If you saw Avengers: Endgame, then you probably talked about it with your friends afterwards. The story. The action. The loss. And the ass. Yes. That ass.

Iron Man (Robert Downey Jr) comments on Captain America’s old suit, saying it does nothing for his derriere. A bit of a harsh comment. Fortunately, Ant-Man (Paul Rudd), has Captain America’s back(side), and jumps in with “As far as I’m concerned, that’s America’s ass.” A few scenes later and Captain America (Chris Evans), has a close fight with himself from the past, and wins. Before walking off, he looks at himself, and says “That is America’s ass.”

If you haven’t seen Avengers: Endgame, then that probably sounds like the strangest paragraph you have ever read. Quite understandable. Iron Man. Ant-Man. Captain America fighting himself. But, it’s a little nod of appreciation. It is a fine ass. (If you have seen it, then no doubt that you agree.) And it does link (almost) to this wonderful Pleistocene beast, so forgive the odd introduction.

Captain America (Chris Evans) commenting on his own rear end.

In the beautiful tropical rainforests of North America, 52 million years ago, we see evidence of the very first horses. Much smaller than today’s familiar species, the dawn horse, Eohippus, was tiny – about the size of a wolf. Eohippus had four toes (horses today move on one toe), and would have been quite fast moving through the thick vegetation. This little beast spread far from North America through to Europe, and many different species evolved from isolated populations.

Horses were a hugely diverse group, with numerous species around from 52 million years ago until just a few thousand years ago. North America had several species trotting across the landscape, whilst South America had none. For over 30 million years, South America was an isolated, drifting landmass. It’s own unique flora and fauna evolved here (including sloths and armadillos). South America was slowly drifting northwards towards North America, and around 2.8 million years ago underwater volcanoes and sediment build up created a link to the two huge landmasses. This new link allowed animals from North America to move into South America, and vice-versa.

The Great American Interchange: the animals in blue had moved from North America into South America; the animals in green had moved from South America into North America. (Image Public Domain)

Several species of horse travelled down into South America, with one species evolving that links to our slightly unusual introduction.

As species moved between the landmasses, some became isolated from other populations, and evolved into new species. In South America an unusual horse evolved, Hippidion devillei. As ever with the naming of extinct species, there is some debate. Some researchers prefer to place it in the genus Onohippidium. The distinction? A small indent in their snout indicating a different genus. Those in the Hippidion camp argue that this is too small and variable to warrant a whole different genus. And the debate continues.

Hippidion or Onohippidium, this horse evolved around 2.5 million years ago in South America. Fossils found at Tarija, in Bolivia, indicate that the short legs were adapted to living on hilly environments, rather than open plains. It was also pretty distinct from other horses around. With shorter legs, it looked more like an ass* than a horse. It had a very elongated nasal bone, which hints at a prehensile lip for feeding on trees and shrubs.

America’s ass, Hippidion, with it’s short, stocky legs, and prehensile upper lip. (Image Twilight Beasts)

Eagle eyes readers, who still have America’s ass on their minds (I don’t blame you), will be wondering why I tenuously linked Captain America’s derriere to a horse that evolved in South America. It wouldn’t have worked, but (fortunately I get to keep the introduction), some populations of Hippidion devillei did move north, into the Americas. Fossils have been discovered in California, so this wonderful little ass-like horse, was America’s first ass.

Fossils recovered from sites in South and North America are not very common, suggesting that this species was not as abundant as other species found. This little animal vanished just 8.000 years ago. It’s not a simple story of how. Cut marks have been found on some fossils, showing that humans butchered them – and possibly hunted them. It also seems Hippidion wasn’t around in very large numbers, and the climate was changing which would have affected the already small populations.

Dozens of species of horse galloped, trotted, and scurried, in the northern hemisphere for over 50 million years. Today, there are just 7 species surviving (1 horse, 3 donkeys, and 3 zebra), none of which are native to the Americas where they originated from. Hippidion wasn’t strictly an ass, but it wasn’t far off. It was closely related to modern day horses. It looked like an ass. A mighty fine ass.

Written by Jan Freedman (@JanFreedman)

*Horse is an interesting noun because it generally describes any animal in the Equidae family. It’s a common name, not a scientific name. Any extinct species in this family, we will call it a horse. Common names to animals are given by people to identify them in their local environment. (A ladybird in England, is a ladybug in American. A plant species can have several different common names depending on where you live.) The scientific name tells us what species it is. The surviving members of the horse family have common names, quite simply because they have been around for people to name them: horse, zebra, and donkey. Donkeys, or asses, are in the same genus as horses around today. So, when describing extinct species, we use modern equivalents to compare them too. Hippidion and all the other extinct ‘horses’ don’t have a common name. It wasn’t a donkey. But it also wasn’t a horse. It did, however, look like an ass.

Further reading:

Alberdi, M. T., & Prieto, A. (2000). ‘Hippidion (Mammalia, Perissodactyla) de las Ceuevas de las provincias de Magallanes y Tierra de Fuego.’ Anales Instituto Patagonia, Serie Cs. Hs. (Chilie). 28. pp.147-171. [Full article]

Der Sarkissian, C., et al. (2015). ‘Mitochondrial genomes reveal the extinct Hippidion as an outgroup to all living equids.’ Biological Letters. 11. 20141058 dx.doi.org/10.1098.rsbl.2014.1058

Croft, D. A. (2016). Horned Armadillos and Rafting Monkeys. Indiana University Press. [Book]

Macfadden, B. J. (2010). ‘Pleistocene horses from Tarija, Bolivia, and validity of the genus Onohippidium (Mammalia: Equidae).’ Journal of Vertebrate Paleontology. 17 (1). pp.199-218. [Abstract only]

MacFadden BJ (2013) ‘Dispersal of Pleistocene Equus (Family Equidae) into South America and Calibration of GABI 3 Based on Evidence from Tarija, Bolivia’. PLoS ONE 8(3): e59277. doi:10.1371/journal.pone.0059277

MacFadden, B. J., & Shockey, B. J. (1997). ‘Ancient feeding ecology and niche differentiation of Pleistocene mammalian herbivores from Tarija, Bolivia: morphological and isotopic evidence.’ Paleobiology. 23(1). pp77-100. [Abstract only]

Orlando, L, et al. (2008). ‘Ancient DNA clarifies the evolutionary history of American Late Pleistocene Equids.’ Journal of Molecular Evolution. 66(5). pp.533-538. [Abstract only]

Prado, J. L., & Alberdi, M. T. (). Fossil Horses of South America. Springer Link. [Book]

Sanchez, B., Prado, J. L., & Alberdi, M. T. (2006). ‘Ancient feeding, ecology and extinction of Pleistocene horses from the Pampean Region, Argentina. Rev. Asoc. Paleontol. Argent. 43(2). pp.427-436. [Full article]

Shockey, B. J., et al. (2009). New Pleistocene cave faunas of the Andes of Central Peru: radiocarbon ages and the survival of low latitude, Pleistocene DNA. Palaeontologia Electronica. pp.1-15. [Full article]

Weinstock, J., et al. (2005). ‘Evolution, systematics and phylogeography of Pleistocene horses in the New World: a molecular perspective.’ PLoS Biology. 3(8). e241. [Full article]