The Prairie Hopcavy

 Though one of the defining characteristics of the Athyrmatherians is their modular, zooid-based physiology, this trait is not as ubiquitous as it may seem. Some lineages, such as the Polycarnivorans, are diontogenetic, with a growth series that diverges into two separate adult morphs. The secondary adult morph is a singular animal that never becomes a zooid, retaining the relatively unmodified bauplan of its larval stage via neoteny. In most cases, these "paedomorphs" are nothing more than an alternate growth stage. However, there is a very notable exception. In the shadows of much larger modular creatures, Athyrmagaia's rodent and insectivore niches are filled by the diminutive Monotherians, a diverse superorder of Euathyrmatherians that have completely forgone the modular adult growth stage in favor of being fully functional, independent animals. Seemingly evolving from the same branch as the Polycarnivorans, it is not entirely understood how the Monotherians evolved or when. One of the most commonly accepted theories is that these animals evolved as a result of the paedomorphic and composite adult stages of an ancestral species becoming reproductively isolated from one another, a phenomenon that occasionally happens among extant Polycarnivorans today. The earliest known Monotherians in the fossil record were subterranean, mole-like animals similar to the paedomorphs of modern Polycarnivorans, but over millions of years of evolutionary time. the order has achieved a staggering level of biodiversity. They are currently one of the most speciose groups of Euathyrmatherians alive today.

One order, the Eumonotherians, looks remarkably similar to mammalian rodents at a passing glance. They walk on four limbs, have bodies covered in insulating fur, and have a set of what resemble "incisors" in the front of their mouth. Also, unlike many other Athyrmatherians, Monotherians have managed to evolve true viviparity, and give live birth rather than lay eggs. Their surface level resemblance to our rodents, however, is merely a form of convergent evolution. Upon closer inspection of their external and internal anatomy, their Euathyrmatherian affinities become crystal clear. Though they possess an internal hydroxyapatite skeleton, they are not vertebrates, and actually belong to Athyrmagaia's "arthropod" phylum, the Stegospondyla. Rather than a proper spinal column, their backbone is comprised of two stiffened girdles formed from fused, internalized tergites, which are connected at the middle of the body by a ball-and-socket joint. This backbone is much more rigid than a proper vertebrate spine, a consequence of their ancestors having heavily compressed bodies that relied on their modularity for body flexibility. Due to this, their bodies are poorly optimized for mammalian style cursoriality, so rather than running, Monotherians have adopted a frog-like saltatory (hopping) gait, possessing long frog-like hind legs with elongated toes. Their ears also lack pinnae, appearing as a pair of similarly frog-like tympanum (external eardrums) behind their eyes. These ears have evolved independently of the limb-derived auripods of their modular ancestors. What appear to be their "incisors" are actually insect-like mouthparts, consisting of an upper labrum and two lower mandibles capable of independent movement. Unlike other Euathyrmatherians, however, the horny parts of the jaws do not contain a bony core, and instead grow from sockets at the tips of the jaws. These pseudo-teeth are constantly growing and self-sharpen with use. Forever freed from the developmental constraints of modular zooid specialization, the forelimb digits (which are fused into singular claw-like bones in most other Euathyrmatherians) have secondarily re-evolved articulated joints via a process analogous to hyperphalangy. Unlike diontogenetic paedomorphs, which have an extremely simplified through-gut, Monotherians have re-evolved a more organized and specialized digestive system, complete with a stomach and intestines. As a consequence of their ancestors being mole-like fossorial creatures, Monotherians have relatively poor color vision compared to their relatives, and ground-dwelling predators that specialize in preying on them do not need to use biliverdin for camouflage.

In the great Western Weave of Borea, one of the most common animals is the prairie hopcavy (Batrachocavia gramineus), an herbivorous, cavy sized Eumonotherian that resembles a hybrid of a toad and a hamster. Hopcavies are crepuscular, being most active during sunrise and sunset. In between these periods of activity, they sleep in underground burrows to conserve energy and avoid the mid-day predators. Their main source of food and water are the leaves and seed-bearing florets of star grass, and they play a part in dispersing their seeds by defecating. These animals are mostly solitary, but in the cases where they meet one another willingly, they mate for life. Prairie hopcavies are a potential food source for a multitude of predators, being one of the main prey items of choice for a species of large predatory flier that patrols these open landscapes. Fortunately, hopcavies are more than capable of evading danger and defending themselves. Their powerful hind legs allow them to jump great distances, and their chisel-like mandibles (which are specially adapted for chewing through tough PLA-based vegetation) are sharp and powerful enough to leave painful, bloody bite wounds on a would-be predator.