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.

Ensidelphis

Last week’s weird-snouted Furcacetus wasn’t the only recently-discovered ancient platanistoid dolphin that deserves some attention.

Ensidelphis riveroi was described in the same paper, and also lived in the coastal waters around Peru during the early Miocene, about 19 million years ago. It was a little less closely related to its modern river-dwelling cousins than Furcacetus, and was slightly larger, estimated to have measured about 3m long (9’10”).

But what made it weird was its incredibly long snout, lined with around 256 tiny sharp teeth, which also curved markedly to the right side along its 55cm (1’10”) length.

Expectation vs reality

With only one known skull of Ensidelphis it’s impossible to tell if this was a natural condition for the species or if it was some sort of anomalous individual. It doesn’t seem to be a deformation of the fossil, at least.

Similar unusual right-side bending has been seen in the skulls of a few individuals of modern South Asian river dolphins, franciscanas, and Amazon river dolphins, possibly caused by injuries at a young age being exaggerated as the animals grew. However, many other platanistoid dolphins (especially squalodelphinids) are known to have naturally had similar bends in their snouts – but always to the opposite side, curving to the left instead of the right.

But naturally bent or not, what might Ensidelphis have been doing with that incredibly lengthy snoot?

Its long slender jaws would have had a fairly weak bite, so it probably wasn’t able to catch large prey, and it had a very flexible neck. Possibly it swam along near the seafloor using its snout to probe and sweep around in the sediment for buried small prey.

Modern South Asian river dolphins swim along on their sides while doing this – almost always on their right sides, interestingly enough – and if Ensidelphis did the same sort of thing then a snout bent in that direction might have been an advantage.

Furcacetus

The two living subspecies of the South Asian river dolphin are the last surviving members of a lineage known as the Platanistoidea, an early evolutionary branch of the toothed whales. This group was once much more diverse and widespread than their modern representatives, found in oceanic habitats around the world from the Oligocene to the mid-Miocene.

Many of them had forward-pointing protruding teeth at the tips of their snouts, resembling those of some plesiosaurs or pterosaurs, suggesting they were a convergent adaptation used for snagging hold of slippery soft-bodied aquatic prey.

Furcacetus flexirostrum is one the newest additions to this group, named and described in late March 2020. It lived in Pacific coastal waters around Peru during the early Miocene, about 19-18 million years ago, and was about the same size as modern South Asian river dolphins at around 2.3m long (7’7″).

And it had a uniquely-shaped snout for a cetacean, curving upwards for most of its length but then turning downwards right at the tip, which along with large forward-pointing teeth gave its jaws a vaguely crocodilian appearance.

A closeup view of the jaws of Furcacetus.

Much like slender-snouted crocodilians and spinosaurids, this arrangement would have allowed Furcacetus to make quick bites at small-fast-moving prey like fish and crustaceans.

Weird Heads Month #27: The Weirdest Wildebeest

Earlier in this series we saw some ruminants with bizarre-looking headgear, but there was another species in that group that evolved a completely different type of strange head.

Rusingoryx atopocranion was a close relative of modern wildebeest that lived during the late Pleistocene, around 100,000 to 50,000 years ago. Its fossil remains are known from the Kenyan part of Lake Victoria, on Rusinga Island – an area which wasn’t actually an island at the time due to lower lake levels, and was instead part of a hot dry grassland environment.

Standing about 1.2m at the shoulder (~4′), it had an oddly-shaped skull with a pointed snout and a highly domed forehead. But this wasn’t the thick bony dome of a headbutting animal – this structure was narrow and fairly fragile, and had looping nasal passages running through it.

Instead it was something never before seen in any mammal: a bony nasal crest convergently similar to those of hadrosaurid dinosaurs.

Juveniles had less developed crests, developing them as they matured, and one skull that may represent an adult female also has a smaller crest, suggesting that this feature was sexually dimorphic.

Based on just the anatomy of the nasal passages Rusingoryx may have honked at a frequency similar to a vuvuzela, but the added length of its vocal tract could have lowered this pitch even further, closer to infrasound ranges – so more like a tuba! Such low frequencies can travel very long distances and are also below the hearing range of many carnivores, and would have effectively allowed Rusingoryx to shout at each other in “stealth mode”.

Weird Heads Month #25: The Case of the Missing Trunk

The rhino-like toxodontids from earlier in this series weren’t the only weird-headed South American ungulates. Another group known as the litopterns evolved in a different direction, becoming long-legged fast-moving animals convergently filling the same sort of ecological niches as modern horses, deer, bovids, camelids, and giraffids.

Macrauchenia patachonica was one of the strangest members of this lineage, living from the Late Miocene to the end of the Pleistocene, between about 7 million years ago and just 12,000 years ago.

It stood around 1.8m at the shoulder (5’11”) and resembled a large camel or llama with thee-toed hoofed feet, but its head was… confusing.

Its skull had a bizarre combination of features, with a shape closer to a sauropod dinosaur than a mammal, a cartoonish-looking set of teeth, and its nostrils set up high above its eyes, more like a cetacean blowhole than a terrestrial herbivore.

Due to its retracted nostrils it’s commonly been restored with an elephant-like or tapir-like trunk. And while a trunk gives Marauchenia a wonderfully weird and memorable appearance, there’s just one problem with that interpretation.

There’s no evidence for it.

Aside from its nostrils being far back on its head, it didn’t have any other features associated with anchoring the complex musculature of a trunk. In fact, a recent study found that its skull characteristics were much closer to those of moose than tapirs!

It seems more likely that it had a moose-like bulbous fleshy nose – possibly giving it an enhanced sense of smell or functioning as a resonating chamber – perhaps with slightly retracted external nostrils like a giraffe or sauropod to prevent it from being stabbed in the nose when feeding on spiky vegetation.

Whatever it was doing with its weird schnoz, it was clearly a highly successful species, since it was found across most of South America in a wide range of habitats.

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 #19: Sword-Snouted Whales

Cetaceans are just weird animals in general. Fully aquatic mammals best described as “fat screaming torpedoes“, with bizarre head anatomy and their nostrils pulled up to the top of their heads behind their eyes. Some of them are among the largest animals to ever exist, some of them can live to over 200 years old, and some can dive to incredible depths below the ocean surface.

And they’re all descended from tiny deer-like creatures, with their closest living relatives being hippos and other even-toed ungulates.

Some ancient cetaceans were particularly odd-looking, evolving walrus-like tusks or elongated chins – or in the case of Eurhinodelphis longirostris here, an incredibly long swordfish-like snout.

Living during the mid to late Miocene, about 14-7 million years ago, Eurhinodelphis ranged across the Mediterranean and the North Atlantic, with fossil remains known from Western Europe, Turkey, and the East Coast of the United States. It was a fairly small dolphin-like cetacean about 2m long (6’6″), and was part of a lineage of early toothed whales called eurhinodelphinids.

Its upper jaw was around five times longer than the rest of its skull, and toothless past the point where the lower jaw ended. Much like the modern billfish it resembled, it probably used its snout to slash at fast-moving fish, stunning them and making them easier to catch.

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.

Weird Heads Month #13: Many-Horned Mammals

The dinoceratans were a lineage of hoofed herbivorous mammals whose evolutionary affinities are a little uncertain, but may have been related to the South American meridiungulates. Found in Asia and North America from the late Paleocene to the late Eocene, they had bulky rhino-like bodies and were some of the largest terrestrial animals of their time.

Eobasileus cornutus was one of the biggest of them all, measuring around 2.1m tall at the shoulder (~7′) and living in the Western United States during the early Eocene, about 46-40 million years ago.

And it had a very odd-looking head, with six blunt ossicone-like horns, large sabre-like fangs, bony flanges on its lower jaw, a concave forehead, and a proportionally tiny brain for its body size. The horns and fangs were sexually dimorphic, much smaller in females, suggesting they were mainly used for display or combat between males.

Weird Heads Month #09: Butterfly Faces

The nose-forks and head-crests we saw last time weren’t the only unusual headgear in ancient ruminants.

The giraffoids are represented today by just pronghorns, giraffes, and okapi, but in the past they were much more diverse, modifying their prongs and ossicones into multiple sets of horns, or into deer-like and moose-like antler shapes.  

And Prolibytherium was probably the most striking of the lot.

Two different species have been identified, with Prolibytherium magnieri here living in North Africa during the early-to-mid Miocene, about 17-16 million years ago. Its exact evolutionary relationships are uncertain but it was probably part of a group called climacoceratids, deer-like giraffoids which often had thorny branching ossicones that resembled antlers.

It stood around 1.2m tall at the shoulder (~4′), and exhibited dramatic sexual dimorphism – females had slender forked horn-like ossicones, while those of the males flared out into large wide flat shapes that resembled butterfly wings.

Heavy reinforcement in the bones of the back of the males’ skulls helped to support all the extra weight of those huge ossicones, and if they actually used the structures to fight with each other then this may have also provided some protection or shock absorption.