Yrp

The yrp (Thagmocephalus longipes) is a common, though quite unusual member of the Onychognatha, about the size of a kangaroo mouse. It is part of the more “primitive” Archaeocephalia, but still differs dramatically from the typical body plan. The jaw-limbs have expanded and become cheliceres. Furthermore, two of the post-oral segments have shifted forwards and fused with the skull through thagmosis, the four arms becoming additonal mouthparts, while the body is now entirely carried by the hindmost, almost birdlike and incredibly long legs with digitigrade feet. This has affected respiration too, the yrp having only two large lungs, instead of the usual six small ones. What is left of lung segments I and II appears to have been absorbed into the throat and repurposed into producing more complex chirping sounds than what is usually heard in other archaeocephalians. The antennae have also retracted into the skull to form sensory pits. This case of extreme cranial thagmosis is in many ways comparable to the condition seen in khaldanes, though it is unlikely that the two are related, given the fact that khaldanes are cuneocephalians. The underlying genetics that lead to thagmosis in onychognaths may however still be a shared, inherited trait.

Despite its erect stance, the yrp is an ectotherm, as in the benevolent Martian gravity such a body posture can be maintained even with lower metabolisms. Yrps use their complex jaw apparatus to feed on the bodies and appendages of succulent fractarians, as well as soft sporians. It is hunted by various onychognaths, verticutian spirifers and species of Hortax. Yrps are usually gregarious, though do not exhibit all-too-complex social behaviour. Disagreements are usually resolved through swift kicks in the head with the long legs. The long-tailed kyphus (Thagmocephalus? macropodoides), a closely related yrpoid, is even capable of balancing on its tail to strike out with kangaroo-kicks.

What the prominent bony protrusions on a yrp’s back are for is not known. Some form of social display is likely. It has been discovered, by complete accident, that they seem to have spots only visible in ultraviolet, though it is unknown if yrps can actually see in that part of the spectrum. It also has been proposed that the increased surface area might help the Yrp fill up more of the faint Martian sun’s heat in the early morning hours, though one would think that an actual sail made of skin would be more practical for that purpose. Similar dorsal spines are also found in various fossil taxa, such as Placodontosaurus. These are usually reconstructed as having supported a skin sail, emulating common reconstructions of terrestrial sailbacks, but there is a prominent camp of astropaleontologists which argues that the yrp would serve as a better model for comparison and that the spines of these extinct organisms were "naked" too. This is ultimately hard to prove and may also be criticized, on the grounds that the giant thagmasaurs were very different in size and physiology from a small archaeocephalian such as this.  

The origin and etymology of the name "Yrp" remains mysterious. In fact, this is the case with most of the weird common names of Martian organisms, be it skrael, netchu or hrypidex. Most of these names can be traced back to one Timothy K. Lustig, great-grandson of the famous astronomer Michael Lustig. Timothy was the first to write a major textbook on Martian fauna, which was used for decades in various school and university curricula. In said textbook, he gave all of the creatures such strange common names and due to the long use of said book as reference material, they stuck around. But Lustig never annotated what the names meant, what they were based on or why he chose them. Some have theorized that these were nicknames that the first astronauts on Mars came up with and which Lustig simply adapted, but there is no evidence for this. Some claim that there is no special meaning behind them and Lustig just made them up willy-nilly to sound strange and "space-y". Nobody has been able to ask him directly about this, because he died shortly after publication, while on an expedition to the island Ryl Madol. Rather appropriately, nobody also seems to remember anymore what the K. in his name stood for.

The intelligence of yrps is currently a debated question. The small brain size would suggest reptilian or even insectoid levels of cognition, but in a particular case a single yrp was once observed looking at itself in the reflective wheel rim of our Mars buggies, deliberately using its mirror image to scratch off a bit of dust behind its left eye. This seems to have been an anomaly, however, as in no subsequent controlled attempt has any yrp ever passed the mirror test or similar cognitive assessments. Thus, the search for intelligence on Mars continues. We better find it soon, because there is bugger all down there on Earth.