mammal – Nix Illustration

Arsinoitherium

Arsinoitherium zitteli was a large herbivorous mammal living in what is now northern Africa during the late Eocene and early Oligocene, about 36-30 million years ago.

Despite looking like a double-horned rhino this resemblance was only superficial, and for most of the 20^th century it was actually the only known representative of an entire order of mammals – the embrithopods – with its wider evolutionary relationships being unknown. Since the 1970s, however, more members of this group have been discovered and embrithopods are now understood to be afrotheres, a very early offshoot of the tethythere lineage, with their closest living relatives being modern elephants and sirenians.

Arsinoitherium was by far the most abundant embrithopod, with numerous fossil remains making it one of the most completely known African fossil mammals. It stood around 1.8m tall at the shoulder (6′), similar in size to modern white rhinos, and would have been a massively-built slow-moving animal with elephant-like columnar limbs.

Its pair of enormous nose horns (and smaller brow horns) were structurally more similar to those of bovids than rhinos, with large hollow bony cores that probably bore thick keratinous sheaths that would have increased their apparent size even more. Both males and females appear to have had these horns, and muscle attachments at the back of the skull suggest Arsinoitherium could powerfully swing its head upwards – possibly wrestling with each other in combat over territories, competing for mates, or in establishing dominance hierarchies.

Flandriacetus

Flandriacetus gijseni was an early beaked whale that lived during the late Miocene, about 8 million years ago, in nearshore marine waters covering what is now the Netherlands.

Around 4m long (~13′), it had a long snout lined with small sharp teeth – unlike modern beaked whales which are mostly toothless – and much like its close relative Messapicetus it probably led a more dolphin-like lifestyle feeding on small fish near the surface.

It’s currently the youngest known example of a long-snouted stem beaked whale, a holdover from a time when these cetaceans were much more ecologically diverse than they are today.

Postschizotherium

Postschizotherium intermedium was a large hyrax that lived during the late Pliocene and early Pleistocene, about 2.5-2.2 million years ago, in what is now northern China.

About 1.5m long (~5′), it had very high-set and sideways projecting eye sockets similar to those of modern hippos, indicating it probably had a similar sort of semi-aquatic lifestyle. The shape of its skull also suggests it may have had a short tapir-like trunk.

Much like modern hippos or capybaras Postschizotherium probably spent much of its time wallowing in bodies of water, and emerging onto land to graze on grasses. Its habitat would have been humid forest and grasslands, alongside other animals such as large horses and bovines, one of the last chalicotheres, woolly rhinos, beavers, macaques, bears, big cats, early lynxes, and scimitar-toothed cats.

Megabalaena

Megabalaena sapporoensis was a member of the balaenid baleen whale lineage, related to modern right whales and bowhead whales. Living in marine waters covering what is now northern Japan during the late Miocene, about 9 million years ago, it helps to fill in a significant gap in the fossil record of this group.

Known from a partial skeleton about 12.7m long (~42′), it was much larger than earlier balaenids, but smaller than modern forms. It also had a narrower flipper shape compared to its modern relatives, a less arched jaw, and its neck vertebrae were only partially fused.

Modern right whales are slow-swimming ram feeders, but since Megabalaena was less specialized for this particular filter feeding style it’s unclear what its ecology was.

Spectember/Spectober 2025 #05: Strangels

We’re continuing into Spectober with another anonymous request, this one asking for: “Hominids that evolved with 6 limbs, 2 of which are wings. Limb placement is up to you.”

A digital sketch of a speculative six-limbed hominind, pictured in both tree-climbing and gliding poses. It has a monkey-like face with very large eyes, opposable thumbs and first toes, no tail, short fur over its body – and an extra pair of "arms" hanging just behind its shoulders that end in stubby fingerless hands and support a large gliding membrane. In the aerial pose it's shown grabbing hold of its wing-limbs to hold them in place for gliding.

Living several million years in a possible future, Anomalangelus anthropogenis is a bizarre case of a six-limbed primate.

Its additional pair of upper limbs originate from a heritable form of notomelic polymelia, similar to the Developmental Duplications condition seen in our time’s domestic cattle. It has a rather chimeric genome that appears to contain ancestry from several different hominid lineages, suggesting that this strange little creature actually descends from something that was originally genetically engineered.

It’s a tiny dwarfed species, only about 25cm long (~10″), with a highly arboreal omnivorous loris-like lifestyle, spending most of its life clambering around in trees. Although its extra limbs can’t move independently, lacking a lot of functional musculature and nerve connections, they’ve been exapted into brightly-patterned display structures and also serve as attachment for a large gliding membrane – Anomalangelus uses its forelimbs to grab hold of these “wings” while airborne, spreading them out and stabilizing them to form a composite wing.

Spectember 2025 #04: Kerguelen Kingdom

A couple of anonymous submissions asked for “Kerguelen fauna before it sank 20 million years ago” and “a predator which prowled Cenozoic Kerguelen before it sank”:

[Context: The Kerguelen Plateau is today almost entirely underwater, but during the Cretaceous Period much larger parts were above water as island landmasses. Initially forming during the Gondwanan breakup of what would become Australia, India, and Antarctica, it was eventually left isolated in the forming southern Indian Ocean, and due to long-term volcanic hotspot activity it may have gone through as many as three different periods of rising and sinking before finally almost completely submerging about 20 million years ago.]

In the early Cenozoic, around the time of the Paleocene-Eocene Thermal Maximum, there’s less of Kerguelen’s land above water than there was in the Cretaceous, but the climate is warm-temperate and conifer forests cover the plateau’s islands.

A digital sketch of a speculative descendant of enantiornitheans. It's a large chunky shaggy-feathered flightless bird with a small head, toothed jaws instead of a beak, a long neck, vestigial wings with small claws, thick legs, and a pair of ribbon-like tail feathers that end in wider spaded plumes.

Enantiochen reliquia is a flightless bird standing around 1.5m tall (~5′), and its toothy jaws, wing claws, and ornamental ribbon-like tail plumes on males identify this species as an enantiornithean, descended from a small flighted form that just barely survived through the K-Pg extinction here.

It’s the largest current inhabitant of the Kerguelen plateau, a browsing herbivore filling a similar ecological role to the later moa of Aotearoa.

A digital sketch of a speculative descendant of gondwanatherian mammals. It vaguely resembles a rabbit with small pointed ears, with a deep snout, long digging claws on its digits, a short thin tail, and long hind feet.

Cuniculitherium kerguelensis is another Mesozoic holdover, a gondwanatherian mammal — although a little less unique than Enantiochen since other gondwanatheres still also survive in early Cenozoic South America and Antarctica.

About 40cm long (~1’4″), it’s a rather rabbit-like burrowing herbivore with long hind feet and a fast bounding gait, traits its lineage originally evolved to evade unenlagiine theropods and small terrestrial crocodylomorphs prior to the end-Cretaceous extinction. Those predators are gone from Kerguelen now, but…

A digital sketch of a speculative descendant of australobatrachian frogs. It has a large head, short chunky limbs, and an elongated body with a row of bony amor plates along its spine.

Daptobatrachus archaeotropus is a descendant of australobatrachian frogs. In a case of island gigantism it’s close to the size of a cat, around 50cm long (~1’8″), and with its elongated body, stubby hind legs, and a row of osteoderms down its back, it almost looks like a throwback to the Permian.

Although unable to hop, it’s an ambush predator capable of raising itself up for very brief bursts of crocodile-like galloping, preying on pretty much anything it can potentially fit into its mouth.

Spectember 2025 #01: The Creeping Whale

Another September, another #Spectember, and maybe, just maybe, one day I’ll finally finish getting through the speculative evolution concept submissions you all gave me several years ago.

(Also, a reminder: I’m not taking further requests!)

As with the previous couple of years I’m not setting a definite posting schedule; it’ll just be whatever I can manage to get done during the month.

So, let’s get started with an anonymous submission that requested a “secondarily terrestrial cetacean similar to Cartorhynchus and Sclerocormus”:

A digital sketch of a speculative secondarily-amphibious early whale. It has a short blunt snout with nostrils in front of its large eyes, a humped back, large flippers, a vestigial-looking dorsal fin, and a fluked tail.

Repocetus aigialonatus is a 2.5m long (~8′) Late Oligocene cetacean closely related to mammalodontids — early baleen whales with toothy jaws — living around the mostly-submerged continent of Te Riu-a-Māui Zealandia.

Its ancestors hunted in shallow waters around the low-lying islands, occasionally semi-beaching themselves in pursuit of penguins or to escape from larger marine predators. This eventually led to Repocetus regaining some degree of terrestrial locomotion ability, able to galumph somewhat like modern seals using a combination of undulating its body and pushing off using flippers with powerful shoulder muscles.

It’s slow and awkward, but there are no terrestrial predators to threaten it — and so it’s also reverted to giving birth on the safety of the shore.

Like its mammalodontid relatives it has large eyes and a fairly short snout. It occupies a similar ecological niche to the modern leopard seal, using large sharp teeth to grip and tear at large prey. While it mainly feeds on large fish, it will also use its amphibious abilities to charge onto shore to raid beach-nesting bird colonies or to take advantage of other beached cetaceans.

Notiomastodon

Notiomastodon platensis was a gomphothere – a relative of modern elephants – that lived across much of what is now South America from the mid-Pleistocene to the early Holocene, between about 800,000 and 10,000 years ago.

Similar in size to an Asian elephant, it stood around 2.5m tall at the shoulder (~8’2″) with a domed head and thick tusks that varied in length and curvature between different individuals. It had a stockier build than modern elephants with thicker and slightly shorter limbs, and fossilized footprints suggest it had five nails on its front feet and at least three on the hind feet.

Isotope analysis and wear analysis of Notiomastodon’s teeth suggest it was a generalist browsing herbivore, with different populations adapting their dietary habits to local conditions. As one of the largest South American herbivores of its time it was probably an important seed disperser for plants such as bamboo and palms – and some of the plants that once depended on it may now be “evolutionary anachronisms“.

Protemnodon

Protemnodon viator was a large macropod that lived in what is now western and southern Australia during the late Pleistocene, around 50,000 years ago.

Although it was built more like a giant wallaby, ancient mitochondrial DNA has shown that its closest living relatives are actually modern grey kangaroos.

Estimated to have weighed about 170kg (~375lbs) – twice as much as the largest modern red kangaroos – it would have stood up to 2.4m tall (~8′) on its hind legs. But unlike its living relatives Protemnodon’s limb proportions indicate it wasn’t a very efficient bipedal hopper, instead probably mostly moving with a bounding or galloping quadrupedal gait.

Its forelimb anatomy also suggests it was a good digger, and strongly curved claws on its hind feet may have helped provide grip on uneven ground.

Panochthus

Panochthus tuberculatus was a large glyptodont – a group of giant heavily-armored armadillos – that lived in central and southern South America during the late Pleistocene, about 800,000-12,000 years ago.

Around 3.5m long (~11.5′) and 1.5m tall (~5′), it was similar in size to a modern rhino (or a small car), and its large domed “shell” made up of numerous small bony osteoderms made it resemble a mammalian tortoise. Its skull was short and deep, with ever-growing grinding teeth and downwards-flaring cheekbones that anchored powerful jaw muscles. A preserved hyoid apparatus indicates that Panochthus also had a more flexible tongue than some other glyptodonts.

The base of its tail was segmented into rings that allowed it to flex, while the end of the tail was fused into a solid bony tube that was probably studded with large keratinous knobs or spikes.

While these sort of tail weapons in glyptodonts have been proposed as being anti-predator defenses, biomechanical studies suggest they required precise aiming to be most effective and weren’t well-suited to fending off fast-moving attackers. Instead they may have been more specialized for fighting each other in ritualized forms of combat – an idea supported by injuries in fossil carapaces that appear to have been caused by blows from opponents’ tail clubs.