Ruminantiforme Skull Morphology

 

Ruminantiforme Euathyrmatherians are superbly well-adapted for herbivory, and no other part of their skeletal anatomy reflects this better than their skulls. From the outside, the head of an animal such as Xenobos almost resembles that of an Earth tetrapod, with the horny beak calling to mind herbivorous ornithopod dinosaurs in particular. However, upon inspection of the bones, the skull morphology is unmistakably that of an Athyrmatherian. What appears to be the animal's jaw is actually a specialized pair of forelimbs called "gnathopods", and the maxilla is actually a hinge-jointed upper mandible called a "labrum". Though Ruminantiformes have mastered the ability to slice and chew the tough leaves of Olekirkophytes, they don't have any true teeth in the traditional sense.  In place of incisors, they possess a broad, external horny beak at the tips of their jaws, formed from the rhampotheca of the labrum and the "nails" of the gnathopods. In place of actual molars, they possess an alternative processing system formed from the actual lower mandibles and flat, molar-like papillae on their palates. Both the papilla and the mandibles form four grinding plates sheathed in iron oxide-reinforced keratin. Much like the teeth of rodents, the papillae and the sheathes of the lower mandibles are constantly growing, so Ruminantiformes keep this growth in check by bruxing. To ensure their food doesn't fall out of their mouths, Ruminantiformes have evolved a "cheek".

Due to their very alien oral anatomy, the method by which Ruminantiformes chew their food is hard to compare to any Earth analogues. The general chewing style of the internalized lower mandibles closest resembles an inverted version of that of a hadrosaur, with the inside surface of the mandibles sliding and grinding against the outside surface of the papilla. Unlike hadrosaurs, however, the mandibles move in an elliptical cyclic motion, moving forward and grinding against the papillae as they move backwards. This allows Ruminantiformes to chew and swallow their food at the same time, and they repeatedly regurgitate the partially digested cud to repeat the process until the food is ready for further stages of digestion.