While this animal might look like some sort of deer or horse, it was actually only distantly related to any modern hoofed mammals.
This is Thoatherium from the Early Miocene (~17-16 mya) of Argentina. About 70cm long (2′3″), it was related to the weird llama-like Macrauchenia and was part of an extinct group of ungulates (the Meridiungulata) which evolved during South America’s time as an isolated island continent.
It was adapted for fast running, with long legs and only a single horse-like hoof on each foot – but it was even more one-toed than modern horses are, having no remaining “splint bones” from vestigial side toes.
Living about 30 million years ago in shallow coastal waters around the southeast USA, in what is now South Carolina, it was a member of one of the very earliest groups of toothed whales known as the xenorophids. Although only very distantly related to modern forms, xenorophids show evidence of being able to echolocate, suggesting the ability was probably ancestral to all toothed whales.
Estimated to have measured about 1m long (3′3″), Inermorostrum had a very short downturned snout and was completely toothless – specialized adaptations for suction feeding on small soft-bodied creatures on the seafloor.
Unusually for a toothed whale it also had proportionally large infraorbital foramina, openings in the bones of its snout for blood vessels and nerves to pass through. This suggests the presence of well-developed fleshy lips and possibly whiskers (as illustrated here), or maybe even an electroreceptive sense similar to some modern dolphins.
Ampelomeryx ginsburgi, a palaeomerycid ungulate from the Early Miocene of France (~17 mya). About the size of a deer, around 1m tall at the shoulder (3′3″), it was a distant relative of modern giraffids.
Males sported three distinctive ossicone-like ‘horns’ – two over their eyes and a third forked one at the back of the skull – and protruding tusks like some modern deer, which probably served a similar purpose in fights against each other.
Globicetus hiberus, a 5m long (16′4″) beaked whale from the Atlantic coast of Portugal and Spain. Its fossils can’t be easily dated since they were fished up from the seafloor, but it was probably around Early-to-Mid Miocene in age (~20-14 mya).
Its skull sported an odd bony sphere at the base of its snout, just in front of the melon, which appears to have been larger and more prominent in males than in females. Many modern beaked whales also have sexually dimorphic crests, ridges, and domes in their skulls, and these structures may function as sort of “internal antlers” – a display structure the whales can “see” via echolocation that signals their size, strength, and health to each other.
Almost a century later that one skull is still all we have. And despite this animal’s popularity among paleo-fans, we actually know very little about it.
It was originally classified as a mesonychian, leading to the many many depictions of it as a sort of “big bad wolf”. But more recent studies have placed it in the even-toed ungulates instead, with some suggestions that it might be most closely related to entelodonts, hippos, and whales.
Although it was certainly a big animal, it may not have been the giant “super predator” it’s often depicted as – its teeth aren’t particularly specialized and resemble those of entelodonts, suggesting it may have been more of an opportunistic omnivore than a dedicated carnivore.
Without more material we just don’t know for certain. So, frustratingly, the rest of Andrewsarchus’ body remains a mystery.
I’ve reconstructed it here based on one of its more obscure possible relatives: the anthracotheres, a group which may have been closely related to modern hippos. Scaling its body proportions to these animals produces rough measurements of about 1.45m tall at the shoulder (4′9″) and 3m long (9′10″), or about the same size as some of the big entelodonts or large modern bears.
Horse evolution is often represented as a simple progression from Eohippus* to modern Equus, but it was actually a lot more complicated than that – and some ancient horses had some very odd things going with their snouts…
(* For a long time Eohippus was considered synonymous with Hyracotherium, but more recently has been split back off as its own genus again.)
Pliohippus sp. skull and head reconstruction
Pliohippus, from the Middle Miocene of North America (~15-12 mya), and several of its other close relatives had especially large, deep recesses in their skulls, usually referred to as “preorbital fossae”.
And the purpose of these holes is still unknown. Although superficially similar depressions are seen in various other perissodactyl groups, they vary in position and structure and probably weren’t all homologous.
Ideas have included resonating chambers, some sort of glands, inflatable sacs, or attachment sites for complex lip musculature.
Hippidion sp. skull and head reconstruction
Meanwhile Hippidion from the Pleistocene of South America (2 million – 10,000 years ago) had especially long and domed nasal bones. This must have supported an enormous nasal area – possibly giving it a saiga-like air-conditioning system, a highly sensitive sense of smell, or perhaps even some sort of prehensile proboscis-like snout.
Unless we find some exceptional soft-tissue preservation, the facial anatomy of these equids is going to remain enigmatic.
Homalodotherium, a South American notoungulate mammal from the Early-to-Middle Miocene of Patagonia (~20-15 mya). Standing about 1.4m tall at the shoulder (4′7″), it seems to have convergently evolved to fill the same selective browsing niche as the North American chalicotheres and the later giant ground sloths.
Despite being an ungulate it had claws rather than hooves, and walked plantigrade on its hind feet but digitigrade on its front feet. It would have been capable of rearing up bipedally to pull down branches with its long forelimbs, with the shape of its nasal bones suggesting it may have also had a prehensile upper lip to help it strip off vegetation while feeding.
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