Lesser Bennu

If it walks like a duck and quacks like a duck… it might still be a Martian. Bennus are members of a fascinating clade of periostracan antitrematans, the Nothornitha, which evolved a warm-blooded metabolism and have learned to walk in an erect gait on their hands. Their periostracum has grown even thicker than in more primitive relatives like the Kratox and, to better conserve body-heat, has developed a short pelt of hair-like fibres, similar to those found on the shells of some Earth-gastropods. This further conceals the form and sutures of the underlying skeleton, which is more like that of a lophophoran than a vertebrate.

The Lesser Bennu is a chicken-sized omnivore that wanders through the shrublands and steppes in search of anything edible, which it cracks open with its fairly robust beak. Its tunicine tail is curiously strongly reduced, though it still sports bristles. Despite its bird-like physique, it is a largely silent animal, capable of only faint hisses and snorts. Most of the communication is instead done over the striped dewlap hanging from the proboscis and clattering sounds made by clapping the scolecodont beak. When encountered by our astronauts, these bennus show little fear and are rather inquisitive. The Portuguese and Dutch may have observed similar behaviour when first encountering the dodo. Curiously though, these animals do have natural predators and other bennu species do show an appropriate fear-response when seeing us, so one wonders why this one does not.

Fat-tailed Kratox

The Antitremata are a rather successful clade for a planet as harsh as Mars. Perhaps their most successful class are the so-called Periostraca. These differ greatly from both the Ortholitha (of which the Zhor is a part of) and the ancestral, sessile forms of the phylum. In the Periostraca, the ancestral valves have lost their clam-like opening mechanism and instead fused together to form a carapace not unlike that of a tortoise. As the name implies, this carapace has been partially internalized by being overgrown by a thick periostracum (a type of skin common to shelled animals) and in some places even with muscle. The stalk has also given up its anchoring-function and has instead become a muscular tail. Its cuticle has evolved into solid rings, making it resemble the exoskeletal tails of crustaceans. In some periostracans the tail bears protective setae.

Remaining inside the carapace are only three permanent openings, out of which highly modified lophophores emerge. Two on the side have lost all feeding function and have instead become legs. Unlike in ortholiths, these limbs are not exoskeletal but instead supported by an internal, apatitic skeleton, making them resemble more the limbs of vertebrates. At the front (and also above the cloaca) extends a muscular proboscis, likely homologous to that seen in the Zhor and Shellubim. It is rather flexible and supported by a very spine-like endoskeleton. Some periostracans are capable of retracting the proboscis into the shell, much like a turtle. At the end of the proboscis is a genuine jaw. It is composed of large, tooth-like elements that have fused together into a beak, somewhat similar to the jaws of polychaete worms or chaetognaths. Behind the beak, four slits represent sense organs tasked with hearing, smelling and breathing.

The pseudo-head at the end of the neck contains neither a brain nor eyes. These are instead located inside or on top of a bulge growing out the front of the dorsal valve. Periostracans seem to have evolved their eyes independently from ortholiths and shellubim, as they are not soft eyes growing from the fleshy mantle, but instead complex mineral structures directly embedded into the shell. These likely evolved out of rock-eyes similar to those seen on the Chiton slug from Earth. Periostracans generally have six eyes, likely due to those eyes’ immobility. Two simple ones at the back of the cranial bulge, likely to look backwards, and two complex ones with an hourglass-shaped pupil at the front. Many also have an additional, simple pair on the bottom valve between the legs and the proboscis.

In some species, such as the Fat-tailed Kratox seen here, the brain under the cranial bulge can reach reptilian levels of relative size. This is more than enough to stalk through the desert sands in search of soft-bodied prey. Here it is seen cutting into a fyrm, a hairy desert-diplognathan. The Kratox, being an ectothermic, egg-laying ambush-predator, often hides beneath the sands, waiting for prey to crawl by. Apart from circulates it also feeds on smaller onychognaths, dust slugs and trichordates. Itself, it is preyed on by larger onychognaths and periostracans, which the pointy setae on its tail help defend against. When feeling threatened by our astronauts, it emits a frog-like, reverberating chirp and prominently waves its tail at us. Why the Kratox has such a fat tail compared to other periostracans is not known. Possibly, under the tunicine-rings it hides fat and water reserves for hard times, of which there are many on Mars.

Spectacled Chirorbite

Chirorbites are likely the most derived members of the superphylum Pseudarticulata. It was first believed that pseudarticulates, despite their unusual morphology, are members of the Martian animal kingdom, as they are heterotrophic and mobile, at least at some point in their lives. This assumption turns out to have been wrong.

Having already met animals that behave like plants, it should maybe not come as a surprise that on Mars there are also “plants” that behave like animals. It now seems clear that pseudarticulates are unrelated to Arezoa, but instead are deeply nestled within the Fractaria, a kingdom which otherwise only includes immobile organisms with no nervous system that live autotrophically or through passive filter-feeding. The commonalities are not only evidenced by their characteristic glide-symmetry but now also molecular data.

Pseudarticulata likely descend from an aquatic ancestor not too different from the frondomorph monvexillans in the foreground. These organisms originally were sessile autotrophs, filter-feeders or osmotrophs their whole life, their chiral body-isomers being constructed of ciliated filtering-chambers and held up by a hydroskeleton. While it is effective for such a lifestyle, such a bodyform is restrictive when wanting to develop more complex morphologies and behaviours. The majority of Fractaria have worked around this by opting for a clonal, colonial lifestyle, such as the two organisms seen here at the back. This represents a path of increasing complexity through external compartmentalization. Pseudarticulata on the other hand went down the other path, the one of internal compartmentalization. Their ancestor must have been a single frondose-organism, which, instead of being anchored straight on the ground, lied flat on the sea floor with its leaf-surface and was capable of crawling across it using cilia, likely using the filtering chambers at the bottom surface to feed on algal mats, while those at the top became specialized in respiration. The former holdfast, now being the first thing that comes into contact with the world as the body crawls forward, soon began being studded with sensory cells, the beginnings of encephalization.

Organisms such as the Spectacled Chirorbite represent a major improvement on these primitive beginnings. Out of sensory and contracting cells, already present in the earliest fractarians, has now evolved a nervous system, with muscle-tissues to control, evolved completely independently from the Arezoa. The small channels which used to connect the chambers of each isomer have become a true, zigzagged through-gut. The formerly gelatinous body is now covered in a tunicine exoskeleton. The chambers at the bottom sole of the body have given up their digestive function. Instead, they now house hydraulic tubefeet, evolved out of extensions of the hydroskeleton’s mesenteries. Instead of cilia, the Chirorbite now uses these to walk and can also fully retract the tubefeet into their pores, much like a starfish. The primitive cephalon has become a proper head segment, housing little tasting, smelling, and hearing feelers and, most conspicuously, eyes. Underneath lies a crescent-shaped mouth with two hardened lip-plates. The chirorbites use this shearing-structure to feed on low-growing vegetation, including their botanical cousins, as well as very small trichordates, spirifers and onychognaths. Its most common prey is the Menamin, which is also a pseudarticulate, but worm-like with no exoskeleton and limbs.

In perhaps a twist of irony, chirorbites are among the few Martian organisms, along with aspiderms, to have evolved liquid-filled lens-eyes, not unlike yours or mine. These eyes, usually five of them, are covered underneath a translucent tunicine window of the exoskeleton. For unknown reasons, the chirorbite also seems to have eyelid-like membranes underneath this hood to close its eyes. It is funny how an organ this basic can generate much sympathy as Man interacts with his fellow alien. Looking at the Chirorbite’s face, one may feel like catching a glimmer of consciousness, perhaps even a soul, behind those eyes, which one does not perceive when looking at the inanimate compound-eye of an arthropod. Yet, the “brain” of the Chirorbite is a ganglion the thickness of a lentil and it is confronted with minds that are as distant to its consciousness as its is against the vegetative relatives it feeds on. An intellect, which is as vast as it is cool and unsympathetic, now rears its malevolent jaw apparatus into the picture, leaning on avipodous legs over a chirorbite, helpess after it had been turned onto its back. The polychaete-like grin of the rannu is made all the more sinister by its maw having no eyes to speak of, just four black holes representing the nostrils and earholes. Man is unfortunately more likely to find his equal among the stars in such minatory beings.