“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
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 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.
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 abroreal. 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.
