocorax goletensis (C)
Phalacrocorax chapalensis (C)
All remaining teratorn (Teratornithidae) spp.
Aiolornis incredibilis (C)
Cathartornis gracilis (C)
Oscaravis olsoni (C)
Teratornis merriami (C)
Teratornis woodburnensis (C)
Several New World vultures (Cathartidae) spp.
Pleistocene black vulture (Coragyps occidentalis ssp.) (C)
Megafaunal Californian condor (Gymnogyps amplus) (C)
Clark's condor (Breagyps clarki) (C)
Cuban condor (Gymnogyps varonai) (C)
Several Accipitridae spp.
American neophrone vulture (Neophrontops americanus) (C)
Woodward's eagle (Amplibuteo woodwardi) (C)
Cuban great hawk (Buteogallus borrasi) (C)
Daggett's eagle (Buteogallus daggetti) (C)
Fragile eagle (Buteogallus fragilis) (C)
Cuban giant hawk (Gigantohierax suarezi) (C)
Errant eagle (Neogyps errans) (C)
Grinnell's crested eagle (Spizaetus grinnelli) (C)
Willett's hawk-eagle (Spizaetus willetti) (C)
Caribbean titan hawk (Titanohierax) (C)
Several owl (Strigiformes) spp.
Brea miniature owl (Asphaltoglaux) (C)
Kurochkin's pygmy owl (Glaucidium kurochkini) (C)
Brea owl (Oraristix brea) (C)
Cuban giant owl (Ornimegalonyx) (C)
Bermuda flicker (Colaptes oceanicus) (C)
Several caracara (Caracarinae) spp.
Bahaman terrestrial caracara (Caracara sp.) (C)
Puerto Rican terrestrial caracara (Caracara sp.) (C)
Jamaican caracara (Carcara tellustris) (C)
Cuban caracara (Milvago sp.) (C)
Hispaniolan caracara (Milvago sp.) (C)
Psittacopasserae
Psittaciformes
Mexican thick-billed parrot (Rhynchopsitta phillipsi) (H)
Several giant tortoise spp.
Hesperotestudo (H)
Gopherus spp.
Gopherus donlaloi (H)
Chelonoidis spp.
Chelonoidis marcanoi (H)
Chelonoidis alburyorum (H)

The survivors are in some ways as significant as the losses: bison (H), grey wolf (C), lynx (C), grizzly bear (C), American black bear (C), deer (e.g. caribou, moose, wapiti (elk), Odocoileus spp.) (H), pronghorn (H), white-lipped peccary (H), muskox (H), bighorn sheep (H), and mountain goat (H); the list of survivors also include species which were extirpated during the Quaternary extinction event, but recolonised at least part of their ranges during the mid-Holocene from South American relict populations, such as the cougar (C), jaguar (C), giant anteater (C), collared peccary (H), ocelot (C) and jaguarundi (C). All save the pronghorns and giant anteaters were descended from Asian ancestors that had evolved with human predators. Pronghorns are the second-fastest land mammal (after the cheetah), which may have helped them elude hunters. More difficult to explain in the context of overkill is the survival of bison, since these animals first appeared in North America less than 240,000 years ago and so were geographically removed from human predators for a sizeable period of time. Because ancient bison evolved into living bison, there was no continent-wide extinction of bison at the end of the Pleistocene (although the genus was regionally extirpated in many areas). The survival of bison into the Holocene and recent times is therefore inconsistent with the overkill scenario. By the end of the Pleistocene, when humans first entered North America, these large animals had been geographically separated from intensive human hunting for more than 200,000 years. Given this enormous span of geologic time, bison would almost certainly have been very nearly as naive as native North American large mammals.

The culture that has been connected with the wave of extinctions in North America is the paleo-American culture associated with the Clovis people (q.v.), who were thought to use spear throwers to kill large animals. The chief criticism of the "prehistoric overkill hypothesis" has been that the human population at the time was too small and/or not sufficiently widespread geographically to have been capable of such ecologically significant impacts. This criticism does not mean that climate change scenarios explaining the extinction are automatically to be preferred by default, however, any more than weaknesses in climate change arguments can be taken as supporting overkill. Some form of a combination of both factors could be plausible, and overkill would be a lot easier to achieve large-scale extinction with an already stressed population due to climate change.

South America

South America suffered among the worst losses of the continents, with around 83% of its megafauna going extinct. Extinctions are thought to have occurred in the interval 13,000–10,000 years Before Present, coincident with the end of the Antarctic Cold Reversal (a cooling period earlier and less severe than the Northern Hemisphere Younger Dryas) and the emergence of Fishtail projectile points, which became widespread across South America. Fishtail projectile points are thought to have been used in big game hunting, though direct evidence of exploitation of extinct megafauna by humans is rare. Fishtail points rapidly disappeared after the extinction of the megafauna, and were replaced by other styles more suited to hunting smaller prey. Humans have traditionally been less cited as a causal factor in the extinctions than in North America, though some recent scholarship is beginning to challenge this.

Ungulates
Even-Toed Hoofed Mammals
Several Cervidae spp.
Morenelaphus
Antifer
Agalmaceros blicki
Odocoileus salinae
Various Camelidae spp.
Eulamaops
Stilt legged llama Hemiauchenia
Stout legged llama Palaeolama
Odd-Toed Hoofed Mammals
Several species of tapirs (Tapiridae)
Tapirus rondoniensis
Tapirus cristatellus
All Pleistocene wild horse genera (Equidae)
Equus neogeus
Hippidion
Hippidion devillei
Hippidion principale
Hippidion saldiasi
All remaining Meridiungulata genera
Order Litopterna
Macraucheniidae
Macrauchenia
Macraucheniopsis
Xenorhinotherium
Proterotheriidae
Neolicaphrium recens
Order Notoungulata
Toxodontidae
Piauhytherium (Some authors regard this taxon as synonym of Trigodonops)
Mixotoxodon
Toxodon
Trigodonops
Carnivora
Feliformia
Several Felidae spp.
Saber-toothed cat (Smilodon) spp.
North American saber-toothed cat (Smilodon fatalis)
South American saber-toothed cat (Smilodon populator)
Patagonian jaguar (Panthera onca mesembrina) (some authors have suggested that these remains actually belong to the American lion instead)
Caniformia
Canidae
Dire wolf (Aenocyon dirus)
Nehring's wolf (Canis nehringi)
Protocyon spp.
Protocyon trogolodytes
Protocyon tarijense
Dusicyon avus
Pleistocene bush dog (Speothos pacivorus)
Arctoidea
South American short-faced bear (Arctotherium spp.)
Arctotherium bonairense
Arctotherium tarijense
Arctotherium wingei
Rodents
Giant vampire bat (Desmodus draculae)
Neochoerus
All remaining Gomphotheridae spp.
Cuvieronius
Notiomastodon
Xenarthrans
All remaining ground sloth genera
Megatheriidae spp.
Eremotherium
Megatherium
Nothrotheriidae spp.
Nothropus
Nothrotherium
Megalonychidae spp.
Ahytherium
Australonyx
Diabolotherium
Megistonyx
Valgipes
Mylodontidae spp.
Catonyx
Glossotherium
Lestodon
Mylodon
Scelidotherium
Scelidodon
Mylodonopsis
Ocnotherium
All remaining Glyptodontinae spp.
Doedicurus
Glyptodon/Chlamydotherium
Heteroglyptodon
Hoplophorus
Lomaphorus
Neosclerocalyptus
Neuryurus
Panochthus
Parapanochthus
Plaxhaplous
Sclerocalyptus
Several Dasypodidae spp.
Beautiful armadillo (Dasypus bellus)
Eutatus
Pachyarmatherium
Propaopus
All Pampatheriidae spp.
Holmesina (et Chlamytherium occidentale)
Pampatherium
Tonnicinctus
Birds
Psilopterus (small terror bird remains dated to the Late Pleistocene, but these are disputed)
Various Caracarinae spp.
Venezuelan caracara (Caracara major)
Seymour's caracara (Caracara seymouri)
Peruvian caracara (Milvago brodkorbi)
Various Cathartidae spp.
Pampagyps imperator
Geronogyps reliquus
Wingegyps cartellei
Pleistovultur nevesi
Crocs & Gators
Caiman venezuelensis
Chelonoidis lutzae (Argentina)

The Pacific (Australasia and Oceania)

A scarcity of reliably dated megafaunal bone deposits has made it difficult to construct timelines for megafaunal extinctions in certain areas, leading to a divide among researches about when and how megafaunal species went extinct.

There are at least three hypotheses regarding the extinction of the Australian megafauna:
that they went extinct with the arrival of the Aboriginal Australians on the continent,
that they went extinct due to natural climate change.

This theory is based on evidence of megafauna surviving until 40,000 years ago, a full 30,000 years after homo sapiens first landed in Australia, and thus that the two groups coexisted for a long time. Evidence of these animals existing at that time come from fossil records and ocean sediment. To begin with, sediment core drilled in the Indian Ocean off the SW coast of Australia indicate the existence of a fungus called Sporormiella, which survived off the dung of plant-eating mammals. The abundance of these spores in the sediment prior to 45,000 years ago indicates that many large mammals existed in the southwest Australian landscape until that point. The sediment data also indicates that the megafauna population collapsed within a few thousand years, around the 45,000 years ago, suggesting a rapid extinction event. In addition, fossils found at South Walker Creek, which is the youngest megafauna site in northern Australia, indicate that at least 16 species of megafauna survived there until 40,000 years ago. Furthermore, there is no firm evidence of homo sapiens living at South Walker Creek 40,000 years ago, therefore no human cause can be attributed to the extinction of these megafauna. However, there is evidence of major environmental deterioration of South Water Creek 40,000 years ago, which may have caused the extinct event. These changes include increased fire, reduction in grasslands, and the loss of fresh water. The same environmental deterioration is seen across Australia at the time, further strengthening the climate change argument. Australia's climate at the time could best be described as an overall drying of the landscape due to lower precipitation, resulting in less fresh water availability and more drought conditions. Overall, this led to changes in vegetation, increased fires, overall reduction in grasslands, and a greater competition for already scarce fresh water. These environmental changes proved to be too much for the Australian megafauna to cope with, causing the extinction of 90% of megafauna species.
The third hypothesis shared by some scientists is that human impacts and natural climate changes led to the extinction of Australian megafauna. About 75% of Australia is semi-arid or arid, so it makes sense that megafauna species used the same fresh water resources as humans. This competition could have led to more hunting of megafauna. Furthermore, homo sapiens used fire agriculture to burn impassable land. This further diminished the already disappearing grassland which contained plants that were a key dietary component of herbivorous megafauna. While there is no scientific consensus on this, it is plausible that homo sapiens and natural climate change had a combined impact. Overall, there is a great deal of evidence for humans being the culprit, but by ruling out climate change completely as a cause of the Australian megafauna extinction we are not getting the whole picture. The climate change in Australia 45,000 years ago destabilized the ecosystem, making it particularly vulnerable to hunting and fire agriculture by humans; this is probably what led to the extinction of the Australian megafauna.

Several studies provide evidence that climate change caused megafaunal extinction during the Pleistocene in Australia. One group of researchers analyzed fossilized teeth found at Cuddie Springs in southeastern Australia. By analyzing oxygen isotopes, they measured aridity, and by analyzing carbon isotopes and dental microwear texture analysis, they assessed megafaunal diets and vegetation. During the middle Pleistocene, southeastern Australia was dominated by browsers, including fauna that consumed C4 plants. By the late Pleistocene, the C4 plant dietary component had decreased considerably. This shift may have been caused by increasingly arid conditions, which may have caused dietary restrictions. Other isotopic analyses of eggshells and wombat teeth also point to a decline of C4 vegetation after 45 Ka. This decline in C4 vegetation is coincident with increasing aridity. Increasingly arid conditions in southeastern Australia during the late Pleistocene may have stressed megafauna, and contributed to their decline.

In Sahul (a former continent composed of Australia and New Guinea), the sudden and extensive spate of extinctions occurred earlier than in the rest of the world. Most evidence points to a 20,000 year period after human arrival circa 63,000 BCE, but scientific argument continues as to the exact date range. In the rest of the Pacific (other Australasian islands such as New Caledonia, and Oceania) although in some respects far later, endemic fauna also usually perished quickly upon the arrival of humans in the late Pleistocene and early Holocene.

Marsupials
Various members of Diprotodontidae
Diprotodon
Hulitherium tomasetti
Maokopia ronaldi
Zygomaturus
Palorchestes ("marsupial tapir")
Various members of Vombatidae
Lasiorhinus angustidens (giant wombat)
Phascolonus (giant wombat)
Ramasayia magna (giant wombat)
Vombatus hacketti (Hackett's wombat)
Warendja wakefieldi (dwarf wombat)
Sedophascolomys (giant wombat)
Phascolarctos stirtoni (giant koala)
Marsupial lion (Thylacoleo carnifex)
Various members of Macropodidae
Procoptodon (short-faced kangaroos) e.g.
Procoptodon goliah
Sthenurus (giant kangaroo)
Simosthenurus (giant kangaroo)
Various Macropus (giant kangaroo) spp. e.g.
Macropus ferragus
Macropus titan
Macropus pearsoni
Protemnodon (giant wallaby)
Troposodon (wallaby)
Bohra (giant tree kangaroo)
Propleopus oscillans (omnivorous, giant musky rat-kangaroo)
Nombe
Congruus
Various forms of Sarcophilus (Tasmanian devil)
Sarcophilus laniarius (25% larger than modern species, unclear if it is actually a distinct species from living Tasmanian devil)
Sarcophilus moornaensis
Monotremes: egg-laying mammals.
Echidna
Murrayglossus hacketti (giant echidna)
Megalibgwilia ramsayi
Birds
Pygmy Cassowary (Casuarius lydekkeri)
Genyornis (a dromornithid
Giant malleefowl (Progura gallinacea)
Cryptogyps lacertosus
Dynatoaetus gaffae
Several Phoenicopteridae spp.
Xenorhynchopsis spp. (Australian flamingo)
Xenorhynchopsis minor
Xenorhynchopsis tibialis
Reptiles
Crocs & Gators
Ikanogavialis (the last fully marine crocodilian)
Pallimnarchus (Australian freshwater mekosuchine crocodiian)
Quinkana (Australian terrestrial mekosuchine crocodilian, apex predator)
Volia (a two-to-three meter long mekosuchine crocodylian, apex predator of Pleistocene Fiji)
Mekosuchus
Mekosuchus inexpectatus (New Caledonian land crocodile)
Mekosuchus kalpokasi (Vanuatu land crocodile)
Varanus sp. (Pleistocene and Holocene New Caledonia)
Megalania (Varanus pricus) (a giant predatory monitor lizard comparable or larger than the Komodo dragon)
Snakes
Wonambi (a five-to-six-metre-long Australian constrictor snake)
Several spp. of Meiolaniidae (giant armoured turtles)
Meiolania
Ninjemys

Relationship to later extinctions

There is no general agreement on where the Quaternary extinction event ends, and the Holocene, or anthropogenic, extinction begins, or if they should be considered separate events at all. Some have suggested that anthropogenic extinctions may have begun as early as when the first modern humans spread out of Africa between 100,000 and 200,000 years ago, which is supported by rapid megafaunal extinction following recent human colonisation in Australia, New Zealand and Madagascar, in a similar way that any large, adaptable predator moving into a new ecosystem would. In many cases, it is suggested even minimal hunting pressure was enough to wipe out large fauna, particularly on geographically isolated islands. Only during the most recent parts of the extinction have plants also suffered large losses.

Overall, the Holocene extinction can be characterised by the human impact on the environment. The Holocene extinction continues into the 21st century, with overfishing, ocean acidification and the amphibian crisis being a few broader examples of an almost universal, cosmopolitan decline of biodiversity.

Hunting hypothesis

The hunting hypothesis suggests that humans hunted megaherbivores to extinction, which in turn caused the extinction of carnivores and scavengers which had preyed upon those animals. This hypothesis holds Pleistocene humans responsible for the megafaunal extinction. One variant, known as blitzkrieg, portrays this process as relatively quick. Some of the direct evidence for this includes: fossils of some megafauna found in conjunction with human remains, embedded arrows and tool cut marks found in megafaunal bones, and European cave paintings that depict such hunting. Biogeographical evidence is also suggestive: the areas of the world where humans evolved currently have more of their Pleistocene megafaunal diversity (the elephants and rhinos of Asia and Africa) compared to other areas such as Australia, the Americas, Madagascar and New Zealand without the earliest humans.

Circumstantially, the close correlation in time between the appearance of humans in an area and extinction there provides weight for this scenario. The megafaunal extinctions covered a vast period of time and highly variable climatic situations. The earliest extinctions in Australia were complete approximately 50,000 BP, well before the last glacial maximum and before rises in temperature. The most recent extinction in New Zealand was complete no earlier than 500 BP and during a period of cooling. In between these extremes megafaunal extinctions have occurred progressively in such places as North America, South America and Madagascar with no climatic commonality. The only common factor that can be ascertained is the arrival of humans.
This phenomenon appears even within regions. The mammal extinction wave in Australia about 50,000 years ago coincides not with known climatic changes, but with the arrival of humans. In addition, large mammal species like the giant kangaroo Protemnodon appear to have succumbed sooner on the Australian mainland than on Tasmania, which was colonised by humans a few thousand years later.

Extinction through human hunting has been supported by archaeological finds of mammoths with projectile points embedded in their skeletons, by observations of modern naive animals allowing hunters to approach easily and by computer models by Mosimann and Martin, and Whittington and Dyke, and most recently by Alroy.

A study published in 2015 supported the hypothesis further by running several thousand scenarios that correlated the time windows in which each species is known to have become extinct with the arrival of humans on different continents or islands. This was compared against climate reconstructions for the last 90,000 years. The researchers found correlations of human spread and species extinction indicating that the human impact was the main cause of the extinction, while climate change exacerbated the frequency of extinctions. The study, however, found an apparently low extinction rate in the fossil record of mainland Asia.

Overkill hypothesis

The overkill hypothesis, a variant of the hunting hypothesis, was proposed in 1966 by Paul S. Martin, Professor of Geosciences Emeritus at the Desert Laboratory of the University of Arizona.

Objections to the hunting hypothesis
The major objections to the theory are as follows:
There is no archeological evidence that in North America megafauna other than mammoths, mastodons, gomphotheres and bison were hunted, despite the fact that, for example, camels and horses are very frequently reported in fossil history. Overkill proponents, however, say this is due to the fast extinction process in North America and the low probability of animals with signs of butchery to be preserved. A study by Surovell and Grund concluded "archaeological sites dating to the time of the coexistence of humans and extinct fauna are rare. Those that preserve bone are considerably more rare, and of those, only a very few show unambiguous evidence of human hunting of any type of prey whatsoever."
Eugene S. Hunn points out that the birthrate in hunter-gatherer societies is generally too low, that too much effort is involved in the bringing down of a large animal by a hunting party, and that in order for hunter-gatherers to have brought about the extinction of megafauna simply by hunting them to death, an extraordinary amount of meat would have had to have been wasted.

Climate change hypothesis

At the end of the 19th and beginning of the 20th centuries, when scientists first realized that there had been glacial and interglacial ages, and that they were somehow associated with the prevalence or disappearance of certain animals, they surmised that the termination of the Pleistocene ice age might be an explanation for the extinctions.

Critics object that since there were multiple glacial advances and withdrawals in the evolutionary history of many of the megafauna, it is rather implausible that only after the last glacial maximum would there be such extinctions. One study suggests that the Pleistocene megafaunal composition may have differed markedly from that of earlier interglacials, making the Pleistocene populations particularly vulnerable to changes in their environment.

Some evidence weighs against climate change as a valid hypothesis as applied to Australia. It has been shown that the prevailing climate at the time of extinction (40,000–50,000 BP) was similar to that of today, and that the extinct animals were strongly adapted to an arid climate. The evidence indicates that all of the extinctions took place in the same short time period, which was the time when humans entered the landscape. The main mechanism for extinction was probably fire (started by humans) in a then much less fire-adapted landscape. Isotopic evidence shows sudden changes in the diet of surviving species, which could correspond to the stress they experienced before extinction.

Evidence in Southeast Asia, in contrast to Europe, Australia, and the Americas, suggests that climate change and an increasing sea level were significant factors in the extinction of several herbivorous species. Alterations in vegetation growth and new access routes for early humans and mammals to previously isolated, localized ecosystems were detrimental to select groups of fauna.

Some evidence obtained from analysis of the tusks of mastodons from the American Great Lakes region appears inconsistent with the climate change hypothesis. Over a span of several thousand years prior to their extinction in the area, the mastodons show a trend of declining age at maturation. This is the opposite of what one would expect if they were experiencing stresses from deteriorating environmental conditions, but is consistent with a reduction in intraspecific competition that would result from a population being reduced by human hunting.

Increased temperature
The most obvious change associated with the termination of an ice age is the increase in temperature. Between 15,000 BP and 10,000 BP, a 6 °C increase in global mean annual temperatures occurred. This was generally thought to be the cause of the extinctions.

According to this hypothesis, a temperature increase sufficient to melt the Wisconsin ice sheet could have placed enough thermal stress on cold-adapted mammals to cause them to die. Their heavy fur, which helps conserve body heat in the glacial cold, might have prevented the dumping of excess heat, causing the mammals to die of heat exhaustion. Large mammals, with their reduced surface area-to-volume ratio, would have fared worse than small mammals.

A study covering the past 56,000 years indicates that rapid warming events with temperature changes of up to had an important impact on the extinction of megafauna. Ancient DNA and radiocarbon data indicates that local genetic populations were replaced by others within the same species or by others within the same genus. Survival of populations was dependent on the existence of refugia and long distance dispersals, which may have been disrupted by human hunters.

Arguments against the temperature hypothesis
Studies propose that the annual mean temperature of the current interglacial that we have seen for the last 10,000 years is no higher than that of previous interglacials, yet most of the same large mammals survived similar temperature increases.

In addition, numerous species such as mammoths on Wrangel Island and St. Paul Island survived in human-free refugia despite changes in climate. This would not be expected if climate change were responsible (unless their maritime climates offered some protection against climate change not afforded to coastal populations on the mainland). Under normal ecological assumptions island populations should be more vulnerable to extinction due to climate change because of small populations and an inability to migrate to more favorable climes.

Increased continentality affects vegetation in time or space
Other scientists have proposed that increasingly extreme weather—hotter summers and colder winters—referred to as "continentality", or related changes in rainfall caused the extinctions. The various hypotheses are outlined below.

Vegetation changes: geographic
It has been shown that vegetation changed from mixed woodland-parkland to separate prairie and woodland. This may have affected the kinds of food available. Shorter growing seasons may have caused the extinction of large herbivores and the dwarfing of many others. In this case, as observed, bison and other large ruminants would have fared better than horses, elephants and other monogastrics, because ruminants are able to extract more nutrition from limited quantities of high-fiber food and better able to deal with anti-herbivory toxins. So, in general, when vegetation becomes more specialized, herbivores with less diet flexibility may be less able to find the mix of vegetation they need to sustain life and reproduce, within a given area.

Rainfall changes: time
Increased continentality resulted in reduced and less predictable rainfall limiting the availability of plants necessary for energy and nutrition. Axelrod and Slaughter have suggested that this change in rainfall restricted the amount of time favorable for reproduction. This could disproportionately harm large animals, since they have longer, more inflexible mating periods, and so may have produced young at unfavorable seasons (i.e., when sufficient food, water, or shelter was unavailable because of shifts in the growing season). In contrast, small mammals, with their shorter life cycles, shorter reproductive cycles, and shorter gestation periods, could have adjusted to the increased unpredictability of the climate, both as individuals and as species which allowed them to synchronize their reproductive efforts with conditions favorable for offspring survival. If so, smaller mammals would have lost fewer offspring and would have been better able to repeat the reproductive effort when circumstances once more favored offspring survival.

In 2017 a study looked at the environmental conditions across Europe, Siberia and the Americas from 25,000 to 10,000 YBP. The study found that prolonged warming events leading to deglaciation and maximum rainfall occurred just prior to the transformation of the rangelands that supported megaherbivores into widespread wetlands that supported herbivore-resistant plants. The study proposes that moisture-driven environmental change led to the megafaunal extinctions and that Africa's trans-equatorial position allowed rangeland to continue to exist between the deserts and the central forests, therefore fewer megafauna species became extinct there.

Arguments against the continentality hypotheses
Critics have identified a number of problems with the continentality hypotheses.
Megaherbivores have prospered at other times of continental climate. For example, megaherbivores thrived in Pleistocene Siberia, which had and has a more continental climate than Pleistocene or modern (post-Pleistocene, interglacial) North America.
The animals that became extinct actually should have prospered during the shift from mixed woodland-parkland to prairie, because their primary food source, grass, was increasing rather than decreasing. Although the vegetation did become more spatially specialized, the amount of prairie and grass available increased, which would have been good for horses and for mammoths, and yet they became extinct. This criticism ignores the increased abundance and broad geographic extent of Pleistocene bison at the end of the Pleistocene, which would have increased competition for these resources in a manner not seen in any earlier interglacials.
Although horses became extinct in the New World, they were successfully reintroduced by the Spanish in the 16th century—into a modern post-Pleistocene, interglacial climate. Today there are feral horses still living in those same environments. They find a sufficient mix of food to avoid toxins, they extract enough nutrition from forage to reproduce effectively and the timing of their gestation is not an issue. Of course, this criticism ignores the obvious fact that present-day horses are not competing for resources with ground sloths, mammoths, mastodons, camels, llamas, and bison. Similarly, mammoths survived the Pleistocene Holocene transition on isolated, uninhabited islands in the Mediterranean Sea and on Wrangel Island in the Siberian Arctic until 4,000 to 7,000 years ago.
Large mammals should have been able to migrate, permanently or seasonally, if they found the temperature too extreme, the breeding season too short, or the rainfall too sparse or unpredictable. Seasons vary geographically. By migrating away from the equator, herbivores could have found areas with growing seasons more favorable for finding food and breeding successfully. Modern-day African elephants migrate during periods of drought to places where there is apt to be water.
Large animals store more fat in their bodies than do medium-sized animals and this should have allowed them to compensate for extreme seasonal fluctuations in food availability.

The extinction of the megafauna could have caused the disappearance of the mammoth steppe. Alaska now has low nutrient soil unable to support bison, mammoths, and horses. R. Dale Guthrie has claimed this as a cause of the extinction of the megafauna there; however, he may be interpreting it backwards. The loss of large herbivores to break up the permafrost allows the cold soils that are unable to support large herbivores today. Today, in the arctic, where trucks have broken the permafrost grasses and diverse flora and fauna can be supported. In addition, Chapin (Chapin 1980) showed that simply adding fertilizer to the soil in Alaska could make grasses grow again like they did in the era of the mammoth steppe. Possibly, the extinction of the megafauna and the corresponding loss of dung is what led to low nutrient levels in modern-day soil and therefore is why the landscape can no longer support megafauna.

Arguments against both climate change and overkill
It may be observed that neither the overkill nor the climate change hypotheses can fully explain events: browsers, mixed feeders and non-ruminant grazer species suffered most, while relatively more ruminant grazers survived. However, a broader variation of the overkill hypothesis may predict this, because changes in vegetation wrought by either Second Order Predation (see below) or anthropogenic fire preferentially selects against browse species.

Hyperdisease hypothesis

Theory

The hyperdisease hypothesis, as advanced by Ross D. E. MacFee and Preston A. Marx, attributes the extinction of large mammals during the late Pleistocene to indirect effects of the newly arrived aboriginal humans. The hyperdisease hypothesis proposes that humans or animals traveling with them (e.g., chickens or domestic dogs) introduced one or more highly virulent diseases into vulnerable populations of native mammals, eventually causing extinctions. The extinction was biased toward larger-sized species because smaller species have greater resilience because of their life history traits (e.g., shorter gestation time, greater population sizes, etc.). Humans are thought to be the cause because other earlier immigrations of mammals into North America from Eurasia did not cause extinctions.

Diseases imported by people have been responsible for extinctions in the recent past; for example, bringing avian malaria to Hawaii has had a major impact on the isolated birds of the island.

If a disease was indeed responsible for the end-Pleistocene extinctions, then there are several criteria it must satisfy (see Table 7.3 in MacPhee & Marx 1997). First, the pathogen must have a stable carrier state in a reservoir species. That is, it must be able to sustain itself in the environment when there are no susceptible hosts available to infect. Second, the pathogen must have a high infection rate, such that it is able to infect virtually all individuals of all ages and sexes encountered. Third, it must be extremely lethal, with a mortality rate of c. 50–75%. Finally, it must have the ability to infect multiple host species without posing a serious threat to humans. Humans may be infected, but the disease must not be highly lethal or able to cause an epidemic.

One suggestion is that pathogens were transmitted by the expanding humans via the domesticated dogs they brought with them, though this does not fit the timeline of extinctions in the Americas and Australia in particular.

Arguments against the hyperdisease hypothesis
Generally speaking, disease has to be very virulent to kill off all the individuals in a genus or species. Even such a virulent disease as West Nile fever is unlikely to have caused extinction.
The disease would need to be implausibly selective while being simultaneously implausibly broad. Such a disease needs to be capable of killing off wolves such as Canis dirus or goats such as Oreamnos harringtoni while leaving other very similar species (Canis lupus and Oreamnos americanus, respectively) unaffected. It would need to be capable of killing off flightless birds while leaving closely related flighted species unaffected. Yet while remaining sufficiently selective to afflict only individual species within genera it must be capable of fatally infecting across such clades as birds, marsupials, placentals, testudines, and crocodilians. No disease with such a broad scope of fatal infectivity is known, much less one that remains simultaneously incapable of infecting numerous closely related species within those disparate clades. On the other hand, this objection does not account for the possibility of a variety of different diseases being introduced around the same era.
Numerous species including wolves, mammoths, camelids, and horses had emigrated continually between Asia and North America over the past 100,000 years. For the disease hypothesis to be applicable there it would require that the population remain immunologically naive despite this constant transmission of genetic and pathogenic material.
The dog-specific hypothesis cannot account for several major extinction events, notably the Americas (for reasons already covered) and Australia. Dogs did not arrive in Australia until approximately 35,000 years after the first humans arrived there, and approximately 30,000 years after the Australian megafaunal extinction was complete.

Second-order predation hypothesis

Scenario
The Second-Order Predation Hypothesis says that as humans entered the New World they continued their policy of killing predators, which had been successful in the Old World but because they were more efficient and because the fauna, both herbivores and carnivores, were more naive, they killed off enough carnivores to upset the ecological balance of the continent, causing overpopulation, environmental exhaustion, and environmental collapse. The hypothesis accounts for changes in animal, plant, and human populations.

The scenario is as follows:
After the arrival of H. sapiens in the New World, existing predators must share the prey populations with this new predator. Because of this competition, populations of original, or first-order, predators cannot find enough food; they are in direct competition with humans.
Second-order predation begins as humans begin to kill predators.
Prey populations are no longer well controlled by predation. Killing of nonhuman predators by H. sapiens reduces their numbers to a point where these predators no longer regulate the size of the prey populations.
Lack of regulation by first-order predators triggers boom-and-bust cycles in prey populations. Prey populations expand and consequently overgraze and over-browse the land. Soon the environment is no longer able to support them. As a result, many herbivores starve. Species that rely on the slowest recruiting food become extinct, followed by species that cannot extract the maximum benefit from every bit of their food.
Boom-bust cycles in herbivore populations change the nature of the vegetative environment, with consequent climatic impacts on relative humidity and continentality. Through overgrazing and overbrowsing, mixed parkland becomes grassland, and climatic continentality increases.

Support
This has been supported by a computer model, the Pleistocene extinction model (PEM), which, using the same assumptions and values for all variables (herbivore population, herbivore recruitment rates, food needed per human, herbivore hunting rates, etc.) other than those for hunting of predators. It compares the overkill hypothesis (predator hunting = 0) with second-order predation (predator hunting varied between 0.01 and 0.05 for different runs). The findings are that second-order predation is more consistent with extinction than is overkill (results graph at left).

The Pleistocene extinction model is the only test of multiple hypotheses and is the only model to specifically test combination hypotheses by artificially introducing sufficient climate change to cause extinction. When overkill and climate change are combined they balance each other out. Climate change reduces the number of plants, overkill removes animals, therefore fewer plants are eaten. Second-order predation combined with climate change exacerbates the effect of climate change. (results graph at right).

The second-order predation hypothesis is supported by the observation above that there was a massive increase in bison populations.

Arguments against the second-order predation hypothesis
The multispecies model produces a mass extinction through indirect competition between herbivore species: small species with high reproductive rates subsidize predation on large species with low reproductive rates. All prey species are lumped in the Pleistocene extinction model.
The control of population sizes by predators is not fully supported by observations of modern ecosystems.

Arguments against the second-order predation plus climate hypothesis
It assumes decreases in vegetation due to climate change, but deglaciation doubled the habitable area of North America.
Any vegetational changes that did occur failed to cause almost any extinctions of small vertebrates, and they are more narrowly distributed on average.