Atsugornithes, a once paraphyletic clade, as they descend directly from populations of Old World Dracornithides, but are now united under the previous clade. Atsugornithides are usually called the "New World Dracornithides", following a radiation in the Paleocene that saw them colonising the Western Continent of Achigo, which at the time, was populated by basal Xenotherians, Metatherians, Multituberculates, and Notosuchians, and quickly established a foothold on the available niches made available by the extinction event.
Gigornithimorpha, a large clade that encompasses Achigo's unique "ratites", the "Achigoan Titan Birds", also sometimes called "Achigoan Ratites" despite not being Palaeognaths. They represent the oldest lineage of Atsugornithes.
Fuwatiidae, includes the monotypic genus Fuwatia fogualense, living exclusively on the island of Fogual. These small herbivorous birds evolved from flying ancestors, but secondarily lost the ability to fly. They are low browsers that forage on the forest floor and on low woody plants, using their wing claws as means for predator defense.
Gigornithiformes, large and truly flightless. These birds are largely adapted for herbivory, but still retain large arms with sharp claws for predator defense.
Smilonychidae, named for the genus "Smilonyx", meaning "Knife Claw". Smilonychines are less derived than their cousins, having sloping postures and less flexible, slightly shorter necks and long legs. They are omnivores that like to eat woody plants, fruit, crustaceans (inc. insects), fish, and opportunistically feed on smaller animals and carrion. They are essentially the bears of the Gigornithiformes. Some have also developed rudimentary pygostyles, although these are only for carrying ornate and heavy display feathers and not flight.
Gigornithoidea, group including most of the Gigornithids. Defining traits of this clade are a horizontally-held posture due to hip alignment favoring that position, an adaptation for running, the development of the arctometatarsus (fused metatarsal bones), stiffer and longer tails, and more flexible necks with unique dentition.
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Pandromaves, including most of Achigo's native bird diversity. The Pandromaves, in general, inhabit the niches of Phasianids, Anserans, Passerines, and some others.
Dromaves, the "Running Birds". These are typically small Dracornithides that are convergent with Phasianids. They are capable of flight, but it's not exactly what they are known for.
Nakraves, including most of Achigo's "Passerine" mimics. Their diversity is somewhat limited because of existing Passerines that inhabit the continent. The word "Nakra" comes from a specific native Achigoan word roughly meaning "Sharp Bite".
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Atsugoraptoria, the "New World Groundhawks". Once again an example of convergent evolution within the Dracornithides, as these Groundhawks are only very distantly related to the Groundhawks in Deraniila, having evolved this body plan independently. They only have very minor differences in anatomy, namely having re-evolved the saurischian hip position. While most Dracornithides have a backwards-facing pubis, Atsugoraptorians have a forward-facing one, like their non-avian ancestors. This is largely an evolutionary leftover from a time when the Atsugoraptorians were the main dominant predatory clade alongside Notosuchians, leading to a heavier focus on size due to their dominant positions and room for larger muscle attachments. This lasted until the Miocene, which wiped out most of the large Atsugoraptorians and wiped out Achigo's native Notosuchian predators, leading to a faunal turnover that saw the dominant carnivorous Xenotherians rise to power. This did not diminish their diversity, but rather meant they couldn't fill the large predatory niches they once did.
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