When the ancestors of Steppesteppers first entered the larger Panapterran ecosystem, their unique anatomy afforded them greater speed than any other animal. They could outrun terror kiwis, which previously had been the fastest terrestrial animals on the planet. This allowed their herds to dominate the grasslands, as they were nearly untouchable unless already injured or caught off guard. But nature abhors a vacuum, and if no other bird or rat could fill the niche of steppestepper-hunter, then it would have to be filled from within that lineage itself. The straightbilled steppestepper of four million years ago already ate some amount of small animals to supplement its grass diet, and its descendants have become more and more finely adapted for carnivory, with the largest specializing entirely on eating its own cousins. Meanwhile, more generalistic Carnivorn (Carnivornis) species have charted a different path.
The Slinkstepper (C. minimus) is the smallest member of its genus. From an ancestral steppestepper weighing over 60 kilograms, it has miniaturized by a factor of ten and now stands only about 40 centimeters high. This trend is ongoing; in another few million years, most of its progeny will be smaller still. The decrease in body size has come about due to the abundance of small prey on the basket-grassland. Slinksteppers, with their fast, agile legs and narror, sharp-tipped beaks, can feed on many different species, ranging from large arthropods to other rodents and kiwis up to half its own size. It can even jump quickly and precisely enough to catch large flying insects that it flushes from dense foliage. It is not an obligate carnivore (though some populations seems to be heading in that direction), eating nutritious plant matter of all kinds. It no longer possesses a cropping bill for eating large volumes of grass or a stomach capable of digesting it, but a well-placed peck can break open hard seeds, roots, and stems just as easily as it can dispatch giant castlebugs.
The Slashstepper (C. maximus), on the other hand, is a fully dedicated meat-eater, with less than 5% of its diet coming from plants. Its overall body size is similar to the ancestral steppestepper, though its proportions have changed in accordance with its new carnivorous lifestyle; its head and neck are more robust, its torso is slimmer, and its tail is longer and stiffer than earlier species. Unlike other members of the straightbilled steppestepper clade, it no longer kills by stabbing with a sharp, pointed beak. Converging with the largest terror kiwis and the Phorusrhachids of Earth, it has instead developed a strongly hooked upper bill to tear flesh from bone. The rear part of the beak also has a curved cutting surface for tearing into soft flesh from the side, a trait useful for inflicting long, shallow gashes while both predator and prey are running at full speed.
Slashsteppers have another feature that sets them apart from other Polyovapterygids. Because their long, inflexible tails make mating difficult, males have evolved their previously vestigial forelimbs into simple claspers. These claws are long but blunt, designed to harmlessly cling to the top of the female while her partner maneuvers the back of his body underneath hers. While these are among the most functional wings any Apterran bird has yet evolved, they can never evolve into load-bearing or flight-capable limbs, for all the arm and hand bones have fused into a single structure. This improves grip strength but limits the range of motion to a simple arc. The arms can be extended out to the side or pulled inward so that the curved claws meet in the middle, but no other motion is possible. The female, for her part, has a pair of much smaller but not entirely useless claspers. Unlike the male, she has no instinct to wear her claws down to a rounded tip, so they grow into a set of secondary weapons. When she pounces on her prey, these sharp points help her maintain her grip, freeing her legs to kick at the underside of her victim while her beak slices and pulls apart flesh from above. Males can attempt this same maneuver, but they typically slide right off their struggling victim, for their claspers only work on a willing, stationary target.