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?