Armored Serestriders

Serina remains in a relatively stable, warm and highly biodiverse period which has allowed many animal lineages which appeared near the beginning of the era to persist and continue to evolve for a long and uninterrupted period of time. Among such old, still successful groups we find the pachyderm serestriders, a group of giant herbivorous aardgeese native to South Anciska and Stehvlandea which were notable for the evolution of large armored osteoderms throughout their skin as adults, used to deter large predators (primarily tyrant serins, bone-crushing predators descended from the skykes, which appeared in the Cryocene and now include the largest carnivorous land predators ever to live on Serina.) Pachyderm serestriders were primarily browsers of forest trees, with long upright necks, and also defended themselves from attackers with a massive spiked hand, formed from the fusion of the wrist.

Fifty million years is a long time, and it has been long enough for this clade to diversify into several dozen living species. During times of changing conditions, such as occurred between the Cryocene and the Thermocene, radical new adaptations that may confer slight advantage more easily take hold, meaning species change more rapidly and clades very different from their ancestors, such as the first pachyderm serestriders, may appear. Yet because it has been a stable epoch with little change of selective pressures since, few of the further evolved species of are radically different from their ancestors. They remain large to gigantic, herbivorous animals that rely on spiked defenses to avoid predation, with most species living in groups for further protection, and like their even earlier Cryocene ancestors, they are able to reach such sizes because they have long since ceased to brood their own eggs, instead burying them in warm earth to develop. This has also given rise to the lower metabolic rate of all serestriders, as the young develop at ambient temperatures.

One of the pachyderm serestrider groups to have changed most from its ancestors are the six or so odd species of armored serestriders, also known as cragbacks. These birds have taken the defenses of their group to its logical conclusion, with their entire body surface covered in bony plates and large, deadly spikes that leave them almost impossible for even the most powerful land carnivores to grapple with and subdue. Yet these defenses come at a cost - they are extraordinarily heavy. Cragbacks thus are considerably smaller than their ancestors, only eight feet at the back, and weigh about as much as a rhinoceros. Notice the use of the back to measure their height instead of the head; another difference in this group co-evolved alongside a reduction in height is a change in feeding behavior and therefore posture. Cragbacks are grazers, indiscriminately cropping all manner of plants from ground level to about eight feet up, and so their necks are generally carried at a horizontal, counter-balanced by an elongated stiff pygostyle "tail" similar to but convergently evolved with those of the vivas. Shuffling along in large herds in loosely forested savannahs, cragbacks are most often left alone by even the largest avian predators, but in the unlikely event one tries its luck, they have another defense up their sleeves. The spiked wrist of their ancestor has been modified into a many-spiked mace, no longer built to stab but instead to pummel the skull of anything dumb enough to try and bite the most heavily-armored birds ever to exist. Yet as predation attempts on the mature adult are so rare, the primary use of this mace is against one another; male cragbacks are viciously possessive over harems of females and fights over breeding rights can be brutal, with opponents swinging their heavy spiked necks at one another, kicking with the hind legs and smashing one anothers' skulls with their wrists. Most adult male cragbacks are killed by other males in such brutal mating contests, but not usually before they have managed to spread their genes and ensure the survival of the next generation. As each male guards groups of many females, there are always more single males waiting in the wings to take over when one dies.

Adult cragbacks have few enemies other than each other, and so their populations are limited almost entirely otherwise by high juvenile mortality, as chicks are hatched small, feathered and dependent only on their speed to escape their enemies. As in their predecessors, cragbacks only develop their osteoderm armor and lose their plumage gradually and with age. The most vulnerable period in their lives is when they are awkwardly too armored to run as quickly as before, yet not well-enough defended to rely on passive defense alone, and it is at this unfortunate adolescent age most are taken prey by the tyrant serins.