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.
New year, new PBS Eons commission roundup day!
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.
These large-headed predators were part of the hyaenodont lineage, evolutionary cousins to modern carnivorans that convergently developed similar shearing carnassial teeth in their jaws. Hyainailourids originated in Africa during the late Paleocene or early Eocene, and repeatedly dispersed into Eurasia and North America before eventually going extinct in the mid-Miocene.
Kerberos was one of the earliest of its kind known from Europe, living in Southern France during the mid-Eocene about 41-38 million years ago. It was close in size to a small American black bear, standing around 65cm tall at the shoulder (2’2″), not nearly as large as some of its later relatives but still making it one of the biggest carnivorous mammals in Europe at the time.
It was a heavily-built animal with a fully plantigrade posture, and would have been an active apex predator hunting similarly-sized early ungulates. While it wasn’t anatomically specialized for fast running it didn’t really need to be – it’s important to remember that modern bears have a similar chunky flat-footed build and yet can move surprisingly quickly.
Its incredibly powerful jaw muscles and premolar teeth adapted for bone-cracking also suggest it ate like a hyena, efficiently consuming entire carcasses.
Once again it’s a PBS Eons commission roundup day!
The docodonts were a group of mammaliaformes (close relatives of the earliest true mammals) which lived across North America, Europe, and Asia from the Middle Jurassic to the Early Cretaceous. Originally only known from teeth and jaw fragments they were traditionally thought to be fairly generic shrew-like insectivores, but more recent discoveries of better fossils have revealed they were actually much more diverse, occupying ecological niches ranging from squirrel-like tree-climbers to mole-like diggers to beaver-otter-like swimmers.
Most of the more complete fossil material of these animals comes from the mid-Jurassic of China, but one species from elsewhere is also known from a partial skeleton.
Haldanodon exspectatus here lived in central Portugal during the Late Jurassic, about 155 million years ago. Around 15-20cm long (6-8″), it had small eyes and short chunky well-muscled limbs with the front paws adapted for digging. Since it inhabited a very swampy environment it probably wasn’t a pure mole-like burrower – extensive tunnels would have constantly flooded – but it may have instead been a similar sort of semi-aquatic animal to modern platypuses and desmans, foraging for invertebrates in the water and excavating burrows in the banks.
Roughened areas of bone on its snout may also have supported a patch of tough keratinous skin, which would have helped protect its face while digging.
During the mid-Pleistocene, between about 900,000 and 500,000 years ago, the Mediterranean islands of Malta and Sicily were connected and shared a unique ecosystem made of up a mix of weird endemic species. While the tiny elephants and giant swans are probably the most famous, there were also several other unusual animals such as dwarf hippos, huge owls, large cranes, giant tortoises, and big lizards.
And also massive rodents.
Leithia melitensis, the Maltese giant dormouse, was descended from garden dormice, but thanks to the lack of large land predators on Siculo-Malta it was able to evolve a much much larger body size – about 60-70cm long (2′-2’4″), almost the size of a cat.
Recent reconstructions of its skull have shown it was also proportioned differently compared to its tiny modern relatives, more chunkily built with a shorter and wider snout, bigger teeth, and thicker cheekbones that must have anchored some incredibly powerful muscles for chewing. It may have been eating a much more herbivorous diet than other dormice, processing a lot of tough fibrous vegetation.
It’s another 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.