Hyopsodus

Back during the early Eocene, around 50 million years ago, global temperatures were much warmer than today, and in North America tropical and subtropical rainforests extended as far as Alaska.

And one of the most abundant animals in these balmy ecosystems was a small mammal called Hyopsodus, an early type of ungulate that was probably part of the perissodactyl lineage, closely related to the ancestors of modern horses.

Many different species of this genus have been discovered, ranging from rat-sized to cat-sized. Remains of Hyopsodus account for up to 30% of fossils in some locations, with tens of thousands of specimens known – although most of them are isolated teeth and jaw fragments.

(The illustration here depicts Hyopsodus wortmani, a 30cm/12″ long species which lived about 50-46 million years ago across the Western and Southern USA.)

More substantial skeletal remains of this little mammal are very rare, and initially seemed to show a long weasel-like body that resulted in Hyopsodus being given the nickname of “tube-sheep”. But more recent specimens have given us a better idea of its proportions, and it wasn’t really tubular at all. Instead it was probably built more like a cavy or a hyrax, with a more chunky body and a spine held more strongly curved.

Its teeth suggest it was a generalist omnivore, probably mainly eating a mixture of vegetation, fruits, seeds, insects, and occasionally smaller animals, and while its limbs were proportionally short it was likely still quite an agile fast-moving animal. It also appears to have had some ability to dig, and may have sheltered in burrows similarly to modern groundhogs.

But one of the most surprising things about the “tube-sheep” comes from studies of its braincase via CT scans of its skull. Its brain was unusually large for its size, and had enlarged areas associated with good senses of smell and hearing – and notably one sound-processing region (known as the inferior colliculus) was developed to a degree similar to those seen in echolocating animals.

Analysis of its ear bones suggest it wasn’t highly specialized for echolocation like bats, but may have still been capable of a more basic shrew-like version, using it for close-range navigation.

Weird Heads Month #23: Dome-Headed Claw-Horses

Much like Platybelodon from a few entries back, chalicotheres look like a fictional creature design rather than something that actually existed.

These animals were odd-toed ungulates related to modern horses, tapirs, and rhinos, who ranged across Africa, Eurasia, and North America for a large chunk of the Cenozoic. Instead of hooves they had large claws on their feet, and they appear to have occupied the same sort of ecological niche as ground sloths or therizinosaurs – sitting or rearing up on their hind legs to browse on high vegetation, using the hook-like claws on their forelimbs to pull down and strip branches.

There were two different lineages of chalicotheres which developed along slightly different evolutionary paths: the knuckle-walking gorilla-like chalicotheriines and the more goat-like schizotheriines.

Tylocephalonyx skinneri here was one of the latter group, known from the Miocene of North America about 16-13 million years ago. Standing about 2m tall at the shoulder (6’6″), it had the same sort of chunky body as other schizotheriines and walked around with its large front claws held up to keep them raised away from the ground.

But there was also an unusual feature on its otherwise rather horse-like head – a large bony dome on top of its skull, like a mammalian version of a pachycephalosaur.

It probably used its dome in the same way as the dinosaurs it convergently resembled, headbutting or flankbutting in fights with each other.

Weird Heads Month #16: Big Honking Snoots

The dinoceratans featured here a few days ago were some of the first large mammalian herbivores to evolve in the Cenozoic, but during the Eocene they were joined by another group: the even bigger brontotheres.

Part of the odd-toed ungulate lineage, brontotheres convergently resembled rhinos but were actually much more closely related to horses. And much like the dinoceratans they also had some unusual heads, with some species evolving concave foreheads and sexually dimorphic ossicone-like pairs of blunt horns on their noses.

But others went really weird.

Embolotherium andrewsi lived in Mongolia during the late Eocene, around 37-34 million years ago. Standing around 2.5m tall at the shoulder (8’2″), it was one of the largest brontotheres and also one of the oddest-looking.

It had a large bony “battering ram” at the front of its snout, formed from modified nasal bones – and while some reconstructions tend to shrinkwrap this structure as a horn, the fact that the nasal cavity appears to have extended all the way to its tip suggests that it was actually supporting a huge bulbous nose.

Since Embolotherium also doesn’t seem to have been sexually dimorphic like other brontotheres, its enormous ridiculous-looking snoot may instead have been a resonating chamber used for sound production and communication.

Lophialetes

Much like how hyraxes were once far more diverse than their modern representatives, some ancient members of the tapir lineage were similarly weird.

Lophialetes expeditus was one of these odd tapir-relatives, living in Mongolia and China during the mid-Eocene about 48-37 million years ago. Standing around 50cm tall at the shoulder (1’8″) it had a build more resembling a deer or a horse than its pig-like modern cousins, and it was adapted for fast running in open plains, with long slender legs and three-toed hoofed feet that bore most of its weight on the middle digit.

Its skull had a nasal region similar to both modern tapirs and saiga antelope, suggesting the presence of a short trunk-like nose – but since some of its closest relatives didn’t have nearly such well-developed snouts, it seems that Lophialetes evolved its trunk separately to modern tapirs.

Diplacodon gigan

Diplacodon gigan, a brontothere from the Early Eocene of Wyoming, USA (~46-42 mya). Standing around 2.1m tall at the shoulder (~7′) it was named after the kaiju Gigan for its relatively large size – not quite as big as some later brontotheres, but still about 20% larger than other known species of Diplacodon.

It had a pair of blunt bony projections on its snout which would have been covered with skin in life, similar to the ossicones of modern giraffids, with males having larger “horns” than females.

Despite looking very similar to rhinos, brontotheres were actually much more closely related to horses, with the resemblance being a result of convergent evolution for the same sort of big-tanky-herbivore ecological niche.

Phenacodus

Phenacodus primaevus, a mammal from the Late Paleocene to Middle Eocene of North America and Europe (~60-48 mya). About 1.5m long (5′), it’s thought to have been one of the earliest known odd-toed ungulates, walking on its middle three hoofed toes.

Its teeth were adapted for a diet of mostly plant matter, although it may also have been opportunistically omnivorous.

Another species in the same genus, Phenacodus intermedius, had a skull structure that suggests it might have had a muscular prehensile upper lip – or perhaps even a short tapir-like proboscis.