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.

Arctic Sortax

 

Peering beneath the ice caps and glaciers of the north pole of Mars can reveal an assortment of many strange creatures. One of them is the sortax, a flattened, cushiony creature with a strangely bifurcated spine out of which grow a tadpole-like tail and two eyestalks. A common urban legend on Earth regarding this animal is that when an astrobiologist, usually only named “Dr. Morris”, if named at all, first discovered this creature, while livestreaming his expedition as a public NASA broadcast, he shouted “Fuck, another new phylum!”. The resulting PR debacle then led to him being fired in most versions of the story. This is almost assuredly a pop cultural myth that is entirely fabricated. The true discoverer of the sortax was a woman by the name Eryx Burton, and there is no comparable recording in any of NASA’s archives (for how the myth may have formed, see Busch 2338).

More importantly, Burton never made the claim that the sortax belongs to a separate phylum, as examination quickly uncovered its ties in the Martian tree of life. The animal’s endoskeleton is made of apatite and silicon dioxide and the anatomy of the tail is identical to the arms of trichordates such as the hortax. The tri-radial cloaca on top of the body is also another dead ringer. It strongly suggests that the sortax is a trichordate, but has shifted away from the ancestral radial symmetry of its starfish-like ancestors towards a bilateral one. Two arms stiffened and became structural supports for an expanded gut and mantle, their tips becoming mainly sensory organs, while the third became a tail. The three-pronged mouth shifted from the bottom of the body towards the front. This is certainly not as extraordinary as it may seem, as on Earth many radial animal groups also have members with bilateral elements, such as ctenophores with two tentacles, or sand dollars and irregular sea urchins among the echinoderms. Like in the latter example, the anatomy of the sortax and its relatives may have originally arisen as an adaptation towards horizontal digging in marine sediments, where a radial body is less beneficial than an axial, streamlined one.

Though it is primitive in its aquatic habitat, the sortax likely does not represent this ancestral state, as it largely lives on top of the sediment and only digs itself into the sand when danger, such as arctic rhosons, presents itself. When on the search for food, the sortax moves by hovering through slight, carpet-like undulations of its flabby mantle, the tail only being used when quick bursts are required. Its main prey include the various worm-like organisms that inhabit the subglacial silt: circulates, pseudarticulates and mollizoans. These are usually devoured through suction feeding, whereby the prongs of the mouth open fast enough to create a brief vacuum. Swallowed whole like spaghetti, the prey is further processed by a masticating stomach.

It is interesting to note that the eyestalks retain some form of independence, similar though not to the same degree as the arms of more classic trichordates. While looking for prey, usually one eye faces forward to focus, while another one surveys the area. Unlike the arms of the hortax, these do not alternate in their tasks but seem dedicated to their roles, though with variation between individuals. In other words, some sortax are right-“handed”, while others are left-“handed”. It would be interesting to further study if the rest of the nervous system has also retained a degree of decentralization or has instead undergone more streamlining during bilateralisation.

Since its discovery, it has been recognized that many strange problematica found across Mars are likely relatives of the sortax, something substantiated by recent molecular analyses (Schaf 2337). This likely monophyletic clade has been named Furchordata and is now largely recognized as a distinctive class among the Trichordata. Trichordates, due to the shape of their cloaca, leave behind peculiar trilateral excrements, which in Furchordata specifically get twisted into strange spires due to their more complex guts. Coprolites of exactly this shape have been found in Middle Athabascan sediments, indicating that furchordates first emerged shortly after the End-Cydonian mass extinction event (Brot 2339).

References:

• Busch, Briegel: Marsgeschichten und andere tolle Sachen, Berlin 2338.
• Brot, Bernd: Mist. Die Ultimative Enzyklopädie, Beck 2339.
• Schaf, Tchili: Testing the Furchordata hypothesis using sequenced Martian genome data, in: Current Astrobiology, 465, 2337, p. 13 – 23.