The Last Writings of the Dawn-Thinker

A huge Moon loomed above the sky as a faint, young Sun touched the horizon. Waves crashed against the beach, strange, gelatinous trees were gently rocking in the wind. High up in the sky floated triradial polyp-like creatures, preyed on by flying disks. Their wings were made of feather-like growths that were actually fleshy in nature. “Fleathers” if you will. The ground was covered in a dense mesh of purple rhizomes, forming an everchanging, evershifting “spongeland” instead of grassland. Embedded in the spongeland was even stranger vegetation, cones connected by strings like pearl-necklaces, spirally algae, long stalks that ended in egg-like “flowers” and large transparent orbs that were suckled on by asymmetrical worms. Only here and there was the spongeland broken up by gigantic, three-sided pyramids and extravagant houses built out of bricks and large stone blocks. Their ornamentation was exquisitely extravagant, quite byzantine one might say, with gilded arches and little frescos depicting history and mythology at every corner. On one of the roofs sat a slug-like creature, its belly armed with hundreds of tiny stubby legs. Its head was an elongated tube, adorned by one huge eye made of a silicate disk. With its tendrils and tentacles it played a baroque tune on a concertina-like instrument, singing to the people below the roofs like a muezzin, telling them to pray to their gods.

This was not an alien planet, but Earth itself, approximately 1.7 billion years before the modern day, deep in the Proterozoic. Except for the algae, none of the creatures here were of the multicellular life we are familiar with. The polyps, worms and the musician were not animals, the trees and cones were not plants, the rhizomes were not fungi. They were all stem-eukaryotes or even multicellular bacteria, descending from experiments in multicellularity that long predate the fauna and flora that would arise in the Cambrian. 400 million years earlier, their evolution was boosted during a quick oxygenation event, leading to a first fauna of macroscopic slime-mold-like flowers and polyps, which greatly diversified in the course of evolution into the wide biodiversity seen here on display. But today only that very first primitive generation, known as the Franceville biota of Gabon, would be preserved as enigmatic fossils, continuing to puzzle humans but ultimately being overlooked in the grand history of life as little more than curiosities.

Out of his window, Ptahhatp watched the serene scene. But whereas it used to fill his being with calm, he now watched the horizon with melancholy in his hearts. Ptahhatp spent a lot of time thinking about the world, about philosophy. He was a scribe of the Society of Sohon, one of many intellectual gentlemen’s clubs. Ptahhatp’s civilization has had many ups and downs, a history even longer than humanity’s. But it had already hit a ceiling millennia ago. The long line of gelatinous trees, with their leathery skin instead of solid bark, did not turn into coal upon fossilization. Algae had simply not existed for long enough or in great numbers yet for their remains to turn into sizeable deposits of petroleum. Living trees were sacred to the dawn-creatures, one needed to make a prayer each time one wanted to fell one. So, all in all, there simply was not enough with which to fuel an industrial revolution. For the better part of a millennium now, Ptahhatp’s society was stuck in an elongated equivalent of the early 18^th century. The height of technology were pocket-watches and elaborate crank-operated automata, imitating people and the fleathery flying disks. They served as little more than entertainment and luxury for the high society.

With no real vision towards the future, Ptahhatp’s society became expert antiquarians, obsessed with the past, “new” movements, be it in art, philosophy, politics or religion, simply being cyclical renaissances of old ideas being brought back in new ways. His Society of Sohon, named in honour of a famous archaeologist, was one of many orders of antiquarians, which met each month to discuss their findings and share and reproduce their texts, much like the royal societies of Enlightenment Scotland. It was one of his favourite hobbies during retirement, now that he no longer had the capacity to go on his adventures. Ptahhatp used to be a polymath, like some sort of Precambrian Shen Kuo, having experienced many different things during his political career under the Emperor. He built canals, oversaw trade and taxes, worked as a royal astrologer and mathematician, drew maps of the realm, led armies into war… it would be easier to list the things he did not do. But now he was living a calm life in his big, old mansion, writing poetry. Until now.

The previous night, he was plagued by a strange dream, a nightmare even. Everything he knew, all the world, was encroached by a veil, not of darkness but of blinding white. Huge walls of ice, thrice as tall as the tallest pyramids, relentlessly marched towards the equator, burying all beneath them until the whole globe appeared like a ball of snow. Drifting solemnly through the emptiness of space. Ptahhatp’s disembodied mind floated atop the ice-sheets, seeing all of history beneath him. Eventually the ice melted and returned to the poles, but when it did, nothing beneath remained. The mighty glaciers carved away the entire world, not just the surface, but also all the rock formations holding eons of life’s history inside them. All the buildings were gone. All the flora and fauna were gone. All the mighty monuments and ruins were gone. All the fossils were gone. It was as if his entire world had never existed. Eroded away by the abyss of time.

Dreams held great meaning to Ptahhatp. Ironically for a person obsessed with the past, he felt as if he had been cursed with visions of the future. But he had never dreamt this far in time before. What was he to make of it? He looked around his chamber, onto the shelf with all the little antiquities, reliquaries and fossils and contemplated the likelihood of them having been preserved, found and brought here. Each one, even the most mundane piece of fossil plankton, is nothing short of a small miracle. The odds of them surviving into the modern day against all the destructive forces of time were astronomically low and now they are just sitting there on his shelf. But they will not survive forever. No matter how good he and his descendants take care of them, they will be destroyed one day. Everything will be destroyed one day, fading into oblivion. Even Earth will one day be gone, with perhaps nobody else in the universe ever knowing that it existed. All the life, all the cultures, all the works of this little pale blue dot… gone forever.

As he looked at his collection, Ptahhatp slowly went through a crisis of faith. What is the point of him preserving history if none of it can be preserved forever? For whom is he doing all of this? Just for himself? He, who cannot take any of this with him into oblivion? Not far from where he lived there was a crimson pyramid, so old that no carving on or in it survived into his time. Nobody knew who built it anymore, what ancient king may have been buried inside. Only the red sandstone blocks remained and in a few thousand years they would be gone too. If even the mighty works of god-kings will fade, what chance does he as a mere historian have that any of his works will be preserved across time?

And he looks out the window again. Into the Sun on the horizon, the lush spongeland, the undulating gelatine trees, the merry musicians on the house roofs. The joy and laughter of the people. This is the present. This is what he actually lives in. There is no past and there is no future for him to experience, only the now. In a flurry of inspiration, his tendrils pick up an ink-tipped fleather and he writes down a poem, unusually for him written in prosaic rhyme:

“What is better?

To have lived and left no letter?

To have legend and no life?

Living but an endless strife?

To become a memory,

Known but for mortality?

Burn my works, smash my bones!

What worth they are once I am gone?

All these things are but loans.

Death is all I own. 

 

I am but sole witness

Of my life in stress.

There is no reason and no rhyme,

Everything just flows with time.

 

If a Beyond there is,

With my goods I cannot depart.

And so these I should not miss,

But one thought I will impart:

 

Mourn me, do not.

Cry for me, do not.

Search for me, do not.

 

Beyond death, you need not plan.

To be happy is all you can.”

 

He goes out to play with the musicians in the street. He does not make history today, but he does make his day.

Awbar

When people think of extinct life, they usually have images of fossils and artistic reconstructions in their head. Extinction is a phenomenon seemingly relegated to the far past, to dinosaurs and mammoths. In reality, extinction happens all the time, throughout the present. It is a process as natural as life and death itself. Yet, it leaves us mourning when it happens in front of our own eyes.

The awbar were a fascinating species which the first astronauts encountered on Mars, including myself on some of my early missions. They lived in a peculiar area of the Argyre Basin. Mars lacks a global magnetic field like Earth does, making it an all-around more irradiated and hostile place. However, some areas contain highly magnetized rock formations, which have managed to save some remnants of the prehistoric magnetosphere, creating local shields against UV and other harmful radiation from space. In these so-called UV-oases, flora and fauna can lead a more sheltered life and attain higher biodiversity than in other areas of the planet. The awbar lived in one such oasis - only one – together with the organisms it depended on.

The awbar is thought to have been a goniopod, a group of dinosauresque deltadactylians, but unlike its bigger cousins, the cecrops and syncarpus, it was generally not included within the more exclusive Thecocerata, as it lacked the characteristic hornlets inside of its beak. This decision has often been criticized, as the lack of that trait may instead have resulted from its specialized diet. Other unique traits were that it felt comfortable walking both on two and three legs and that it exhibited multioculy (having more than two eyes), a trait otherwise rare in goniopods.

It was a nimble creature, able to fit inside a human hand. From its back grew a fleshy fin, adorned with a peculiar oval spot. Undoubtedly this served some display function, but what exactly is now forever uncertain. Awbar lived in close association with a plant dubbed the sporangobush, a type of fractarian. Its sporangia ended in hairy bulbs, each hair drenched in some kind of viscous liquid. Awbar were most often seen climbing up the bushes and licking these furballs with their long, retractable tongue. Many authors have assumed that this could have been a symbiotic relationship. Assuming the liquid produced by the sporangia was some kind of nectar, the Martian may have been lured into licking up the plant’s spores. Inside the stomach and guts of the creature, these spores may have combined with those of other sporangobushes and exited the body through excretion, already fertilized. It is impossible to test any such hypotheses anymore, however. There may not have been a mutual benefit at all to such behaviour, the creature could have been licking the sporangia for reasons entirely unintended by the plant. Perhaps the liquid was toxic or unappealing to some herbivores but was unintentionally alluring to the little creature, the same way spicy plants on Earth have unintentionally garnered the attention of humans. Or the relationship between the organisms was much more intricate and complicated than we can ever imagine, seeing as how little we still know about these ecosystems.

The extinction of the awbar was not brought about by a catastrophe like the dinosaurs’ or through human interference like the dodo’s. It was the end of a slow process already well on its way long before man set his foot on the red planet. The magnetization held within the surrounding rocks had simply begun to fade. With each passing year, the local magnetosphere grew weaker and more radiation reached the soil. The changes must have been incremental at first. With each blooming, the number in each organism’s generations must have grown less, rates of cancer and other ailments must have risen and gradually lowered their lifespan. The margins and tall hills of the oasis became barren first, the eggs of the sporangobushes and tube-cycads in the soil simply failing to germinate. These blank spots were then quickly colonized by more UV-resistant flora and planimals from outside the region, like chiropedes and the aggressive red weed. From that point on, the collapse of the previous ecosystem progressed at a geometric rate, as now the local organisms did not only face environmental degradation but also competition from outsiders they would have normally been able to outbreed. Local nekhbets failed to spawn and were gradually replaced by wadjets and more delicate spongisporians died from mutations before they could bloom, losing ground to their thorny upland counterparts. The ecosystem transformed and many were simply not able to adapt quickly enough to the changes. It was a prolonged evolution of the landscape, observed by us humans over a span of about twenty years. When the shield was finally gone, very little remained of the previous ecosystem. The last sporangobushes failed to reproduce and aged into misshapen mutants before mercifully fading away. The last awbar was already sighted five years before their extinction.

It is a curious feeling, to know that these little creatures used to crawl over my feet one day and are now forever gone. Though less spectacular than the great fossils dug out from the ground, their loss is a much more personal one. A more painful one. It is the difference between reading about Abraham Lincoln’s assassination in a schoolbook and seeing your own father pass away at the hospital. The many questions you ask yourself. Was this inevitable? Were there ways I could have helped? Why didn’t I try to help? Why did I not do more with the time we were given together? But such things, speculating about changing a past that can no longer be changed, hypothetical realities, is a futile misery. There was nothing I could have done. The magnetized rocks would have faded regardless of me being there or not and none of our expeditions were ever equipped to preserve species. We were just there to observe and study. And by the point I knew my father was sick, it was already too late for us to bond in the way be both wished we would have. Years of neglect had eroded any emotional foundation that could have been built upon. He was my father, and a good one at that, but he was never my friend.

Placodontosaurus spinosus

Among the extinct lifeforms of Mars, the “Tagmasauria” rank quite high up in popularity (rivalled only by the giant nothornithes of the Hylozoic), due to their diversity, size and perceived similarity to Earth’s dinosaurs (or rather what we used to think dinosaurs were like in the past). But, unlike Dinosauria, Tagmasauria is not a true clade. It is a polyphyletic umbrella-term that has historically been applied to any megafaunal onychognaths from the Thermozoic, This includes many groups of varying ancestry, from quite baroque archaeocephalians with arthropod-like characteristics, to more reptilian or even avian cuneocephalians. Sometimes the label is even applied to giant deltadactylians of the Hylozoic. 

A more distinct group among the tagmasaurs that probably does have a monophyletic origin are the Lagopodia. These are a group of huge, bipedal and herbivorous cuneocephalians. They likely had their origin in a faunal turnover during the Middle Cydonian that saw the extinction of many earlier archaeocephalians. One of the better-known lagopods is Placodontosaurus from the Late Cydonian Basmachee Rocks. Its skeleton preserves most of the spinal column, ribcage, pelvis, some parts of the limbs, the impressive dorsal spines, a complete mandible and a large part of the lower jaw, which have all given us great insight into the appearance and biology of these creatures.

Diet and lifestyle

Unlike almost all modern onychognaths, early cuneocephalians possessed teeth. These were composed not of hydroxyapatite like ours, but rather of biosilicon, much like the eye-disks of onychognaths. This made them incredibly durable, but also energetically expensive to grow. In addition, silicon dioxide is piezoelectric, so, while it remains untested, there was perhaps a chance that creatures such as these could have accidentally electrocuted themselves by just clicking their teeth. These two reasons are perhaps why many lineages later independently lost this trait in favour of keratinous beaks. Placodontosaurus is named so for its large, flat teeth, which were accompanied by rake-like ones at the snout-tip. It is generally interpreted as a high-browser, using the rake-like teeth to strip off leaves and branches off fractarian and arephyte trees and then grinding them down with the placodonts. A competing hypothesis is that, much like the Oloropoda, a group of lagopods that Placodontosaurus was closely related to, it was an amphibious animal and instead used its teeth to pluck shelled prey off the lake floor and crush it open. This diet might explain the robustness of the jaws, but there is not much more to corroborate it. The oxbow-shape of the pulmonal ribs indicate that the breathing-holes were located on the back and not, like in many modern onychognaths, on the belly. Nearly all tagmasaurs share this trait and it is usually interpreted as an aquatic adaptation towards life in the primordial swamps and lakes of Mars, but it is possible that this may have simply been an ancestral condition that was retained even in more terrestrial descendants. While its feet were probably broad enough to swim (if skin connected the two toes) Placodontosaurus’ arms were not paddle-shaped like in its derived oloropod-relatives and seem to have instead been rather long and grabby, better suited for drawing near vegetation. What would be helpful to know is what its full tail looked like, but about half of it is missing. In oloropods, the tail is laterally flattened, much like in crocodiles, and if Placodontosaurus also shared this trait it would be a good indicator for an aquatic lifestyle. The lower jaw notably has a central groove, which may have given room for a long, muscular tongue similar to a giraffe, which would make more sense to have for a high-browser.

Gait and Posture

The earliest tagmasaurs were hexapods, though beginning with the Cydonian there appears to have been some sort of “arms race” that drove multiple groups to independently evolve bipedalism for whatever reason. Early on some reconstructions showed these bipeds walking with a horizontally held spine and stiff tail as a counterbalance, much like theropod dinosaurs. Ironically, most evidence instead points towards these tagmasaurs actually having walked much less dynamically, more like tokusatsu-actors in rubber suits. Their tails lacked any ossified tendons and, without constant muscle straining, would therefore have dragged on the ground. Their pelvis is more similar to that of kangaroos than theropods, so at rest the thigh muscles would have been much more comfortable in an upright standing position. Although the low Martian gravity would have maybe allowed it for animals this size, unlike kangaroos, which have long, plantigrade feet, the digitigrade feet of lagopods show no evidence for saltation, so they likely also walked in this stance instead of hopped. Extinct ground sloths or sthenurine walking kangaroos like Procoptodon are perhaps the most comparable. This undynamic gait lines up pretty well with the idea that these aliens were ectotherms with low metabolisms (discussed below) that only achieved this erect bipedal gait through assistance from the planet’s low gravity. Although unrelated to these tagmasaurs, the same principle is observed today on Mars in goniopods and yrpoids.

The tripod-hypothesis has been largely confirmed by a Late Cydonian trace fossil, Urocunichnus, which shows a large tagmasaur, perhaps an oloropod, walking along, clearly creating a rut in the dirt by dragging its tail behind it. Interestingly though, as the distance between footprints increases (indicating an pick-up in speed), the rut disappears. Perhaps like basilisks and other lizards, these tagmasaurs were capable of running for short distances, during which they assumed a more dynamic horizontal posture.

Physiology and Dorsal Spines

In the earliest days of areopaleontology, it was thought that the tagmasaurs may have been endotherms or at least had an elevated metabolism, based off what was known about giant, straight-legged animals on Earth. While there is no doubt that the largest of them must have very likely been mass-homeotherms to some degree thanks to their size alone, there is today very little evidence that any tagmasaurs were tachymetabolic (having a high resting metabolism). On the contrary, most evidence points towards bradymetabolism (low resting metabolism that is only raised in quick bursts when required):

• Histology: Whenever studied, bones of tagmasaurs show strong, closely spaced growth rings, sparse haversian canals and dense collagen layers. This is in stark contrast to the spongy lamellar bone found in modern deltadactylians and nothornithes or mammals and dinosaurs. It points towards tagmasaurs having had only slow very growth rates with low energy expenditures. It likely took them multiple decades to grow to adult size, with some of the largest perhaps living for centuries, like tortoises.
• Ecology: In warm-blooded animal communities, carnivores are vastly outnumbered by herbivores, as they have a high energy need and therefore cannot support large populations. In fossil nothornithe communities, as well as mammal and dinosaur ones on Earth, predator populations are only about one to five percent as large as that of their prey.  In tagmasaur ecosystems, the predator-prey ratio could make up to 10 – 25%, a value which on Earth is closer to that seen in ectothermic arthropods, such as spiders.
• Environment: Most tagmasaurs seem to have lived in humid, tropical environments and many showed aquatic adaptations, suggesting amphibious lifestyles and habitats like those of crocodilians and turtles that are more suited for bradymetabolic organisms. Notably, even though Mars was quite warm during the Late Thermozoic, tagmasaur fossils are exceedingly rare from far southern latitudes and high elevations, suggesting that they were not able to cope well with aridity and low temperatures. Conversely, in said environments, early periostracans, who are hypothesized to have evolved tachmetabolic rates, enjoyed greater diversity than they did in tagmasaur-dominated communities, though they curiously still remained small.
• Atmosphere: While the exact composition remains unknown, most signs point towards the atmosphere of Mars being low in oxygen during the Thermozoic Era and higher in carbon dioxide and hydrogen, similar to the modern one. It seems doubtful that this atmosphere could have supported such large aerobic organisms and more likely that these organisms more heavily relied on less efficient hydrogenotrophy, necessitating lower metabolisms. It would also serve to explain why the more aerobic and endothermic periostracans remained small throughout the era and only attained huge sizes during the more oxygenated Hylozoic.
• Gravity: With no heavy gravity like on Earth, standing and walking on upright legs did not require any significant energy expenditure, so even bradymetabolic organisms were free to enjoy the benefits of said posture. It also required far less energy and smaller hearts to pump blood through large bodies.

In this light, special attention has been given to the prominent dorsal spines of Placodontosaurus and many other tagmasaurs, hypothesized to have supported skinny sails. This feature obviously would have increased the surface area of the body compared to the volume, allowing the organism to more easily pick up environmental or solar heat from its environment. On the flipside, however, such a sail could also be used to argue in favour of endothermy, as such features can also be used to shed off excess heat, much like the large ears of elephants.

This is further complicated by the question of if these dorsal spines even supported a sail. As many researchers have pointed out the, tiny yrp of modern day has almost identical spines growing out of its back, but they stand free with no membrane between them. A growing paleoart trend has become to show Placodontosaurus and others similarly sailless, interpreting the dorsal spines purely as display features. There is no strong evidence either way.

Reproduction

How exactly most tagmasaurs like lagopods reproduced remains a mystery. Probably like most of the more derived onychognaths, some gave live birth. The reduced second arms of forms like Placodontosaurus have sometimes been interpreted as spanning a membrane between them that and the abdomen that could have acted as a pouch like in marsupials, implying some degree of parental care, but direct evidence for this remains sparse. Due to the amphibious habits of many tagmasaurs, a more radical hypothesis has been that at least some of them may have had an aquatic larval stage, with some small amphibious tagmasaurs actually being the tadpoles of larger, more terrestrial species. This idea also remains unsupported.

Ecosystem

Above we see Placodontosaurus, conservatively reconstructed, in an approximation of ancient Basmachee Rocks, which used to be a sub-tropical crater lake. In the background, large shield volcanoes rise and a giant wadjet soars through the air in search of clouds of aeroplankton. Wading through the lake is the lurdupod Astraposaurus,  a huge, amphibious tagmasaur that likely fed on aquatic flora and used its “shoulder-snorkels” to breathe while its head was submerged. Our protagonist is leaning against an early polyfractarian tree, which were now replacing the older arephyte flora and, unlike them, pumping oxygen into the air. The tagmasaur is alerted by something out of frame, perhaps a predator like the bipedal Hyksosaurus or the hexapod Mantidognathus. Upset by the giant shaking its home is a little Platyodon, an early periostracan that may have been clad in fur like its modern descendants.

Their day would eventually come. As volcanism and geosyncline orogenies continued, more and more carbon dioxide was drawn from the air while light-blocking aerosols amassed. The global temperatures grew colder, sea levels lowered and the new change in flora caused a spike in oxygen levels that reduced free methane. The swamps and lakes of the tagmasaurs would soon dry up and perhaps a series of punctuated ice ages would finish the job. The future belonged to endothermic bird-like creatures that would succeed where the insectoid reptilians failed.

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.

Exinct Life

“In Egypt’s sandy silence, all alone,

Stands a gigantic leg, which far off throws

The only shadow that the desert knows

“I am great Ozymandias,” said the stone,

“The King of Kings; this mighty city shows

The wonders of my hand.” – The city’s gone –

Naught but the leg remaining to disclose

The site of this forgotten Babylon.”

- Ozymandias, Horace Smith, 1818.

 

That Mars as we encounter it today is but a shadow of its former self is perhaps an understatement. The planet is but a desolation, dominated by fields of ice, toxic salts and harsh sands, where nary a tree grows and the remaining animals carve out an arduous existence. But wherever we look, we find evidence of a world now lost, one far wetter, warmer and weirder, the world of antiquity. And the great beasts which ruled the lost lands have left behind their bones.

Excavating and studying these remains is a unique challenge. Only very few missions have been sent out by the spacefaring powers with the main goal of astropaleontology. Of these, most work was done by rovers, remote-controlled drones or other robotic spacecrafts, which often have to study fossils in-situ and leave them behind when done. This puts the finds at risk of being destroyed by erosion over the following seasons before they could be studied again. Ideally, but only rarely, actual astronauts can study the fossils in secure conditions at research bases. These are difficult to set up on top or close to dig sites, as fossils often tend to erode out of quite inconvenient places, such as cliffsides, canyon-walls or strewn about the toxic perchlorate wastes. Transporting such fossils back to Earth to be studied under more professional conditions is even more difficult, given the logistical nightmare involved. Only very few space programs have managed this feat.

Due to these factors, our understanding of the past life of Mars is rudimentary in every sense of the word. I would hazard the guess that our current understanding of the antiquity of Mars is akin to the knowledge we had of Earth’s at the end of the nineteenth century, if not more archaic. The field of astropaleontology is still very much dominated by mysteries and fierce debates, large gaps and plenty of room for interpretation. Nevertheless, here is what we think we know about the timeline of events that led to modern Mars:

Pre-Noachian

Mars formed around the same time as the other rocky planets between 4.6 – 4.5 billion years ago out of the accretion disk that orbited the young sun. According to most simulations, the planet should have ended up with a similar size and mass as Earth and Venus, but something went majorly wrong during its development. It is now thought that early Jupiter did not circle the sun at the same orbit as it does today but was instead much closer during this time, perhaps where now Ceres and the asteroid belt are. This did not only disrupt the development of a fifth hypothetical rocky planet, but also gave Mars the “birth defect” that would dominate its entire life, its small mass.

Earliest Mars, much like Hadean Earth, was a hellish place. The entire surface was in only a semi-solid state, the temperature may have been 350 degrees Celsius hot and the early volcanic atmosphere may have borne down with a pressure of up to 200 bars. Giant impactors hit the planet on a daily basis. Some of these were large enough to melt the surface again and strip away parts of the atmosphere. One would think that the next thing to happen would be that the crust slowly cooled and solidified into its current state, but, for reasons still poorly understood, it did not. When its primary crust, which consisted mainly of iron and radioactive materials, first cooled, it seems to have insulated the radioactive materials inside the planet so much that enough heat built up to melt it again. The primary crust was thus liquified and sank all the way to the core. Geologically speaking, this is year zero, as everything that may have existed on Mars’ surface before that point is now forever lost and destroyed. On the positive side, the recycled crust added a lot more fuel to the planet’s core, ensuring its geological longevity and internal stratification.

After this event, when the surface solidified again, it is very likely that Mars developed its most prominent feature, the Great Dichotomy, the difference in elevation between the northern and southern hemisphere. Although the cause for this was linked by some to a giant impact in the north, I believe it far more likely that it was internal. Numerous pieces of evidence strongly suggest that Mars, instead of developing multiple smaller magma plumes and a plate-conveyor-belt like Earth, had one gigantic primary magma plume that controlled a global cycle. The convections of this giant plume added mass from the inside to the southern hemisphere, elevating it, and subtracted mass from the northern one when they sank there again, thinning it. It is unlikely to be a pure coincidence that the line of the Great Dichotomy runs so closely along the planet’s equator, suggesting that this large-scale redistribution of mass altered its orbit and led to what is called true polar wander. This would not be the last time Mars’ global plume would alter the position of its poles.

It is likely that during this time Mars also acquired its two moons Phobos and Deimos. It is unclear if these were asteroids captured by the planet or accreted out of impact debris like Earth’s moon. Giant craters like Argyre and Utopia Planitia were also likely created shortly after the Great Dichotomy began.

Towards the end of the Pre-Noachian it is likely that temperatures cooled down enough for the vapour in the atmosphere to condensate and form the first hot lakes and seas, but these would have been ephemeral. Giant asteroids and planetesimals were still colliding with Mars regularly and some of them produced enough energy to reheat the atmosphere again above boiling point. Nevertheless, some geochemical evidence suggests that life may have already begun during this eon. How the first Martians would have survived such impacts is a mystery. Perhaps they survived in a dormant state on meteoric fragments flung out into orbit before raining down onto the planet again, a sort of self-panspermia. Or perhaps life was just extinguished completely with each giant impact and abiogenesis just repeated itself an uncounted number of times when the conditions presented themselves. It is not as outlandish as it may seem, given our current knowledge that self-replicating nucleic acids can form spontaneously if triphosphates simply percolate through volcanic glass (Jerome et al. 2022), both of which would have been abundant on early Mars.

The transition from the Pre-Noachian to the Noachian is marked by the creation of the Hellas Basin, one of the last great impacts of the solar system. Although not powerful enough to re-melt the entire surface again, the whole planet was covered by its impact debris and reheated the atmosphere above boiling point. Apart from the formation of the crater itself, the impact was also powerful enough to create massive cracks and rifts across the whole globe, which would serve as weak points for later geologic developments for eons to come. It is likely not a coincidence that some of the later giant volcanoes of Tharsis, such as Alba Mons, happen to be at the exact antipode of Hellas. How early life, if it truly existed at this point, may have survived this event remains again a mystery.

Noachian

In the Noachian eon, the planet finally stabilized. Temperatures cooled down again and allowed for the creation of permanent seas and lakes. The carbon dioxide atmosphere may have been thick enough to create pressures of up to 3 bars. Eventually, enough water accumulated to fill up the northern hemisphere, creating the Oceanus Borealis. At times, water-levels may have been high enough that straits directly connected this ocean to the Hellas and Argyre seas. The entire northern hemisphere may have been devoid of landmasses, as Elysium Planitia did not exist yet and the entirety of the Tharsis Plateau had yet to form. The orientation of the planet was also very different. Before Mars underwent true polar wander in the Late Hesperian, it is very likely that the original north pole lied where today is Scandia Colles, while the southern one was at Malea Planum.

How Mars managed to stay warm at the very balmy levels that are evidenced by the Noachian and Early Hesperian strata remains a challenge to explain. Like Earth, Mars should have fallen victim to the fact that the faint young sun was up to 30% dimmer in the past than it is today, meaning the planet should have completely frozen over, even with the potent greenhouse gases we know it had. The possible solution to this problem is that Mars’ atmosphere may have regularly produced condensed carbon dioxide clouds, which are even more effective at trapping the sun’s energy in the atmosphere than CO₂ alone. The wide presence of opaque CO_2-clouds paradoxically means that while early Mars was very hot, it was also quite dimly lit, perhaps even gloomy during daytime.

The Noachian is split into three eras. The Palaeonoachian sees the creation of various inland lakes and seas on the southern highland, in what were previously impact craters from the Pre-Noachian, such as Eridania. During this era also formed the oldest known volcanoes of the planet, all on the southern highland close to Hellas, where they likely formed by magma seeping through the deep cracks created by the impact.

In the Mesonoachian we find the very first solid evidence of life on Mars in the form of stromatolites. Photosynthesis had also evolved by this point, though the geochemistry of these stromatolites suggests that all or nearly all of it was of the primitive sulphurous kind still seen in the arephytes. It is unclear if this altered Mars’ early atmosphere. Some geochemical evidence suggests that the first rhodokaryotes had also evolved by this time.

Mars as it may have appeared during the Noachian and Early Thermozoic. The coastline west of Argyre, where in the future the Tharsis Plateau would emerge, is entirely speculative, as all traces of this area have been buried deep beneath later lava flows. It is possible that instead of just a bay and open sea there may have been whole landmasses here we will never know about. Note also that the projection of this map is technically wrong, as the poles of Mars were in a different position during this time. The original poles (Scandia Colles in the north, Malea Planum in the South) are indicated by stars.

The Neonoachian saw the diversification of the macroareonts. In the Middle Cimmerian, the penultimate period of the Neonoachian. Some of these early macroareonts already strongly resembled the Pocupoa and may have been living on land. The Cimmerian aged trace fossil(?) Daorepichnia has even been suggested to have been produced by a zoomorph macroareont, a sort of hypothetical bacterial slug, but the fossil could simply also be a root structure or the product of water flow. From the same time are known the Xenoamorpha (“foreign without a form”), blob-like shallow-water fossils which are thought to be a waste-basket taxon of early multicellular rhodokaryotes. Among these, especially the Cochleamorpha are strongly suspected to be ancestral to the Arephyta. Another group, the Pseudictyostelia, have been suggested to be the common ancestor of Fractaria, Spongisporia and Arezoa, but the fossils are simply too featureless to tell. An identity as giant flechtoids or as an entirely extinct group is equally likely. A large climatic shift seems to have happened at the end of the Cimmerian, leading to the disappearance of any terrestrial macroareonts and many of the xenoamorphs. The cause may have been an ice age.

The last period of the Neonoachian, the Fractarian, remained largely devoid of multicellular forms, apart from the first Phylloaestia, a group of seaweed-like macroareonts. But in the Mid-to-Late Fractarian finally appear the first definitive members of the Fractaria in the Martian fossil record, which give the period its name. This is a bit unfortunate, as it would imply that fractarians were at their most dominant and successful during this time, when they really had their heyday much later, but the name has stuck, as fractarians are the most readily identifiable fossils from this time. They are mostly represented by archaic monovexillans, with polyfractarians and pseudarticulates appearing only towards the end of the period. They shared the world with the last xenoamorphs, as well as stem-group arephytes, archaeosporians and protosporians. The first definitive arezoan fossils are also known from this time, but they remain indeterminate. Tube-shaped and medusoid forms have been tentatively identified as early mollizoans or “brachiostomans”, but could just as well represent separate radiations that went extinct without descendants. At least one definitive laterazoan is known from the Fractarian with Wrightia quadrata, but it is too featureless to assign it to any further clade. Also mysterious is the supposed trace fossil Spiralopodus, which presents itself as a series of spiral-shaped imprints that are perplexingly only found in terrestrial sediments, at a time when almost all life on Mars was restricted to water. These could have been produced by areont or macroareont colonies, or, as has been proposed, were made by the earliest possible land-arephytes as they rolled through deserts similar to tumbleweeds. The latter seems unlikely, as the closest arephytes that could have produced such forms did not evolve until the Argyrian. More out-there and even less credible hypotheses have been proposed, like it being the genuine “footprints” of a slinky-toy-like organism capable of independent movement.

How drastic the transition from the Fractarian to the Lyotian - and with that the transition from the Noachian to the Hesperian – was, remains a matter of debate. The supposed evidence for a mass extinction at the end of the Fractarian has been shown to be fleeting, as most groups that evolved during the period have made it into the Hesperian without much loss. The transition was instead marked more by a more gradual revolution of marine ecosystem complexity. Some have used this to argue that the Fractarian should be classified as the earliest Hesperian period instead of being the last Noachian one.

Hesperian

 

Organisms are not drawn to scale

The Hesperian is the eon which is most comparable to Earth’s Phanerozoic, as it saw the strongest radiation of multicellular life and megafauna and experienced an unprecedented time of habitability. The eon is generally divided into two eras, the Thermozoic, which is then followed by the Hylozoic. Unlike the largely stable Noachian, the Hesperian is marked by both gradual and drastic changes, continuously evolving from a warm and wet planet dominated by carbon dioxide and hydrogen towards a more dry, cold and oxygenated Mars, though not without interruptions. The name of the Thermozoic is misleading in that aspect, as it would imply that Mars was continuously hot during this era, when really it was punctuated by multiple short-lived ice ages before returning back to a more tropical global climate.

Lyotian

The Lyotian is the first period of the Thermozoic and, much like Earth’s Cambrian, oversaw the appearance and radiation of most known arezoan phyla on Mars. We find familiar forms, such as spirifers, mollizoans, conchocaudatans, cimmerozoan antitrematans, ortholiths and trichordates, as well as the earliest stem-group onychognaths, which consisted of worm-like archaeocephalians that still had all or most of their segments unfused. These lived in early reefs consisting of the earliest eusporians. The Lyotian also saw the evolution of the first true predators, which at this time consisted of diplognathan circulates and large, serpentine aspiderms, like the arthroboids and flagrobrachians. The major planktonic group Litholaria, macroareonts with siliceous shells, also first appeared.

 A ceratotergian spirifer

An archaic aspiderm

There were also various problematica that seem to defy classification. Among them were the Multistomia, bizarre creatures that seem to have had more than one mouth. It is possible that, through the Ambulostellia, they may have been in some form related to the trichordates and hemicalyxians. Also problematic were the Allochordata, little trunked creatures that strongly resembled lancelets. Both groups died out again by the end of the Lyotian, except perhaps for the Pterostomia, which are regarded by some as multistomians. The equally perplexing Urocephalia, which may or may not have been a strange offshoot of the stem-onychognaths, would also survive into the later Thermozoic. Many other groups, like the Sklerotaria, Pediculozoa, Gregorania and Pygopsia died out and can only be mentioned in passing.

 

An ambulostellian

 

A flagrobrachian aspiderm

 

A stem-periostracan

At the end of the Lyotian, many of the stem-group forms of the familiar taxa died out alongside the mentioned problematica. A minor ice age that reduced the number of shallow-water habitats may have been the culprit. Towards the end of the period the first archaic periostracans appeared among the Antitremata.

Argyrian

Life quickly recovered again in the Argyrian period. During this time, the landmasses of Mars were colonized for the first time by early arephytes as well as spongifoliforms. The latter practiced oxygenic photosynthesis and may have begun a partial oxygenation event. Some small organisms also seem to have made the step from water to air directly, as in these strata we find the earliest palunoliths, microscopic fossils of aeroplankton. These were apparently quickly followed by arezoans. Some aspiderms during this time evolved their gills into wings and followed suit, becoming the first tyropterygians. These reigned largely alone over the Argyrian skies, though a few fossils suggest that some mollizoans and pterostomians may have also experimented with becoming airborne. 

 

An entelocranian diplognath

 

An archaic giant ortholith

In the seas, the dominant animals were large ortholiths and diplognaths. In the place of where we would expect “fish-like” lifeforms were the entelocranians, which were diplognathans with strong masticating jaws, an armour-clad body and a tadpole-like swimming tail. These were hunted by giant ortholiths with crocodile-like jaws. Among the benthic life were the crown-onychognaths, which diversified into a variety of crustacean-like forms and weird creatures like the waterbirdesque tyrrallidae. The periostracans also further diversified. Reefs consisted largely of conchocaudatan colonies and polyfractarians.

A cancrisuchian

Among the earliest meadows formed the first terrestrial ecosystems. Among the first arezoans to go on land were groups like the onychognaths, spirifers, trichordates, urocephalians and hemicalyxians. Especially onychognaths saw an early success and large diversification, as they were among the very first Martians to have evolved legs and were therefore pre-adapted for terrestriality, much like the arthropods were on Earth. Unlike arthropods, they were not physiologically restricted in size and by the Late Argyrian we can already find crocodile-sized forms. These were the Cancrisuchia, amphibious creatures which evolved their first limb pair into a prominent pair of raptorial chelae.

A fractarian chiropede

At the end of the Argyrian a significant mass extinction occurred, which largely affected life in the sea but not on land. About 70% of marine species vanished, among them all entelocranians and pterostomians, as well as various aspiderms, ortholiths and spirifers. Conchocaudatan reefs also crashed, while fractarians and periostracans remained strangely unaffected. On land the large cancrisuchians, the Magnastracia, died out as well, leaving only the smaller Microchelia behind. What exactly caused this event is not discernible. It might have something to do with the earliest stages of the formation of Tharsis, as the mantle plume that originally created the Great Dichotomy now moved northwards and punched through thinner crust. The resulting uptake in volcanic activity could have resulted in another climate change many organisms were unable to cope with. But we cannot really say at this stage.

Isidian

Though the climate was apparently colder than in the preceding period, life recuperated in the Early Isidian and on land the first forests appeared. These were characterized by the first large heliophytans, whose wide distribution suggests an abundance of sulphurous molecules in the early Martian atmosphere. In some places, however, forests seem to have been dominated by large spongisporians, whose rhizomes, perhaps in symbiosis with macroareonts, created a vast root network that seems to have carpeted large swathes of Mars in a dense, sponge-like material. Ambulostellia, who had made the step onto land by this time, also became the first large planimals, resembling in some ways the giant wanderstalks of the later Hylozoic. These strange, shifting spongelands were inhabited by giant chirorbites and ududomids, a group of car-sized spirifers, which in turn were hunted by megafaunal urocephalians and craniopods. These seem to have profited from the extinction of the Magnastracia, but their day in the limelight would not last long. 

 

An urocephalian

An archaic craniopod

The microchelians quickly evolved again into large “crocstaceans” to fill the gap left behind by their relatives. At the same time evolved among silverfish-like archaeocephalians the first “Tagmasauria”. This is an informal catch-all term for any of the megafaunal onychognaths that would evolve in the late Thermozoic. Tagmasaurs largely abandoned the crustacean morph and instead developed increasingly more towards reptilian bodyplans. This they did at the expense of the other taxa at the time, outcompeting all the previously mentioned clades back into microfaunal niches. By the beginning of the Late Isidian, all landmasses on Mars were firmly Tagmasaur Country. Isidian tagmasaurs were all a polyphyletic bunch of archaeocephalians. Among them were long-necked amphibians like the Torneriosauria and giant armoured herbivores like the glyptosaurs. These were hunted by the six-legged Pantelopoda, which had modified their cheliceres into a mouth apparatus that resembled mantis arms. Bipedalism had also evolved among these first tagmasaurs, with some forms evolving strange grappling arms. This composition of tagmasaurs would reign until the Earliest Cydonian.

 

A glyptosaur

 

A pantelopod tagmasaur

Among the smaller onychognaths another major radiation was occurring, that of the Stultusauria, which split into the salamandrine Urusauria and the insectoid Dodecapoda. Somewhen in the Mid-to-Late Isidian the first Cuneocephali evolved, likely among the Urusauria.

 A chelonichthyan periostracan

In the oceans, marine life was increasingly dominated by large tyrallids, who had been doing surprisingly well since the end of the Argyrian, despite their leg-stroke swimming technique. They now had to contend with large, streamlined and fast periostracans, which had become the fiercest marine predators the planet had seen up to that point. A small group of periostracans, the Manupterygia, made their first amphibious steps onto land during the Isidian, along with the first terrestrial polyfractarians and shellubim.

Cydonian

Global temperatures seem to have increased again by the Cydonian, the last period of the Thermozoic. Perhaps thanks to the actions of herbivorous tagmasaurs, the spongelands were now largely replaced by mixed forests of arthrophytes and the first polyfractarian scale-trees. The archaeocephalian tagmasaurs were now joined by the first megafaunal cuneocephalians and the two shared the world, until some turnover in the Middle Cydonian led to the extinction of many archaeocephalians. Among this new radiation of tagmasaurs were the large amphibious lurdusaurs and oloropods, as well as the armoured atenosaurs. These were predated on by the bipedal arachnosuchians, who captured their prey with spear-like forelimbs. By the Middle Cydonian also evolved the very first deltadactylians, though they would remain in the shadow of their cousins until the very end of the period.

 

 

A lurdusaur

 

An oloropod tagmasaur

Throughout the Cydonian, the first terrestrial periostracans gradually evolved from amphibians to dry-skinned egg-layers and then, towards the tail-end of the period, into small, arboreal creatures with elevated metabolisms and possibly even fur. Our current data suggests that, up to this point, they were one of the few taxa on Mars that evolved true endothermy. In contrast, all of the tagmasaurs, as indicated by bone histology and predator-prey ratios, seem to have instead functioned as ectotherms, with the largest perhaps being mesotherms. This is somewhat suspect and has been used to suggest that the atmosphere of the Thermozoic did not yet have enough oxygen to allow giant endotherms to exist.

 

An atenosaur

An arachnosuchian

The end of the Cydonian is marked by a massive extinction event. All tagmasaurs vanish completely after having reigned supreme for millions of years. Together with them disappeared the cancrisuchians, urocephalians, tyrallids and ambulostellians. Conchocaudatans, which had slightly recovered since the Argyrian, also went nearly extinct, as did the arthrophytes. 

 An early arboreal periostracan

The causes for this extinction are unclear and it remains a point of contention whether this change was gradual or drastic. Those who argue for a sudden event believe that it was an asteroid impact which killed the Cydonian megafauna, much like what happened to the dinosaurs on Earth, with the culprit being the bolide that created the Lomonosov crater. However, the formation of said crater cannot conclusively be dated to the C-A-Boundary and the claims of iridium found at the boundary are contested. A layer of black ash in some locations has been tentatively dated to the end of the Cydonian, but contains no traces of iridium. Its composition has instead been described by some researchers as “fossilized fallout”, speculated to be volcanic in origin, though the discovery of potential trinitite in the layer has led to wilder hypotheses.

Those who argue for gradualism have come up with multiple explanations. It is undoubtable that volcanism at the end of the Cydonian ramped up significantly, as Tharsis began expanding. Tharsis Tholus and Arsia Mons most likely began forming during this time, as did an entirely new volcanic province in Elysium Planitia, which now began growing out of the sea as a new landmass. The released volcanic aerosols, such as sulphur dioxide, could have darkened the sky in multiple pulses across millions of years, leading to a series of ice ages, which the ectothermic life of the time would have been unable to cope with. The erosion of these new volcanoes, as well as the orogeny in Solis Planum could have also drawn significant amounts of carbon dioxide out of the atmosphere.

 Mars during the late Thermozoic. Original poles are again indicated by stars.

The change in atmospheric composition itself could have also contributed to the extinction, though this idea is not mutually exclusive with a volcanic ice age cycle. After the population crash of the arephytes, polyfractarians quickly took their place and became the main constituents of the global flora. As these, unlike the arephytes, practiced aerobic photosynthesis, this could have led to a significant oxygenation event on land, in which even more carbon dioxide and methane was removed from the atmosphere, while at the same time the metabolisms of the predominantly hydrogenotrophic lifeforms became disrupted. More oxygen in the air, especially coupled with the probable fact that hydrogen was already a major constituent of the early Martian atmosphere, would have also led to many more forest fires than had happened earlier in the Thermozoic. Though this would have obviously become a self-regulating factor in the spread of the scale-trees. Another possible cause could have finally been the partial breakdown of the planet’s magnetosphere, which the growing ozone layer of the planet took time to compensate for.

Athabascan

Life recovered only slowly in the Athabascan, the first period of the Hylozoic era. The Hylozoic is named that way for its large amounts of fossil wood, which came from dead scale-trees, as well as the Dendrotorres and Curatotorres that took the place of the vanished ambulostellians, becoming walking forests. The era is marked by lower temperatures than the Thermozoic, on average perhaps comparable to what Earth experienced during the Pleistocene. The ice caps of Mars likely first developed during the Athabascan.

 

A cuneouran

 

An early nothornithe

The first megafauna to evolve after the extinction were the Cuneoura, a group of rather baroque periostracans that used their fused tails as large skids to slide over the ground. These were soon joined by the very first Nothornitha, which quickly came to dominate Mars towards the end of the period.

An archaic goniopod

Onychognaths as a whole did recover, but (on the mainland) would never reach the huge sizes of the tagmasaurs again. Among the largest were the mangalasaurs, a group of crocodile-like cuneocephalians that took the place of the vanished cancrisuchians and even evolved marine offshoots. The exception to the rule was to be found on Elysium, which by now had developed into a gigantic island that may have been separated from the rest of Mars by sea currents. Here, a few small deltadactylians, who had only a very brief stint at the very end of the Cydonian, found refuge and in their splendid isolation were able to develop into a unique megafauna. Among these were the first goniopods as well as the extinct eborotheres.

First appearing during this period were also the Nergalacantha, a type of proteroareozoan plankton that built its shells out of either calcitic or siliceous materials.

Candorian

 

 Mars during the Early Hylozoic. Original poles are again indicated by stars.

The Candorian was the true nothornithe age, with many of the carnornithes, segnornithes and rhynchornithes rivalling the sizes of Earth’s dinosaurs. Temperatures remained cool, but the air was highly oxygenated, likely much more than today, the growing luminosity of the sun assured more climatic stability and the now shallow boreal ocean was at its most productive. Scale-tree forests were thriving across most of Mars. 

 

A carnornithe

 

A rhynchornithe

 A segnornithe

In said forests, a new type of animal evolved among more archaic periostracans that had remained arboreal. These were the pedicambulates, who now either took to the air or came down from the trees, though they did not yet play a major role in the Candorian.

The period took an abrupt end. The build-up of Tharsis had by now accumulated enough mass on top of the planet that it began deforming the crust. Along fault-lines that were created eons ago by the Hellas impact, the ground gave away and opened up into a series of massive cracks that cut right across the planet. Today we know this scar of Mars as Valles Marineris. Its creation must have released large amounts of volcanic gases into the atmosphere, in addition to the still ongoing Tharsis and Elysium orogenies. The mass of Tharsis was now also so large that it began affecting the rotation of the planet. True polar wander occurred, shifting the planet’s crust by 20 to 25 degrees so that the plateau’s center of gravity would lie directly on the equator. This movement is recorded by the suspiciously straight line that runs from Arsia Mons in the south through Pavonis Mons and then Ascraeus Mons in the north. Both developments must have majorly disrupted the climate.

Consequently, many of the large nothornithes went extinct at the end of the Candorian, with the rhynchornithes vanishing completely.

Kaseiic

Nothornitha as a whole survived past the Candorian, with either the carnornithes or segnornithes giving rise to the new group Avidonta. But they could never replace their losses, which was seemingly exploited by the pedicambulates, who now gave rise to giant tripodal forms, both herbivorous and predatory.

A pedicambulate

The Early Kaseiic was characterized by the formation of Olympus Mons and Alba Mons, the planet’s last and greatest volcanoes. Whatever kept the greenhouse effect alive seems to have now been slowly losing the fight to the volcanic aerosols, while atmospheric loss became more and more of a problem. As the ice caps encroached towards the lower latitudes, the sea level lowered. By the Middle Kaseiic, it sank enough to create a land bridge between the great southern continent and Elysium, finally breaking its isolation. What followed must have been very similar to the Great American Interchange. Although some deltadactylians managed to survive and thrive on the mainland, most of them went extinct, apparently unable to handle the southern immigrants. 

 Mars during the Late Hylozoic.

The Late Kaseiic saw the large-scale decline of sea life, as the shallows receded further and further, while life on land became increasingly more restricted to the lower latitudes. Finally, the Hesperian ended with the complete disappearance of the Oceanus Borealis and life on Mars would never be the same again.

Amazonian

Despite being the eon we still experience on Mars, we know the least about developments during the Amazonian. Barely any fossils are known from between the end of the Hesperian to today. What this at least tells us is that this was a time of very strong erosion across most of the surface of Mars, meaning that recent fossils only had a short lifetime before being destroyed.

Another thing we can say with some certainty is that this was the time during which Mars finally acquired its characteristic red colouration. Yes, Mars was not always red. Most of its bare volcanic rock is actually black, while the carbon dioxide air is inherently colourless and should therefore simply appear blue due to light refraction. All of Mars’ redness is caused by the constant dust that now veils its surface and sky. Almost all of this dust, we now believe, comes from the Medusae Fossae Formation, which lies midway between Elysium and Tharsis and seems to have been created as an Amazonian accumulation of ash and pyroclastic material from both volcanic provinces. As the winds of Mars erode and abrade this formation into tiny dust particles, they collide with each other at high enough energies for chemistry to occur. Over millions of years this turned siliceous ash and magnetite into haematite, which eventually turned all of Mars red.

Mars today

Apart from that we cannot say much about the Amazonian. Did some of the Hesperian megafauna survive into this eon? Did the inland sea of Hellas hold out longer than the northern ocean? Were there intermittent periods where orbital changes made Mars more habitable than it is today? We suspect so, but truly know it, we do not. Which brings us to our next problem:

The Dating Problem and the Long-Amazonian vs. Short-Amazonian debate

As you probably noticed, I have not given any firm dates for any of the periods listed here. That is because I have no firm dates to give. The sporadicity of geological and paleontological work on Mars means that not enough samples have been taken to firmly date the whole geological column on Mars. In addition, cosmic radiation and isotopic differences in the Martian atmosphere have made it difficult to use the same radiometric methods that work on Earth without major distortions. First attempts at radiometrically dating fossils from the Late Kaseiic have all been met with bizarre anomalies that render the results useless, almost as if the aliens had all somehow died in Hiroshima. Much like Victorian era geologists, we are in the dark about the actual age and length of Mars’ time periods.

When dating is attempted with other methods, such as crater counting or luminescence dating, they return some staggering results. It is largely assumed that the Hesperian and Amazonian largely coincide with Earth’s Phanerozoic, meaning they extend only a couple of hundreds of millions of years into the past. However, according to some of the recent works done with these methods, the Noachian eon seems to have ended 2.7 billion years ago and the Hesperian a shocking 2 billion years ago. If this is indeed true, which I highly doubt, it has some rather stupendous implications:

• Complex multicellular life evolved on Mars much earlier than on Earth and very soon after the appearance of the complex cell. This is at least somewhat tangible. The rhodokaryote cell evolved gradually and not through a chance event like the eukaryotic one and may therefore have also evolved much earlier. The earliest multicellular fossils on Earth date back as far as 2.4 billion years ago (Bengtson et al. 2017), which is only 300 million years removed from the appearance of the first eukaryotes (Knoll 2003), but these early radiations seem to have frequently fallen prey to events like the Snowball Earth phases and could not thrive until the Ediacaran. In contrast, Mars may have simply had more luck with its first multicellular radiation.
• Much more challenging is the fact that the lifeforms which exist on Mars today are evidently descended from survivors of the Hesperian, but have not changed all that much in their overall morphology. Over a span of 2 billion years, this is preposterous. It is perhaps somewhat believable for the more conservative forms like mollizoans, but nothornithes and deltadactylians are large, active animals, which we know reproduce, adapt and radiate fast and regularly. It is simply unbelievable that they would have stayed this conservative in their morphology. Samuel Leidy, a proponent of the Long-Amazonian-Hypothesis, has attempted to explain this by suggesting that Amazonian Mars may have gone through multiple long periods of deep freeze, in which most life hibernated, essentially halting the process of evolution until the planet thawed again. In addition, he argued that the nothornithes and deltadactylians that we see today are not as directly linked to their Hesperian forebearers as they seem, but instead convergently evolved out of more archaic Amazonian survivors. Even then, the timespans involved still make this scenario hard to believe.

On the lack of intelligence

Originally, I did not intend to write about this, as it seems like a non-issue, but some people may still ask about this. Apart from Palmer’s claim of supposed tool-use in the Kaseiic Psittacanthropus (which is not at all accepted by the scientific community and has more in common with the Victorian era discussion around eoliths) and of course those infamous canals, there is no evidence that any intelligence ever evolved on Mars that would approach that of Man. If Mars had complex life for about as long as Earth did, this might seem strange to the average reader, but there is really nothing in the mechanisms that govern the process of evolution that would dictate the emergence of intelligence. This becomes clearer the farther out we go from both Earth and Mars, as we come across either hot or cold desolations, where the only forms of intelligence we find are too alien and hostile to create any meaningful bonds with. It is perhaps Man’s destiny to ponder all the questions he asks alone.

References

• Bengtson, Stefan, et al.: Fungus-like mycelial fossils in 2.4 billion-year-old vesicular basalt, in: Nature Ecology & Evolution, 1, 2017.
• Glaser, Etienne: Reanalysis of the Problematica of the Dao Vallis biota, with special reference to the ichnofossil Daorepichnia, in: Strate Station Geological Journal, 466, 2302 p. 78 – 90.
• Jerome, Craig; Kim Hyo-Joong; Mojzsis, Stephen; Benner, Steven; Biondi, Elisa: Catalytic Synthesis of Polyribonucleic Acid on Prebiotic Rock Glasses, in: Astrobiology, 22, 2022.
• Knoll, Andrew: Life on a Young Planet. The First Three Billion Years of Evolution on Earth, New Jersey 2003 (Second Paperback Edition).
• Leidy, Samuel: The Long-Amazonian Hypothesis. Reconciling Martian dating results with the morphologies of modern lifeforms, in: Journal of Astropaleontology, 611, 2340, p. 89 – 111.
• Palmer, Robert: Tool-using culture in Psittacanthropus, in: Journal of Astropaleontology, 607, 2333, p. 55 – 99. (Since retracted)
• Sivgin, T.K.: Life on a Dead Planet. The first 3 billion years of Evolution on Mars, Zürich 2345.