“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 CO2-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
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.
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.
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.
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.
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.
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.
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.
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 been 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.
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 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.
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
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.
In said forests, a new type of animal evolved among more archaic periostracans that had remained abroreal. These were the pedicambulates, who now either took to the air or came down from the trees, through 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.
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