Lifting the veil of the megafaunal extinctions: South American native ungulates

Some memories are simultaneously pleasant and sad: One such is of the many afternoons spent reading about the discovery and then the osteology of South American mammals. It was then that we read with some awe of the great deeds of Florentino Ameghino on the discovery of the mammalian fauna of South America. The pleasantness of those memories comes from being inspired, even a bit awed, by Ameghino’s meteoric and prolific contribution to paleontology, which uncovered a genuinely lost Cenozoic world, full of such unfamiliar mammals and dinosaurs attempting a comeback that the northerner is simply left stunned by their strangeness. Ameghino was the successor of Charles Darwin in South America and the first to really understand the evolutionary theory in Argentina. He came from a low class family and became a self-taught biologist without any formal education while exploring the wilds and collecting fossils as a child in Argentina. Upon reading Darwin’s works he immediately realized its profound significance and wrote that biology had finally become an exact science as he foresaw the role for mathematical methodology in reconstruction of evolutionary histories. While he was widely admired in his country as a brilliant man, even a hero, the heavy hand of the the corpse-cult resulted in him never being fully understood or followed by capable immediate successors. Indeed, many in his country held the view that the evolutionary theory presented by Darwin had “villainous consequences” upheld by the “laughable pride” of his follower Ameghino. In course of his life of 56 years he published 24 volumes of papers, covering over 18000 pages, on fossil vertebrates from Argentina. Among these was his magnum opus “Mammalian Fossils in the Argentine Republic”, the first detailed work on the extraordinary mammalian world of South America of over 1000 pages. Being isolated in the southern hemisphere, with little direct understanding of the northern faunas, he came up with some strange ideas like the origin of humans in South America from a fossil primate, which in reality was an ancestral New World monkey. Nevertheless, Ameghino’s study of humans in South America led to one of the widely debated ideas in archaeology – namely the human hand in the extinction of the megafauna of South America (and more generally in all parts of the world where humans event spread out of Africa).

This dramatic event first discovered by Ameghino may be described thus:
● Around the Pleistocene-Holocene boundary 100% of the mammals 1000 kg or greater in mass abruptly became extinct
● 80% of large mammal species with mass over 44 kg became extinction
● Only a small number of mammals below that threshold and no notable set of plants became extinct.

Interestingly, certain plants adapted to the megafauna continue to linger on as before: the Osage-orange tree with it is giant fruit which were once dispersed by extinct elephants feeding on them; the Cassia grandis tree whose fruits were consumed both by extinct giant sloths and elephants; the honeylocust tree which has large spines high above the ground to deter extinct elephants; the avocados whose fruits were consumed by sloths and thus dispersed – this plant has found a second life with human cultivation.

Thus, this extinction was unlike the other mass extinctions – while the great extinctions like the K-Pg transition was marked by the death of all dinosaurs above a certain mass, it also had mass-independent effects with several small sized vertebrates, plants, mollusks and even microbial eukaryotes taking a noticeable hit. This peculiar pattern of the megafaunal extinction played out in all other parts of the world with the spread of the hominids. The effects were particularly severe outside of Africa and Asia where Homo appeared abruptly. Today the weight of the evidence delivers what to us is a clear verdict: “Homo stands accused in the mass extinction of megafauna.”

One could debate if the extreme form of the hypothesis, sometimes called the “Blitzkrieg” version played a role but the evidence from the Americas does point to relationship between emergence of particular hunting tactics and megafaunal extinction. Genomic evidence suggests that the first, and most prominent thrust of humans in to America was by the group called the First Americans who entered via the Bering land bridge from Northeastern Asia. With the exception of the Eskimos and Na-dene groups, the majority of native Americans have entirely descended from these First Americans. The recent sequencing of the genome of the Anzick-1 boy from 12.7-12.5 kya (belonging to the first archaeologically prominent human culture of North America, the Clovis culture) and comparisons with other native Americans suggests that there was some already diversification of among the First Americans descendants by this time. This suggests that the First Americans might have entered North America around 15 Kya or a little before that. Their descendants rapidly advanced through the two American continents and appear to have been hunter-gathers pursuing a wide range of foraging tactics, which involved some hunting of megafauna coupled with exploitation of marine food along the coast and plant-based subsistence. Among the descendants of the First Americans the Clovis culture emerged in North America around 13 kya. This was marked by the development of a very distinctive type of stone points, the Clovis points, that were hafted onto projectile javelins. The emergence of this culture was accompanied by a major push towards megafaunal hunting and was followed by complete extinction of the megafauna by 10.37 Ky in North America. The influence of the Clovis culture (either through diffusion of technology or the people) rapidly entered Central and South America, where its presence is marked by the emergence of the related Fishtail point that was similarly hafted on projectile javelins. Like their North American Clovis counterparts, the Fishtail hunters clearly targeted megafauna. Not unlike North America, the emergence of the Fishtail point marked the beginning of the end of the South American megafauna. Three items of note might be gleaned from what is currently known regarding the emergence of these cultures: 1) The emergence of the above hunting technologies are archaeologically correlated with a deliberate targeting of large animals by the Paleoamericans in both the northern and southern continents. 2) The Clovis points are typically earlier than their southern Fishtail counterparts. 3) The megafaunal extinction occurs first in the northern and then in the southern continent, but in both cases is preceded by the emergence of the distinctive hafted missile. Thus, irrespective of whether it is considered a Blitzkrieg or not, the emergence of a particular hunting technology and associated tactics, specifically targeting large animals, was a major factor in their extinction in a circumscribed temporal window. Some of these megafauna like the carnivorans and sloths were capable of defending themselves at close quarters – recently there was a report of how a hunter in Brazil was killed by an anteater using its claws – their extinct relatives, the large sloths could have similarly used their claws. However, the use of projectile javelins along with fire by specialist hunters could have over come these defenses. Finally, the rapid depletion of the megafauna might have had a feedback effect on the Paleoamericans with the unified Clovis/Fishtail system breaking up and giving way to a wide diversity of local cultures with not much gene flow between them.

The casualties of the First American invasion of the continents spanned a wide range of mammalian lineages. In the north there were: xenarthrans including several lineages of giant sloths, glyptodonts and armadillos; afrotherians including lineages of elephants (Cuvieronious, Mammut and Mammuthus), perissodactyls including horses and tapirs; Artiodactyls including camels, llamas, cattle, bisons, Ovibos, peccaries, several deer and peccaries; cats including Homotherium, Smilodon, American cheetah and American lion; dog-bears including dire wolves, the short faced bear, and varieties of spectacled bears; giant rodents including the capybaras – Neochoerus and Hydrochoerus. In the southern continent some of the above such as peccaries, certain llamas, the spectacled bear, tapirs, and one capybara survived the onslaught at least in certain localities; however 50-60 species were lost in South America as opposed to the estimated 30-40 in North American. Among these were a huge chunk of xenarthran diversity including several lineages of sloths, glyptodonts and armadillos; elephants like Stegomastodon and Cuvieronious; perissodactyls including horses; some llamas, the deer Morenelaphus and Antiger; similar carnivorans took a hit as the north, including the gigantic short-faced bear Arctotherium; some capybaras; the large New World monkey Protopithecus brasiliensis; most dramatically the South American native ungulates (SANU) represented by forms like Toxodon, Mixotoxodon, Xenorhinotherium, Macrauchenia, Hemiauchenia completely vanished without trace. While many of the extinct megafauna of South America descended from the northern animals, or have at least a few living representatives (the xenarthrans) the SANU have no identifiable relatives, living or extinct, elsewhere in the world. This was the cause of sadness – a veil over the knowledge of their true affinities – their anatomical uniqueness only making things worse.

South America was home to much strangeness over the Cenozoic: The aftermath of the tumultuous closure of the Mesozoic left the marsupials in possession of much of the continent. There they greatly diversified giving rise to several forms among which chiefly, the borhyaenoids occupied the carnivore guilds. These included the early tree-climbing carnivore Mayulestes from Bolivia and the related Allqokirus. They were followed by more advanced forms like the Brazilian Patene from the end of Palaeocene emerge and in the Eocene the borhyaenoid spawned several massive forms such as Callistoe, Arminiheringia and Proborhyaena which was larger than a grizzly bear. By the Miocene they diversified into a range of carnivore niches: otter-like Cladosictis, a marten-like Prothylacinus, a peculiar long-snouted ambush predator Lycopsis, mongoose-like hathlyacynids, leopard-like Borhyaena, saber-toothed Thylacosmilus resembling the saber-toothed cats and the probably bear-like Pharsophorus. The Pliocene however saw their ultimate decline and extinction. Interestingly, despite their diversity they were never solely in possession of the carnivore niche. They were accompanied by the theropods attempting a come back in the form of the phorusrhacid birds. The rest of the South American mammalian radiations were those of placentals. Of these the xenarthrans were an exclusively South American clade, which from early on (i.e. the glyptodonts) as though responding to the predation from the phorusrhacid birds developed armor and even spiked tail clubs, thus converging to strategies of the Mesozoic ankylosaurs against bipedal theropod predators (also mirrored by meiolanid turtles). Some placentals reached South America from the Old World, probably floating across the paleo-Atlantic from Africa. These included the two related clades the rodents and the primates. The rodents, while typically small animals on other continents, appear to have undergone a major ecological release in South America giving rise to gigantic forms like Josephoartigasia (~3 meters; 800 kg or more), Phoberomys (~3 meters; ~700 kg), Telicomys (~2.5 meters; 600 kg) and Chapalmatherium (1.7 m; 200 kg). They appear to have taken the place, in part, of the ungulate herbivores in several South American ecosystems. However, along side them were the SANU which were ecologically indistinguishable from the ungulates of the Old World and North American ecosystems.

Five major lineages of SANU have been recognized: Litopterna, Notoungulata, Astrapotheria, Xenungulata, and Pyrotheria.

continued…