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.