Hoofpoles
The flying ungulates
With the reformation of the polar and alpine ice sheets, the sea level began to gradually decrease during the Biancocene epoch, with an average drop of 50 meters every 5 million years. This process has resulted in the re-emergence of many island systems and the expansion of others, as observed in the two main Antarctic islands and the Infinite Archipelago. However, this phenomenon is not exclusive to Antarctica. Many dead atoll areas in the middle latitudes, originated during the warm climate of the Cambiocene epoch, have become ecological corridors that facilitated the arrival of a new wave of flying vertebrates. With more frequent migratory stops, numerous birds seized the opportunity to colonize the Antarctic continent, again.
Some of these species resemble the taxa we are familiar with from the Holocene epoch, while others exhibit bizarre forms never seen before, such as the hoofpoles (clade Pteroungulatidae).
This group of relatively large flying birds, despite their delicate appearance, seems to have sprung out of a mythological book. At first glance, while they feed in grassy expanses, they may not appear unusual, but upon closer inspection, one notices their hind limbs. These birds possess four fingers, but only two are used for locomotion. The supporting fingers are partially fused at the apex and rest on the ground only with the distal part; the portion that contacts the ground is not fleshy but covered with a keratin layer, forming a hoof-like structure similar to that of ungulates.
The torso of the hoofpole is not semi-erect like most birds, but fully upright, resembling the posture of humans, with the legs positioned distally on the body, akin to loons. Hoofpoles have also a highly unbalanced forward stride that allows them to exploit the acceleration provided by gravity. All these anatomical adaptations make hoofpoles incredible sprinters, capable of reaching speeds over 70 km/h within seconds. Their body is built for running in a straight line, disallowing abrupt changes in direction, which is an adaptation to the vast grassy plains where they commonly reside.
This group of ostrich-like birds is distantly related to rollers, which belong to the order Coraciiformes, from which they have inherited their unusual foot structure (syndactyly). Hoofpoles have greatly benefited from the onset of the Biancocene epoch, which led to the expansion of steppes and savannahs, their favored habitats. While most species have an omnivorous diet, independent lineages have often developed a more plant-based diet.
Despite their remarkable running abilities, hoofpoles are also capable of discrete flight. However, due to their proportions and weight, they require high speeds to take off from the ground. Their flight is predominantly gliding, minimizing the energy demands of this activity. Consequently, running serves as the primary means of escape for hoofpoles, with flight observed only in exceptional cases.
Long-distance flights are mainly undertaken during migratory periods, during which flocks of hoofpoles have been documented flying continuously for up to 12 hours. Prolonged flights of this nature are more common among smaller and more gracile species, as larger species (weighing over 10 kg) tend to engage in shorter flights of up to one hour, alternated with breaks on the ground. It is no wonder, then, that the only colonizing hoofpoles in Antarctica were of moderate size, and only later did they develop larger sizes.
Their incredible colonization and movement capabilities have made hoofpoles a highly successful group worldwide, including in Antarctica. In our frozen continent, approximately ten hoofpole species are known. They are among the most easily detectable macrovertebrates in the trample steppe, forming herds that can reach up to a hundred individuals. The breeding period typically occurs in November, relatively late compared to other bird species. This delay is attributed to the astonishingly rapid incubation of the eggs, which can hatch in as little as fifteen days—an unexpectedly short duration for birds of their size.
The young hoofpoles are born relatively early and are nourished by a secretion produced by the parents' crop, similar to penguins and pigeons. Thanks to this protein-rich nourishment, the young hoofpoles grow quickly and can begin following the herd just a few days after hatching. This rapid growth has evolved as an adaptation to their nesting habits on the ground, as their large size makes them susceptible to terrestrial predators. The fast development of the young, their strong gregariousness, and other anti-predator behaviors (such as creating false nests, grouping eggs in the central part of the colony, and mimetic coloration of the eggs) help reduce juvenile mortality.
Overall, the unique adaptations of hoofpoles have made them remarkably versatile animals. Through their rapid colonization of Antarctica, they have outcompeted several species of birds, including some flightless ones, like herbivoos and (to some extent) terrariders: while the first one were not able to withstand the arrival of these sort of flying ostriches, terrariders were shown a trend toward hypergigantism and aquatic habitats, niches that hoofpoles haven't managed to exploit.
Feet anatomy of a generalized hoofpole
(p=phalanx)