Dedrorax and Zyloron

 
A peculiar difference between Earth and Mars is the ways in which the local fauna has chosen the number of limbs on which to walk. On both planets there are plenty of bipeds, hexapods as well as creatures with no limbs at all. Outside of that, on Earth, animals have eight or four legs, whereas on Mars, tripods have evolved on at least two separate occasions while tetrapods are only secondarily derived from hexapods. What leads to such a difference? A popular explanation might be the differences in gravity, the heavier creatures of Earth requiring more legs for support, but so far nobody has been able to prove a direct causation. If it were true, it seems odd that the animals with the most legs on Earth are also all among the smallest and thus least affected by gravity, while hexapods are also abundant on Mars. Evolutionary history and contingency seems to be an equally strong factor, if not stronger. Periostraca ancestrally had only two limbs and so were restricted to bipedality unless also turning their tail into another limb, making tripodality their evolutionary “end-point”. However, the same restrictions would not have applied to the onychognaths, yet when natural selection called for the reduction of their limbs, they jumped straight from six to three with almost no transitional forms, which suggests there might really be an adaptive advantage to tripodality in the Martian environment.

A world apart from all of these discussions is an animal with no equal on Mars, let alone Earth. The dedrorax is a genuine monopod, at least when it has to be. When slow and idle it slithers across the desert, using its boomerang-shaped headshield to glide above the sand, but when in pursuit of prey or fleeing from predators it erects itself onto its muscular, bony tail, which ends in a three-toed foot, and hops away in wide strides like a kangaroo. When attacking its prey, it will extend a snorkel from beneath its headshield and strike out with a beak. From its back extends a retractable sail held up by bony rods. Perhaps it used in temperature control or social signalling.

Dedrorax are rarely observed and thus little is known about them, including their overall behaviour and reproduction. For a long time it was even unknown to what family tree it even belonged, as it seems to combine traits of many lineages, sort of like a Martian platypus. It has simple been considered incertae sedis. Close comparison to some other strange creatures offers at least some clues. The dedrorax has a partly siliceous endoskeleton and spine-like protrusions supporting its eyestalks, as well as a triradial cloaca in front of its sail. The beak at the end of the proboscis is three-pronged. These are all traits it shares with arezoans such as the arctic sortax. This points towards it being a highly derived member of the Furchordata, perhaps even the most derived trichordate of all time. How exactly this led to its evolution as a monopod is, however, not clear. Perhaps its ancestors were able to erect themselves and strike out on their tails like cobras and some unknown selective force reinforced that ability?

Among the reported prey of the dedrorax is the zyloron, a member of the verticutian dust slugs. It and its close relatives have lost their ancestral pseudopods and instead move much like serpents. They themselves predate on smaller onychognaths. Their armour has been reduced, something they compensate for with speed and being able to quickly bury in the sand.