#spectember 2022

Despite having a convergent resemblance to penguins or gannetwhales, the humdertaker (Suchomergus pollinctor) is actually a distant descendant of modern hummingbirds.

Its ancestor was a hummingbird species similar to the tooth-billed hummingbird, with tooth-like pointed serrations along the edges of its beak – initially developed for fighting, but later co-opted for better catching insects. Some of these hummingbirds shifted their diet fully away from nectar and became aerial hunters more like their relatives the swifts, and one lineage that commonly fed on insects at the water's surface began also snatching up aquatic prey like tadpoles and small fish.

These birds became larger kingfisher-like divers with waterproof feathers, and eventually some became increasingly aquatic, spending most their time swimming at the surface of lakes and rivers like mergansers or loons. These forms rapidly became flightless, specializing for penguin-like wing-based swimming, but due to their front-heavy build and ancestrally small legs also became completely unable to stand or walk on land, instead restricted to an ungainly motion somewhere between a tobogganing penguin and stiff-spined galumphing seal.

Growing up to about 2m long (6'6"), the humdertaker is a semi-aquatic ambush predator occupying a similar ecological role to small crocodilians. With a heavily serrated beak able to hold onto slippery prey, it feeds mainly on fish, amphibians, and crustaceans, but will also opportunistically snatch other animals from the water's edge and scavenge on carrion – and like a giant aquatic version of a shrikes it often caches "larders" of surplus food on snags in the water.

Although it has a much slower metabolism than its nectar-fueled flying ancestors, it has retained some of their other biological quirks. Tolerance for very low oxygen and the ability to enter a torpor-like resting state allow it to hold its breath underwater for much longer than many other air-breathing vertebrates, and to better conserve its energy during periods of food scarcity.

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One offshoot lineage of small humdertakers began using their beaks and wings to excavate burrows into the riverbanks, and gradually specialized more and more for digging rather than swimming.

The humderminer (Iposkapteryx werpachowskii) is an especially mole-like form, a fully subterranean tunneler with reduced eyes and broad shovel-like 'flippers'. Only about 10cm long (4"), it primarily hunts worms and other small underground animals, and has retained somewhat iridescent plumage due to the structural benefits when moving through soil.

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The humdingers are another offshoot from early humdertakers, specializing into even more aquatic lifestyles and eventually moving out from freshwater into the ocean.

Voragornis novaraptoriae here is one of the largest humdingers, about 5m long (16'4"). It's a fully wing-propelled swimmer with a highly streamlined body, and its still-relatively-small legs are adapted into rudders and stabilizers similar to the hind flippers of turtles.

It's an especially accomplished diver, able to reach depths of close to 2000m (~6600 ft), preying on soft-bodied deepwater cephalopods and fish. Lacking echolocation it mainly uses sight to hunt in the deep dark waters, looking for glimpses of bioluminescent prey.

Although humdingers are the equivalent of "whale-sized" compared to their much tinier hummingbird ancestors, they're currently limited from growing much larger due to their reproductive requirements – these almost-fully-aquatic birds do still need to be able to haul themselves out onto land once a year to molt and breed.

Much like penguins humdingers undergo a seasonal "catastrophic molt", replacing all their insulating feathers at once and being stuck on shore unable to swim for a few weeks at a time. This molting coincides with their nesting season, when females use their flippers to dig out a turtle-like nest on the shore – burying the clutches to incubate since their bulky bodies are too heavy to safely sit on them. They'll guard the nests until they've completed molting, but then abandon them, leaving the young to hatch superprecocial and fully independent a few more weeks later.

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This one isn't based on a specific prompt, but instead is a companion piece to a previous one.

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While North American brontotheres were adapting to the spread of grasslands, some of their Asian cousins took a very different evolutionary path through the rest of the Cenozoic.

Some Asian brontotheres became aquatic hippo-like animals, with thick dense bones and flipper-like feet, moving into shallow coastal marine habitats and grazing on seagrasses. They developed a convergently similar lifestyle to desmostylians, fairly slow swimmers that "bottom-walked" along the seafloor to feed – but they weren't in direct competition due to occupying more southerly waters in the west Pacific than the desmostylians, ranging from Southeast Asia down to Oceania.

Like other brontotheres they developed some strange nasal bones (similar to those of Embolotherium) forming a raised snorkel-like nose that allowed them to breathe without fully surfacing.

Many of these aquatic turret-nosed brontotheres eventually declined and went extinct during the Miocene and Pliocene, when cooling climates and dropping sea levels removed many of the shallow seagrass meadows they relied on, and dugongs began competing with them for the same limited food resources.

But one lineage hung on down in Australia.

The false bunyip (Scanduladorsum pumilum) is a diminutive aquatic brontothere, about the size of a pygmy hippo at 75cm tall (2'6"). It has reverted to a slightly more amphibious lifestyle compared to its almost fully-aquatic extinct relatives, and unusually for its kind it's also primarily a freshwater species, living in lakes, rivers, and marshes – but migrating down to congregate in estuaries and shallow marine waters during the breeding season.

It spends daytime submerged underwater with just its "snorkel" nose poking above the surface, grazing on aquatic plants, then emerges onto land during the evening to supplement its diet with soft terrestrial leaves and fruits, moving with a slow sealion-like gait. Generally solitary, but not territorial, it communicates over long distances with other individuals of its kind using surprisingly loud booming calls, amplified by its large nasal cavity.

It also carries a commensal barnacle (Chelonibia brontes), descended from turtle barnacles. This species has evolved a unique tolerance for freshwater, allowing it to stay attached and alive on its host between trips back to more marine conditions.

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These two animals are the descendants of brontotheres and paraceratheres, almost the last living representatives of their kinds, hanging on in the equivalent of modern-day times in a world similar to our own.

But in this timeline some of the North American brontotheres managed to cope with the cooling drying climate of the Oligocene, adapting their teeth and their diet to grazing the spreading scrublands and grasslands. They became smaller and somewhat less bulky, built more like tapirs or wild boars than their previous rhino-like forms, and although they were now able to move faster in the more open environments their main defense was actually their highly social herding behavior – relying on strength in numbers and collectively mobbing predators.

Males also developed increasingly elaborate head ornamentation, like mammalian versions of ceratopsids, with ossicone-like projections on the back of their skulls as well as on their noses.

But they never regained their kind's previous success or diversity, instead remaining as a fairly minor element of the North American plains fauna. They began to decline against the rise of more specialized ungulates and smarter pack-hunting carnivorans, but one lineage did manage to disperse into South America during the Great American Biotic Interchange before the remaining North American brontotheres died out entirely.

Now the crowned brontothere (Brontochlos coronatus) is one of the last remnants of the whole group*, living in the Patagonian Steppe of southern South America. A relict species from the last ice age, under its thick coat of hair it's only about the size of a sheep – around 1m tall (3'3").

(*There is one other still-extant lineage of brontotheres in this world, but that's a subject for another time.)

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Meanwhile the Asian paraceratheres survived and went through a similar shift towards tougher diets and smaller body size. No longer the titans they once were, they never again exceeded the size of the largest horses and instead converged on camelids and macraucheniids.

They reached North America via Beringia in the late Miocene, but struggled to establish themselves alongside the native camelids at the time – and the later camel dispersal into Asia would eventually push the remaining paraceratheres there towards extinction, too. But, much like the brontotheres, a few paraceratheres did make it all the way down into South America and persisted for longer, taking advantage of the ecological vacancies left by the already-dwindling native ungulates.

The woolly paracerathere (Ebursonora glacialis) is one of the last paraceratheres, sharing the same Patagonian Steppe habitat as the crowned brontotheres. Just under 2m tall (6'6"), it has a highly prehensile upper lip and a large bulbous nasal cavity that acts as both a saiga-like air filter and as a resonating chamber.

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Despite sometimes competing with each other for the same food resources, these two species have developed something of a mutualistic relationship with each other. The social crowned brontotheres will often incorporate one or two woolly paraceratheres into their herds – often young "solitary" males of the species – due to the paraceratheres' larger size and better ability to spot threats at a distance. In exchange for acting as an early warning system, the paraceratheres gain the protection of the whole brontothere mob, increasing their own survival odds in a harsh environment still stalked by sabertoothed cats and the occasional terror bird.

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(…There's still over 50 of them left. This is going to take a while.)

So today's concept comes from an anonymous submitter, who requested an arboreal ornithopod dinosaur:

Anevasmasaura polaris is a small tree-climbing ornithopod that inhabits the temperate southern polar forests of the Early Cretaceous, descended from a Leaellynasaura-like ancestor. About 1m long (~3'3"), with over half that being just its long tail, it has a lifestyle similar to modern squirrels and lemurs – spending most of its time leaping around high up in the branches and only rarely venturing down to ground level.

It has zygodactyl feet, with the first and fourth toes reversed, and sharp curved claws that help it climb and grasp onto vertical tree trunks. Its face is short-snouted and superficially owl-like, with large forward-facing eyes that give it both depth perception and good night vision during the long dark polar winters.

Its diet is primarily herbivorous, using its short pointed beak to feed on the fruits and seeds of plants like cycads, ginkgoes, and conifiers, but it will also opportunistically eat invertebrates, eggs, and smaller vertebrates.

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