Zaoulouros hierakonpole

The fossil life of Mars is among its least understood and also one of the most understudied. Not only are excavations difficult to undertake at our current technological level, but many of the petrifications defy classification. What can you really say about a dark patch of carbon imprinted on a piece of rock from aeons ago on a planet you do not even fully understand in the present? It is barely more than a naturally formed Rorschach-test.

That said, some advancements have been made in certain areas. Counted among these are the organisms now referred to as Urocephalia. Originally, these were only known from fragmentary skeletons, largely consisting of torsos and legs, found in Argyrian and Isidian sediments. Evidently coming from organisms with endoskeletons, they were tentatively placed within Onychognatha. But they never quite fit in. For one, onychognath bones are made of apatite with some siliceous elements, whereas the bones of these problematica were made of a mix of aspidin and dentine. The shape of the vertebrae is also off, as in these organisms there are two muscular depressions on the side and their nerve-chord is placed dorsally, whereas onychognath-vertebrae only have a single, central hollow and their nerve-chords run ventrally. Onychognath limbs are complex in that their legs are formed of fused or paired bones, whereas the segments of urocephalian bones are made of single bones. Onychognaths ancestrally had six limbs while these creatures only had four.

Not much could be made about this situation, as the remains were simply too incomplete. Then came Richard Coombs with the discovery of one of these fossils having a somewhat intact head. But this Argyrian-aged fossil, which he named Tiresiacephalos, brought up more questions than it answered. Instead of resembling a typical onychognath skull, the upper jaw or cephalon was a solid, spade-shaped plate, with no internal room for a central nervous system where one would be expected. There were also no holes or indentations for eyes, nothing for ears, and not even apparent attachment sites for antennae. The only distinctive feature was a singular hole at the front of the snout, which Coombs initially interpreted as a cyclopean eye. As this hole is directly connected to the mouth cavity underneath, it is today instead generally interpreted as a breathing orifice or nostril. Also preserved of the head was a single mandible, imbedded with aspidin teeth (differing from the siliceous teeth of extinct onychognaths). Lacking the other mandible, Coombs assumed that the full lower jaw was a single piece that articulated with the upper one vertically, very much as in Earth-vertebrates. This interpretation is still sometimes seen in outdated or misinformed reconstructions.

Based off its limb and vertebral anatomy, Coombs recognized that Tiresiacephalos was related to the other “mystery onychognaths” and that together they must form a distinctive clade of one-eyed, pseudo-tetrapods. In personal communications he had named this group “chariclopoda” and interpreted them as an extinct phylum independent from the onychognaths. However, as he had failed to coin this term in a published scientific paper, it has never become the official name of this clade and has since fallen out of use. He was also not able to explain how these organisms lived and functioned, lacking a brain and most sense organs. He simply joked that this was likely the reason for their extinction.

The most major and most recent step in urocephalian research was made years later at the Hierakonpolis digsite, an Isidian aged Lagerstätte uncovered at Peridier crater in Syrtis Major Planum. Here was found by Greta Verne a huge slab of rock, preserved on which was what appeared to be a nearly complete and alligator-sized skeleton of one of Coombs’ “chariclopods”. 

Various clues indicate that Hierakonpolis used to be a freshwater habitat, as is also evidenced by the ortholith, conchocaudatan and cimmerozoan shells which surround the skeleton. Two large cracks run through the slab, likely produced by later tectonic deformation. Right above the fossil-bearing layer is a wavy sandstone layer, still preserved on the lower right. It could not be determined if the dunes in the sandstone were formed in an aquatic context or in a dry desert one. That they are not antidunes, which are otherwise a common bedform on Mars due to the lower gravity, speaks in favour of an aridification event following the fossiliferous time.

The skeleton itself preserves the full skull, this time with both mandibles. Verne could show that the mandibles articulated with the rest of the skull horizontally instead of vertically and thus functioned more like the mandibles of insects or the lower jaws of a hagfish. The almost window-like appearance of the snout-hole speaks strongly against it being an eye and more likely being an orifice for breathing/smelling. Furthermore, the front limbs were directly attached to the skull, similar to a fish. There was therefore no neck to speak of, differing from Coombs’ original sketches. On the right flank of the body is preserved a patch of aspidin scales. Due to the incompleteness, it is open to interpretation if these scales covered the whole body or only part of it. On the left side of the body both the front- and hindleg are fully preserved, whereas they are disarticulated and incomplete on the right, possibly swept away by currents. The articulation in the dead limbs points towards a respectable degree of flexibility in life.

The most important aspect of the fossil is the tail, which had not been preserved in previous “chariclopod” finds. Its tip is truly extraordinary, with no equal ever seen before or since in any other group. It is constructed of an elongated crescent that spans between it a three-tiered, thin bone-bridge. Around the crescent are six indentations from which spring six long prongs, shaped somewhat like cricket bats or golf clubs. Internal analyses showed that inside the base of the crescent was a hollow cavity connected to the spinal chord, with thin tunnels into the indentations and prongs.

What could be made of such a structure? Given the freshwater context, Verne first speculated that there could have been membranes spanned between the prongs and that the organism may have used it as a fluke to swim. But this seemed unlikely, as it was simply overdesigned for such a purpose. Then she thought that the organism may have held the tail above the body like a scorpion and that the tip was a stinger. The interior cavity could then have served as a venom storage. But the tips of the crescent were fairly blunt and, more importantly, did not actually have a canal through which venom could have flown. That the organ could have been a sort of pincer can also be excluded, as there was no point at which it could articulate. It would have made for a terrible weapon in general, due to the fragile bone prongs, which are even partially broken in the fossil.

Before coming to the final resort of any paleontologist (which is interpreting any unexplainable and elaborate organ as a display device for courtship), one of her colleagues jokingly remarked that the organ reminded him of a primitive tribal mask. That was when the realization came.  She was looking at the organism’s face the whole time. All the previously missing sense organs were actually in this tail-tip. The internal cavity must have housed a nerve ganglion or brain. The six indentations were cavities for the eyes, which in life may have been either liquid-filled like ours or solid disks like in the onychognaths. The prongs could have sensed vibrations in the air or water and be used for hearing. This was essentially a two-headed organism in which the development of the central nervous system happened independently from the mouth. She named the new taxon Zaoulouros, after a traditional mask of the Guro people, and gave Coombs’ previously defined clade the now official name Urocephalia or “tail-heads”.

 Speculative reconstruction in an Isidian landscape, based off the urocephalian hypothesis. Artistic liberty was taken and this interpretation of the organism(s) may differ from others. Also depicted are an archaic craniopod, an ambulostellian, ortholiths and, in the background, a huge ududomid. Isidian flora consisted largely of arthrophytan and heliophytan arephytes.

Verne’s hypothesis was not immediately accepted. An animal that thinks with its tail is unknown, both on modern Mars and the other known biospheres in the solar system. It seems to go against the general rules of encephalization. Though it needs to be said that “rules” in biology should be more accurately called trends, since there can always be outliers. While not to the same degree, decoupling of the mouth and “head” is also observed in other Martian organisms, such as periostracans and some onychognaths. Even on Earth, planarian flatworms have their mouth on their belly, quite far away from the actual head. Verne speculated that the ancestor of the urocephalians was possibly a blind, brainless burrowing animal that originally used its tail to tactilely probe what was going on at the surface. From that point on it could have become more sensitive and specialized, eventually evolving true sense organs and a brain to control said organs, becoming a sort of periscope.

Verne was also criticized for possibly misinterpreting the fossil slab. There is a notable disarticulation and a tectonic break which separates the main body from the tail. Perhaps the two parts may not actually come from the same animal. This was deemed unlikely by Verne, as the bone in the tail is composed of the same material as the rest of the body and said bone material is, among the known arezoans, so far unique to the Urocephalia. Besides that, those who have suggested that the slab preserves two separate creatures have never been able to determine from what kind of known organism the “tail” could have come from.

For these reasons, the urocephalian hypothesis is generally accepted nowadays and newer fossil finds seem to affirm it. However, there still remains a lot of mystery surrounding these creatures. We know nothing about their internal organs. Did they breathe using lungs, gills or something else? How did they reproduce? Where on the body did they reproduce? What was their lifestyle like? Although found in a strong freshwater context, Verne thought the organism was terrestrial in life, as there were no strong aquatic adaptations that could have helped with swimming or diving. The sharp teeth also indicate carnivory. Being about the size of an alligator, this would have made Zaoulouros among the largest predators of its time.

There also still remains the question of phylogeny. Were the Urocephalia a distinct phylum or were they related to other known taxa? Verne’s reinterpretation, especially of the jaw, has given some new evidence for an affinity with the onychognaths, though not necessarily a strong one. It is generally assumed that crown-group onychognaths evolved from a multi-limbed ancestor which through thagmosis fused its arms and legs into the typical chelicerous mouthparts and the paired-bone legs. Possibly, the urocephalians descend from very ancient proto-onychognaths that split off before this limb-thagmosis occurred, explaining the simple branching of the legs. In this view, the urocephalian upper jaw and mandibles are homologous with the onychognathan cephalon and cheliceres.

What speaks against the interpretation of urocephalians as stem-onychognaths is, for one, that their bones consist of a very different material and, for the other, that even the earliest onychognaths already had eyes and antennae on their cephalons. The possible reconciliation for this is that the Urocephalia split off before true ossification of the skeleton occurred in the onychognaths and that the urocephalians may have lost any incipient eyes and antennae on their first heads when they entered Verne’s proposed burrowing stage. What all this would indicate, however, is that the last common ancestor of these two groups must have lived insanely long ago, in the earliest Lyotian if not much earlier, making them still quite far removed. If still alive today, a modern taxonomist would surely classify Urocephalia as its own phylum, just as the velvet worm is recognized as independent of the Arthropoda.

Finally, there is also the question of why exactly they are not with us anymore today. Newer finds have extended the age-range of these organisms from the Lyoatian until the end of the Cydonian, so they must have been quite successful during the Thermozoic and survived a number of mass extinction events. However, a decline can be observed during the Middle Isidian, when tagmasaurs and a second radiation of cancrisuchians begin to diversify. Giant urocephalians like Zaoulouros vanish, leaving behind only smaller forms. These then finally go extinct, together with the tagmasaurs and cancrisuchians, at the end of the Cydonian. Perhaps it was no personal failing, but a statistical casualty of whatever catastrophe or crisis must have occurred at the time.

References:

• Coombs, Richard: The Pseudo-Tetrapods of Mars, in: Astropaleontology, 498, 2320, p. 309 – 315.
• Sivgin, T.K.: Life on a Dead Planet. The first 3 billion years of Evolution on Mars, Zürich 2345.
• Verne, Greta: Zaoulouros and the anatomy and biology of the newly described Urocephalia, in: Astropaleontology, 555, 2335, p. 115 – 140.