Unlike modern baleen whales it was small, about the size of a modern porpoise at around 2m long (6’6″), and the wear on its multi-cusped teeth suggest it was a predator taking slicing bites of fish – possibly using suction-assisted feeding like its close relatives the aetiocetids.
While the largest animal known to ever exist is an aquatic mammal (the modern blue whale), mammals on land have never managed to attain the same sort of massive sizes seen in the sauropod dinosaurs. This is probably due to a combination of factors, including their reproductive strategies, metabolisms, and physiological differences like lacking internal air sacs – but even being limited to overall smaller body sizes, some of the mega-mammals known to have evolved during the Cenozoic were still absolutely enormous.
And one of the largest was Paraceratherium transouralicum.
(The exact name of this animal has a long and complicated history, and in various times and places it’s also been known as Indricotherium, Baluchitherium, and Pristinotherium.)
Found across much of Eurasia during the Oligocene, about 34-23 million years ago, Paraceratherium was part of an ancient lineage of long-legged hornless rhinoceroses. It stood around 4.8m tall at the shoulder (15’9″) – big enough that most modern humans would be able to walk right underneath its belly without even having to duck – and it had elongated limbs and a long neck that gave it an overall appearance much more like a giant weird horse than a rhino.
There was a pair of downward-pointing tusks at the front of its upper jaw, and the shape of the nasal region of its skull suggests its nose formed a short prehensile tapir-like trunk, which would have been used to help grab and strip leaves from high branches.
I’ve also reconstructed it here with a speculative dewlap on its neck, used for both display and thermoregulation.
Early members of this group swam like otters, using a combination of undulating their bodies and paddling with large hind limbs, but somewhere in the Late Eocene they switched over to propelling themselves entirely with their tails and gave rise to even more whale-like forms like the basilosaurids.
And Aegicetus gehennae was right in the middle of that switch.
Discovered in the Wadi Al-Hitan (“Valley of the Whales”) fossil site in Egypt, Aegicetus lived around 37-35 million years ago. It was similarly-sized to earlier protocetids like Georgiacetus, measuring about 3.5m long (11’6″), but its hind limbs were proportionally smaller. Its hips were also completely disconnected from its vertebrae, giving it much more flexibility to undulate its body and tail – and preventing it from supporting its weight on land, suggesting that it spent its entire life in the water.
It wasn’t a direct ancestor to more “advanced” cetaceans, since it lived alongside several species of basilosaurids. Instead it seems to represent a late-surviving example of what the earlier protocetid-basilosaurid transitional forms would have looked like.
Once again it’s a PBS Eons commission roundup day!
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.
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.
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.
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.
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.
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”.
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.
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.
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.