The Selandian

a continuation of the surviving basal paravians

The Isthmus

Location: The Weddellian Isthmus
When: Paleocene, Selandian, 61.2 Million years ago

Nearly five million years have passed since the K-Pg extinction and while no animals come close to rivaling the giants of the Maastrichtian, a race to grow in size to fill less heavily exploited niches has been going on since the moment the world stopped being in climate chaos. Rustling through the brush of a noticeably less overgrown forest than what would have been seen just two or three million years earlier, a herd of the largest mammals in Gondwana grazes. The size of a black bear, with a long fat tail and a body and head reminiscent of a Caviomorph rodent, Antarctotherium (Antarctic Beast), is the largest Gondwanathere to have lived up until this point, and of Allotheria as a whole, only the Taeniolabis would rival it in size. In the hyper-competitive world of small herbivores and facing the threat of increasingly raptorial dracoavians, some of the Gondwanatheres responded by dramatically growing larger to better exploit under-utilized resources as well as outgrow their predators.

Antarctotherium, shows that while the Dracoavians are diversifying, mammals are not simply going back to being small shrew and rodent-like animals

Generalist mixed low browsers and grazers the herd has developed a relationship with the high browsing Eonychus for their mutual protection. The awkward-looking dracoavian stands at nearly 2.5 meters tall and this particular large male stands out even more thanks to its bright crimson wing and tail feather. Though the large male Eonychus (Dawn Claw) towers over the Antarctotheres, it does not weigh significantly more than them, and the combination of its keen eyesight with the herd's protective behavior present a great deal to overcome for any would-be predators… and there are predators in these woods. While the large mammals and bird brows, a pair of predators watch with keen eyes from the distant shadows.

Eonychus named for its large powerful claws, is speculated to have used them to help pull down branches.

Trying to maintain their concealed position, two falcon-like predators stalk the herd. Against even a single adult Antarctothere, the pair of 1.9 meter long and 23 kg predators would be hard-pressed to bring down the large mammal without suffering some debilitating kind of injury and to attack the adult Eonychus would be courting death. No, these two want to exploit the poor color vision of their mammalian prey to bring down one of the juvenile Antarctotheres. However, the Eonychus’ ancestors are the same as the small predators and it does not feel the influence of the Nocturnal Bottleneck that the mammalian herbivores do.[1] Silently, the two predators wait and watch as the herd grows comfortable and thins out following one infant in particular as it strays from the herd. The two predators are a mated pair of Sateopteryx (Sate’s wing). They are the largest terrestrial predator on the isthmus as it is generally too cool for the more robust Sebecids of South America to be comfortable venturing this far south. They do not betray themselves with the soft cooing or shrill shrieks that they might release when at their nest or when defending their territory from a wandering male or female, instead, they silently trade glances and head bobs as they separate to approach the lone juvenile from different angles.

The mated pair eyeing their prey from the shadows

As the pair close in, the male lets out a low whistle sound that causes the ears of the rest of the herd to perk up in alertness as their loose formation begins to tighten rapidly. The throng of moving mammals creates a blanket of visual noise distracting the Eonychus as the female Sateopteryx rushes the juvenile, using her wings to lift her body from the earth and descend upon the young antarctothere with her sickle claws, attempting to restrain the little mammal while berating it with her wings. She is quickly joined by her mate who attempts to aid in the restraint of the juvenile, biting at its muzzle and placing a heavy clawed foot against its neck.

Even with the throng of moving mammals, the Eonychus spots the flapping arms and hears the shrieks as the pair begin to eat their victim alive. The large dracoavian rushes forward its arms outstretched hissing as it drives the predators off. Sending them awkwardly flying away as they use their wings to help clamber into the trees above. Letting out a few territorial hisses and stomping its feet, the Eonychus prances around the mortally wounded juvenile. It is not emotionally invested in the mammal but the strength of the herd acts as a protective wall for its own eggs and chicks when a female eventually comes to its territory looking for a mate. A big male with a large herd is a male that can protect its chicks and dominate a territory large enough to support that herd. The attacks by the Sateopteryx are a threat to the security of the herd and of the male’s sexual appeal, so he will strut and cry around the dying mammal for a time before he eventually withdraws back to the herd and the little predators are allowed to descend from the trees to consume their prey.

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The Diversification of Kairosdracoaves

The rulers of Antarctica
By the Selandian stage of the Paleocene, the clade Kairosdracoaves (Second Chance Dragon Birds) had already begun to radiate into the ancestors of several of the clades that have persisted until the modern day. The basalmost members of Thanatoaves like the Danian Polarvenator, had already emerged in Antarctica and begun to spread through the Weddellian Isthmus into South America and the Tasmanian Gateway into Australia. These small predators were likely still volant, but had already transitioned to becoming largely terrestrial predators, and were part of a grade leading into Culebreformes. This group appears to have either arisen in Patagonia or at least Western Antarctica and been unable to spread back across the continent to Eastern Antarctica and Australia which is why the more basal Thanatoavians like Thanatogallus (Death Chicken) will be the dominant group of thanatoavians in Oceania while South America will be dominated by the Culebreformes until the Middle Miocene. These small predators like Thanatogallus would form the clade Citipodia which gets its name from the fact that these predators retained the longer second phalanx of the second digit and slender subarctometatarsus which allowed them to be faster and more cursorial than the more robustly built Culebreformes like Sateopteryx a trait that will help them thrive when the world begins to get cooler and grasslands begin to spread.

Thanatogallus, a basal Citipodian
Sateopteryx itself is the basalmost known Culebreform. Though only slightly heavier than Polarvenator, its more robust subarctometatarsus would allow it to restrain larger prey, trading speed for power. Unlike some of its descendants, Sateopteryx was not a pack predator however it was a (largely) monogamous species that lived in mated pairs with slightly more robust females than males. This sort of sexual dimorphism will become far more pronounced during the age of Oligocene giants, but for now, the difference is only measured in a kilo or two.

Eophoenix and Eornithodromeus
While the carnivorous Dracoavians were still fairly small in size, the Serpopardiidae had already begun to reach impressive sizes. Eonychus the largest of these early giants had already lost its sickle toe claw and had its fourth toe grow to rest upon the ground to help support its increased weight. While it's very likely that juvenile members of the species would have still eaten insects and small animals, the heavy high browser’s teeth were reduced in size and peg-like in shape to help it process plant matter. In contrast, the less heavily built Eophoenix retained the more gracile build of its ancestors as well as the slender subarctometatarsus. This was not because it was specializing in small speedy prey (though it’s likely that it was an omnivore), but because it preferred speed as a method of predator evasion to size-based intimidation. Alongside these large dracoavians, lived Eornithodromeus a small Dromaeoavian, like Eophoenix, that subsisted primarily on seeds and tough vegetation and had taken the tunnel-dwelling lifestyle of its ancestor Antarctolestes further, by digging its own burrows rather than co-opting existing mammal burrows. Both families of Serpopardiidae (Serpopardinae which Eonychus represents and Dromaeoaves which Eophoenix represents) are South American fauna and not represented in Australia, indicating that like the more robust carnivores, these larger herbivores evolved in South America or the Weddellian Isthmus and did not manage to cross back to Eastern Antarctica and Australia.

Eoanatomimus, the dawn goose mimic
In place of the Serpopardids, the large herbivorous dracoavians of Australia would primarily be descended from animals like the Antarctic Eoanatomimus (Dawn goose mimic) of the Anatomimians. In some ways, Eoanatomimus appears like a slightly more specialized Antarctolestes taking on an even more goose-like appearance as it left behind its terrestrial foraging and turned more towards feeding exclusively on water plants and insects. Prior to the discovery of Antarctolestes, when a Campanian-age Paleodracoavian from Mongolia was discovered, with remarkable similarities to animals like Eoanatomimus it was speculated that Anatomimians represent the basalmost group of Kairosdracoavians. While this may still be true the Mongolian animal represents a convergence upon the goose-like body plan rather than a continuation of it. Alongside the more herbivorous Eoanatomimus, there was an indeterminate Anatomimian discovered in the same formation which has denser bones, likely to allow it to better dive below the surface of the water and engage in subaqueous foraging and predation of small crustaceans.

Eoichthyovenator
The Anatomimians were not the only semi-aquatic dracoavians of Middle Paleocene Antarctica. The quintessentially Antarctic family Antipodidae has its origins in this period with the very cormorant-like Eoichthyovenator (Dawn Fish hunter). This species was still volant and had a fairly long tail, two traits that would be lost by the time of the modern polar Antipodids. A coastal piscivore, Eoichthyovenator, and its kind largely avoided competition with more volant sea birds by adopting increasingly aquatic lifestyles, developing denser bones, and trading their limited ability to fly for increased maneuverability in the sea.

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A Continent with its own Rulers

While mammalian carnivores like Ankalagon were quick to emerge in the Northern Hemisphere after the K-Pg event and there were already a number of megafaunal mammalian herbivores, an outside observer with no knowledge of what the future would hold might have described the Paleocene as the dawn of the second age of reptiles. By the Selandian, a squamate was the single largest carnivore on the planet, one of the largest herbivores was the turtle Carbonemys, and in North America, South America, Europe, and parts of Asia terrestrial crocodilians occupied the roles of top-order carnivore. When the early Culebreformes crossed the Weddellian Isthmus into South America, they did not cross into an environment that was absent a large terrestrial carnivore. Though comparable in length and height to the largest of the South American Sebecids, animals like Sateopteryx weighed dramatically less than the native Notosuchian predators. While their superior lung systems and endothermy gave them better endurance and speed than the heavier crocs, this did not matter in the dense deltaic forests of the Austral Basin where the Sebecids were perfectly adapted to ambush hunting. Even in Patagonia's more open fern prairies, it mattered very little when a more robust Sebecid could trot up to a Sateopteryx kill and simply bully it away from its prey and scarf it down.

As larger mammal and Dracoavian herbivores spread deeper into South America during the Paleocene, both Culebreformes and Sebecids would increase in size to bring down larger prey and compete with each other. However, kleptoparasitism by the larger and more robust crocs would continue to define the relationship between these two competing groups of archosaur predators for some time to come.

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[1]: I am aware that there is still not enough information on the origins of trichromatic vision in some Marsupials to be able to rule out the possibility that non-therian mammal groups like Allotheria had color vision, but I am operating under the assumption that the ancestors of Antarctotherium were nocturnal and that it still has dichromatic vision.