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.

Chiropede Shrubs

Most of the common chirorbites  live lives not unlike arezoans. They are born from simple eggs or through live birth after two parents mate and impregnate each other and go on to live by feeding on smaller animals or plant matter. But not all of their members have forgotten their roots in what is actually Mars’ flora.

One sub-group of the Chirorbita reproduces in a unique way. The chiropedes, which have elongated bodies and often only a single eye, begin their lives as plants. When two adult chiropedes mate, they lay an egg into the ground which hatches into a tiny, worm-like larva that buries its head into the soil and becomes a sessile organism, whose tissues house photosynthetic zooxanthellae. As the frond of this plant grows, it begins producing leaves at its tip, which eventually form into a fan-like canopy. As some of these leaves mature, they begin growing a hardened tunicine exoskeleton, feelers and eyes. Eventually, some of them devour their own zooxanthellae, detach and begin life as their own independent animal, soon about to repeat the cycle. Most chiropedes are herbivores and feed on photosynthetic flechtoids, which is probably where they acquire the photosynthetic cells for the next generation from.

In some ways this bears similarity to the reproductive cycle of the unrelated skolex, but in them the sessile stage is diploid and the mobile one haploid, making them alternating generations like in earth-plants, whereas in the chiropedes both forms are diploid. The difference can be understood in simple terms as follows: If you were a male human and reproduced like a skolex, your sperm cells could undergo mitosis by themselves and become independent organisms once released. If you reproduced like a chiropede instead, it would be your whole penis detaching from your body and becoming independent.

While this seems extraordinarily alien, it is really not much different from the reproductive cycles seen in our own oceans’ cnidarians. There, free-floating jellyfish fertilize eggs, which hatch into a planula larva. Said larva attaches to the seafloor, becoming a polyp. As it grows, the polyp produces more jellyfish in a process called strobilation. Some parasitic flatworms, the cestodes, also reproduce through strobilation. The chiropede clade thus derives its scientific name, Strobilata, from this well-known process.

The existence of Strobilata poses a lot of phylogenetic questions for the Chirorbita. Some studies suggest that the clade might actually be paraphyletic, chiropedes being the ancestors to the more derived euchirorbites, like the spectacled chirorbite. If true, this would mean that strobilation is actually ancestral to the clade but was lost later on in some lineages in favour of a more direct reproduction. This is supported by the fact that some of the more basal pseudarticulates (though not all of them), like the menamin, also reproduce through cestode-like budding.

For the family tree of the Fractaria phylum as a whole, other fascinating possibilities open up. It is generally thought that pseudarticulates and polyfractarians, which are clonal colonial organisms, are only distantly related, sharing a common ancestor among simple, monovexillan fractarians. But two new competing schools of thought have appeared in recent years. One proposes that the pseudarticulates actually derived from polyfractarians whose gonosphores became independent from the whole organism. The second, more popular one, is that polyfractarians derived from a basal strobilating pseudarticulate whose organs one day stopped detaching from the polyp and instead started working together as a single sessile organism, eventually losing all complex traits like eyes, guts or a nervous system in the process. Actual evidence for either position has not yet been gathered.

On a final note, it is interesting that, while still attached to its polyp, a chiropede still has a nervous connection to the polyp and the surrounding chiropedes. One wonders how it must feel like in the final stages before detachment, when the chiropede already has eyes and is wriggling, to be your own being and yet still be part of a larger one. If your hand could think, what would it think?

Hellas Savannah

Hellas Planitia is likely the most distinctive region of the red planet. It is a giant basin that spans 2300 km between its widest ends and its depth reaches over 7 km below the planet’s datum (its equivalent to a sea level). It is large enough that early astronomers could already see it from Earth with their telescopes. Billions of years ago, when the rocky planets were still in their formative phase, the basin was formed by a giant impact, one of the largest in the solar system’s history. As the young and violent Mars entered its aqueous phase, water soon accumulated and Hellas Planitia became home to a vast inland sea, as confirmed by local stratigraphy. However, this sea vanished so long ago that now, even beneath the deepest points of the basin, can be found the petrified remains of land-living animals lying in desertous sediments dating back hundreds of millions of years ago. When the Hellas Sea vanished, it left behind only a vast salt flat, which, after being bombarded by thousands of years of UV-radiation, must have become the largest perchlorate desert that ever existed in the solar system. But even this reign of the toxic sands came to an end. The continuous growing and melting of the ice shields and glaciers of the southern highlands has flooded the basin for hundreds of thousands of years with crushed stones and debris. Over the course of millennia, the salt desert has been buried by deep layers of gravel, on which eventually could grow fertile soil again.

Fig. 2: Extent of the Hellasic Savannah climate. Note that the warmer savannah region is bordered by a ring of more temperate shrubland before transitioning into tundra.

Thanks to a multitude of factors, the Hellas Basin is now the most biodiverse region of the planet and likely represents the last remnant of an otherwise lost ecology. The two main factors that facilitate this condition are the basin’s depth and geography. The region lies so deep beneath the Martian datum that at its bottom, the air pressure is 103% higher than on the average elevation of the planet, or in other words, 1.035 bar, which is actually slightly higher than the average on Earth. Such a thick carbon dioxide atmosphere results in an amplified greenhouse effect, creating temperatures that are much more comparable to those found in the great northern deserts than to those in the tundras which surround the basin. But what distinguishes Hellas from the Boreal Desert is its humidity and vegetation. While rainfall is nearly nonexistent on Mars, the Hellas Basin manages to stay well-hydrated in a quite simple way. Being surrounded on all sides by frozen tundra, the basin becomes a natural drainage area when the top layers of the permafrost thaw every spring. In the basin, the waters seep into the soil and, like in the Swiss Mittelland, swell up the deep layers of ice age gravel, becoming excellent aquifers. Below much of the basin is thus an extensive and quite high water-table that plant-like lifeforms can access throughout the inundation season. Unlike anywhere else on Mars, the Hellas Basin can thus support a savannah-like vegetation-cover. Its dry climatic condition supported by a high water-table is somewhat comparable to parts of the Gran Chaco or, perhaps more aptly, the Late Jurassic Morrison Formation.

The thick air suspended in low gravity also has other ecological influences. Only here are there still clouds of aeroplankton thick enough to blot out the sun. These consist of shellubim- and wadjet- larvae, a multitude of microflier onychognaths, spores and gametes of various sessile organisms and even algae-like macroareonts suspended on tiny balloon-organs. Many of these organisms are fascinatingly bioluminescent, making for spectacular night skies. These clouds support both the wider aerial and terrestrial ecosystem. The perhaps most distinctive type of flora in this savannah are the giant tube-trees, who are among the largest living organisms on Mars. These are spongisporians, which function like a mix between giant lichen and land sponges. In their tissues live various endosymbiotic microbes engaging in photosynthesis, fuelling the huge organism’s resting metabolism in exchange for shelter. But chiefly, these tube-trees feed by filtering the air for aeroplankton with their tube-like outgrowths. Inside the organism are vast canals and tubes manned by rows and rows of hair-like setae, whose motions produce a continuous airflow in and out of the body. Once trapped in this flow, the aeroplankton is siphoned into a cauldron-like cavity to be slowly digested by mild acidic fluids. A long and agonizing death.

The giant tube-trees themselves are important for various other organisms. Various trichordate and spiriferian spongivores feed on their squishy, porous skin, which quickly regrows.  In some areas, the tube-trees grow in dense groups, creating reef-like islands in the middle of the savannah, on which various organisms live and roost.

The other mainstay of the savannah are the scale-trees. Although they resemble coniferous plants from Earth, they are internally quite different. These are fractarian organisms, more specifically polyfractarians. Each tree is actually a clonal colony of multiple individuals, called fractophores, working together as one organism. The condition is somewhat comparable to a Portuguese man o’ war. The tree begins life as a single individual growing from a spore. This is the genophore, from whose bottom then grow multiple connected clones, who develop into rhizophores that build up a root system. From the top of the genophore then grow in an alternating pattern the dendrophores, which build the stem, branches and leaves. Once mature, the top-most dendrophore produces gonophores, whose sole task is reproduction.

Most fascinating about the scale-trees is their solution to transport. Instead of transporting water and nutrients through something akin to a xylem, almost every fractophore possesses a heart-like organ that slowly pumps the fluids through the body. Standing close to a tree, the slow heartbeats of these large organisms are actually audible. Combined with all the other trees and animals across the savannah, this makes for a truly unique soundscape:

This trait of the scale-trees is fascinating for multiple reasons. Fractaria do not ancestrally have muscle-tissues, though they do have placozoan-like precursors to such tissues, which pseudarticulates evolved into true muscles. It would be of high interest to investigate if the muscle-like tissues which power the polyfractarian hearts are homologous with similar tissues found in pseudarticulates or if it is an entirely independent development that arose out of shared building blocks. Furthermore, recent studies (Bomhoff 2339) have found that the fractophores are capable of coordinating their heart-rates as well as fluid-flow in unison across the whole tree’s body. How they are capable of doing that despite not possessing nerve-cells is unknown and demands further inquiry.

Another mystery of the scale-trees pertains to the reproduction of some species. The gonophores of most scale-trees reproduce primitively through exchanging airborne gametes, which then develop into airborne spores that grow into new genophores. However, in some rare species found across the savannah, the spore becomes encased in a woody shell over which then grows a sponge-like coating. These often coconut-sized “eggs”, as they are informally called, then fall on the ground, where many of them rot without developing into a new genophore. There is a distinct possibility that what we are looking at here is the Martian version of a fruit or nut, but there seems to be no animal large or willing enough to eat and disperse these organs, which is maybe why these egg-bearing trees are so rare.

There are other such ecological anachronisms found across the savannah. Many of the giant tube-trees have defensive spikes, derived from their spicule skeleton, growing across their whole height, which seem like good deterrents for large herbivores. Some shellubim also have defensive toxins of such high potency that they seem like overkill for most living animals that could still step on them. It seems that not too long ago, the savannah was still home to megafauna, but it has all vanished, leaving behind a dwarf fauna. This could easily be linked to the worsening conditions of the planet as a whole, but, puzzlingly, calculations done on the vegetation cover indicate that the floral biomass of the Hellas Savannah would theoretically still be capable of supporting populations of much larger animals than can be found today (Schröckert 2340) (the caveat here being that those calculations had to estimate many variables using values from earth-ecosystems, which might not accurately reflect Martian ones). The true cause for this absence therefore remains mysterious. Possibly, the savannah recently went through a harsh dip in habitability, with the flora being able to recover again to previous levels today, while the now impoverished fauna has not kept up. Or this last megafaunal extinction was caused by non-ecological factors, such as suffocation by the gigantic global duststorms, which may have only developed in the last couple of hundred thousand years. Widespread disease caused by limited space may have also been a factor. Or our calculations and observations are simply wrong. Those adaptations we think are ecological anachronisms could simply serve an entirely different purpose that we currently do not realize. Tellingly, there is little direct physical evidence of recent megafauna in Hellas Planitia. The best we currently have is a large fossil scolecodont from some kind of predatory periostracan found in Hellas Chasma, dating back to around 430’000 years ago (Sivgin 2345). From around the same time is a fossil trackway in Hamakhis Vallis, which attests to the existence of a roughly giraffe-sized nothornithe (Krätschmer 2122). Both the tooth and the ichnofossil have frustratingly never been analysed in detail again since their original discovery.

References:

• Bomhoff, Nils: Coordinated flow of coelomic fluid in Titanofractus lanali. Implications for polyfractarian physiology, in: Areobiology Magazine, 67, 2339, p. 28 – 40.
• Krätschmer, Simon: Description of a nothornithe trackway from Hamakhis Vallis, in: Strate Station Geological Journal, 460, 2122, p. 1456 – 1496.
• Schröckert, Daniel: How much can the Martian savannah support? Outline and limits of modelling extraterrestrial ecosystems, in: Astrobiology Magazine, 704, 2340, p. 11 – 23.
• Sivgin, T.K.: Life on a Dead Planet. The first 3 billion years of Evolution on Mars, Zürich 2345.