Friday, 24 September 2021

Species Profile: Riesenkriechhecht

The genus Atractosuchus is widespread across Dome 4, being represented by at least five species that come in varying sizes and forms. These animals I named Kriechhechte, as they greatly resemble gars in the general shape of their heads, but this is just a case of convergent evolution. As their limbs show, these fish are actually lobe-finned and finer anatomical details suggest they might descend tetrapodomorphs, most likely Rhizodontida. While rhizodonts were fairly primitive members of our family tree, the Kriechhechte seem to have convergently evolved some characteristics of true tetrapods. Most conspicuously, the pectoral fins have adapted into a sort of proto-arms capable of raising the fish out of water, though instead of true hands with fingers they still have lepidotrichs. More subtle is the fact that the shoulder-girdle is separated from the skull, giving the animal a true neck. The rest of the body is very conservative by comparison, preserving the general shape of the Paleozoic sarcopterygians.

The most well-known member is the Riesenkriechhecht (Atractosuchus gigas), whose size can vary between 3 and 6 meters, comparable to a saltwater crocodile. Riesenkriechhechte are found in almost all wetland areas of Dome 4, where they act as large riparian predators. While a lot of their prey consists of smaller fish, a common hunting tactic for them is also to ambush terrestrial prey from the water. It is interesting that amphibious fish occupy a niche in Dome 4 that on Earth was usually occupied by large amphibians or reptiles. The dominance of terrestrial placoderms and the lack of tetrapods may have precluded the evolution of crocodile-shaped predators out of terrestrial creatures for a long time, thereby opening this niche to be exploited by these creatures. The fossil record might be telling a different story, however. The strata show that kriechhechte of essentially modern form have existed in Dome 4 for at least 300 million years, but for the majority of that time they were rather small and rare. The role of large, semi-aquatic ambush predators was instead  occupied by haruspicamorph placoderms. About 80 million years ago, however, large, sometimes very large, atractosuchids start appearing, while haruspicamorphs gradually decline. This competition may have been what initially forced haruspicamorphs to adapt to land again in the form of munchers. 70 million years ago a mass extinction swept away all aquatic haruspicamorphs, while atractosuchids went relatively unharmed. For the following millions of years they would rule the waterways unchallenged. Today riesenkriechhechte do however face a perhaps relatively new competitor in the form of the crocomire, which, as big and bulky as it looks, seems to be another placoderm-answer to this riparian lifestyle. As a fully grown crocomire is too big and armoured to be harmed by an Atractosuchus, the two usually leave each other alone, but during mating season crocomires become excessively moody and aggressive and will often let out their anger at kriechhechte that were just minding their own business. In turn the kriechhechte will prey on juvenile crocomires and injured adults. Some kriechhechte have been observed deliberately killing baby crocomires, even when they were close to their mothers or easier prey was available, suggesting that they instinctually sabotage the reproduction of their competition. Crocomires cannot really do the same in turn. Kriechhechte are ectotherms who can lay thousands of eggs upon spawning, while crocomires are mesotherms who give live birth to only a handful of young, so crocomires cannot even dent the population sizes of kriechhechte. The fiercest competition a riesenkriechhecht faces is in reality from its own kind. Fights over carcasses can become very brutal and cannibalism is indeed quite common. On occassion some kriechhechte have nonetheless been observed working together to drown a larger animal, a behaviour reminiscent of Nile crocodiles.

Riesenkriechhechte spend a surprising amount of time out of the water basking on the shore, shifting around with their strong pectoral fins and breathing with their well-developed lungs. They mostly do this with their mouth open so that their teeth and jaws may be cleaned by celaenosaurs and their hide picked clean off parasites by stoutbills (a type of chimaera fly). Despite its terrestrial capabilities, A. gigas never ventures too far from the water and unlike its smaller relatives it is difficult for it to relocate to a new body of water should its old one dry out. Unlike crocomires, who can just walk from place to place, the range of riesenkriechhechte is therefore limited to stretches of water that stay wet all-year-round. In contrast, they do seem to be able to cross saltwater, as they are found on nearly all landmasses in Dome 4.

Thursday, 16 September 2021

Species Profile: Zygoceros

Zygoceros pentacornis is among the most impressive megafauna found so far in Dome 6. The Zygo is part of the krossosaur clade Pachypoda. Pachypodes have an elevated, in some forms truly endothermic metabolism, strongly differentiated teeth and some give live birth. They might be described as the most mammal-like among the krossosaurs and therefore the creatures on the Rhynia, though they nonetheless lack features that would make them comfortable to us, such as fur, external ears or a rhinarium. Especially large, reptilian forms like zygos are perhaps more comparable to ancient therapsids, such as dinocephalians. 

Zygoceros is a browsing herbivore of the bushlands. With a length of 8 meters it is among the largest terrestrial creatures found on the Rhynia, twice as large as a white rhinoceros and well in the size range of some dinosaurs. The genus name derives from its two large, downward-pointed horns growing from the cheekbones. These are used for a variety of tasks. Zygos have been observed plowing the dirt with their horns in order to dig out tubers and roots, but they also use them to fend off predators and even other herbivores encroaching on their personal space. Both males and females possess these horns and were observed using them in territorial disputes. The species name pentacornis derives from the five odd ossicones found on top of the head. Four surround the pineal eye and look very similar to those found on the heads of giraffoids, while an additional Y-shaped protuberance grows from the nose right behind the beak. This strange horn is similar to that of ancient brontotheres. While only the male zygos possess this, it is almost never used in fights, as the structure is made of bone, not horn, and could easily break given the forces these animals can exert. The antler instead acts as a visual signal, with the animal capable to change its colour. It does this with a set of chameleon-like chromatophores controlled by hormones. Such chromatophoric communication is found in some form in various other pachypodes and even parasaurs, such as the headcrest of the arachnosaur, hinting that this may have been an ancestral trait of the cotylosuchians. 

Zygos feed off leafy and shrubby plants, fruits, shoots and sometimes even thorny bushes. They live in bushlands or close to the edges of rainforests and tend to avoid the fern prairies and especially the land-reefs, as their jaws are not adapted to eating hard-shelled brachiophytes. Zygos lead largely solitary lives. They do not have a clear mating season where they congregate, instead mating just occurs when a male and female zygo meet, find each other suitable and the available resources allow it. Afterwards the female lays a clutch of eggs into a mound-nest and closely guards it. While the male generally leaves the female alone after mating, it is notable that it still stays in the general area of the nest (though at a distance) for a couple of months, hinting that it might still be involved with guarding the nest. After hatching, the young closely follow the mother around for a few months until they are able to protect themselves.

Monday, 13 September 2021

Clade Profile: Carrion Trilobites

In some respects, the evolution of terrestrial trilobites likely was easy, in other ways hard. Like their cheliceratan uncles they were able to evolve simple book-lungs by invagination or their gill-arms into their body. However, what greatly held them back from terrestrial niches is the lack of true mouth-parts, compared to the wide range of mandibles and cheliceres of their cousins. The closest thing they have is a hypostome, which acts more like a backwards-pointed spade or shovel rather than a jaw. This is not a big problem when living on the ocean floor, where microscopic detritus constantly rains from above, but land-life does not grant such luxuries. As the myriapods prove, even feeding off leaf-litter requires mandibles. Facing jawed competition from not only centipedes and arachnids but also the great barage of insects, it is perhaps for this restraint why trilobites on Earth were never able to make the big leap onto land, dooming them to an oceanic existence where they would eventually meet their end once marine environments became near-inhabitable in the Permian. The Rhynia however was apparently constructed and seeded before winged insects had even evolved, removing this monumental clade entirely from its ecologies. As we have seen with the chimaera flies, the insane effect of this is that various other, sometimes unexpected animal groups were able to fill the void once terrestrial ecosystems became more complex. Even the trilobites were able to carve up a few niches for themselves, sometimes literally. In these niches they are actually quite successful, as they can be found in five out of six domes and also the lower decks.

While some of these have adapted to feed on soft-bodied earthworms, the majority of terrestrial trilobites are still detritus-feeders, feeding off the soft excrements and droppings of various larger animals through simple sucking- and shoveling-motions. These basically fill out the same ecological niches as dung beetles and scarabs do on Earth. While this may seem like a pitiful existence from a human perspective, it is actually quite a respectable job, as many a great pathogen and toxin meets their end in the robust stomachs of these critters, keeping the ecosystem healthy and safe from serious disease outbreaks.  Still, at least one group of "terrabites" has taken up a more savoury diet. The carrion trilobites are a special clean-up-crew that appears anywhere where a large vertebrate carcass lies. The little critters possess a strongly reinforced hypostome shaped like a wood-carver. They use this to scrape off meat into easily digestible chunks and they are especially good at removing the last bits of flesh from bones. A large swarm of these can turn an ungainly corpse into a beautifully stripped and clean skeleton in under two hours.

Journal Entry 5: First day in the field

Year: 2548

Date: 01.10, Sunday

Location: Base Camp Jackson

My first day inside Dome 4 has certainly been interesting. The base camp was set up by Expedition Team O'Brien (before my arrival) on the northern coast of the Southern microcontinent, the largest continuous landmass of Dome 4, but on a peninsula very close to the "ziggurat island" which connects the dome to the lower decks. We arrived there by a shuttle, which had to fly from the "Alien City" all the way through the tunnels of the Rhynia, as it was feared that boring through the dome-ceiling might seriously damage the biospheres. The camp consists of a set of tents, elevated metal rooms like the ones they use in Antarctica, a generator and a helipad. It is surrounded by a heavy electric fence to avoid any unprepared, hostile encounters with wildlife.

On their first day back to the camp, Susan and Nuradin wanted to go with me to a nearby zosterophyte clearing and catalogue the wildlife and flora we would see that day. Seeing these plants alone was already an amazing experience for me, as they were organisms that had vanished from Earth over three hundred million years ago. These new alien forms reminded me a bit of bamboo. Our operation was quite calm and uneventful that day, though Ma He still was there with his jeep and weapons just in case. I got to see chimaera flies, a shell-snake and was delighted when I got to see my very first living trilobite. Towards the end of our shift a dwarf thelocaud curiously approached the jeep. It was both the most alien and cutest thing I had seen in my life up to that point. It scuttled around us, keeping a safe distance. It let out sounds I could only describe as burping. As we were about to pack up, I felt something crawling up my leg. It was a banded trilo-slug, Susan said, a terrestrial polyplacophoran. I wanted to look at it but she strongly urged me to stand completely still. She did not tell me why as to not further panic me and we just stood there still, waiting for the slug to crawl down my leg and onto the forest floor again. Afterwards she told me that the trilo-slug can inject a toxin with its radula that can send a human into a coma for three days.

Back at the camp, the data-registration in the afternoon was interrupted by the outside sounds of a metallic clanging, a high-voltage zap and Ma cursing like a sailor. We ran outside and saw an almost comically horrific-looking animal that had just made its first experience with an electric fence.  The animal was already in our database and nicknamed “crocomire”. It is a seven meters long placoderm, possibly a basal eusthenopod, with an elongated maw, interlocking teeth, and large, hippo-like tusks in the lower jaw. The tail is a large paddle with spikes. It looks like a demonic tadpole. Seeing this animal at our camp was unusual. Crocomires are semi-aquatic ambush-predators who spend most of their time camouflaged as logs in murky swamps and waterways, while our camp was in a jungle-clearing close to the coast. Susan would later speculate that the animal’s home-pond may have dried up and it was in search of a new one. 

Despite her gnarly looks, the encounter with the fence seems to have given the big girl quite a shock, as she lied on her side, her legs helplessly paddeling in the air and her large maw letting out bellowing cries, perhaps for help or simply out of pain. Two juveniles stood besides her, most likely her young, nudging her to get up again. While they did, they let our distressed chirps, almost like birds. Thankfully the mother managed to prop herself up and once on her feet again, I think you could really see the anger and confusion in her eyes. She stood there for almost five minutes just angrily growling at the fence and, after we locked eyes, us. The feelings of seeing such an animal for the first time in such a way are indescribable. Back on Earth, almost all megafauna I was familiar with I only knew from books and movies as they had gone extinct in the wild. Seeing an animal of this size, this appearance and this ferocity just out in the wild was like seeing a real life dragon or sea serpent. After she calmed down again, the mother disappeared back into the forest with her young.

Friday, 10 September 2021

Species Profile: Arachnosaurus

The person who named the monkey-spider (Arachnosaurus) probably thought they were being quite smart, as in their eyes the animal seemed like a biological inversion of the spider monkey (Ateles). Of course, Arachnosaurus is neither a mammal nor an arthropod but instead a krossosaur of the basilisk order. Basilisks are a very numerous group across Dome 6 and 5, but genetic testing suggests that despite their primitive looks they are quite a recent group, diverging from the other members of Parasauria about 75 to 80 million years ago. Parasauria, along with their sister-clade Pachypoda, belong to the larger clade Cotylosuchia, which is itself the sister-group to the Cancrisauria. If cancrisaurs are the krossosaur-equivalent of amphibians, then cotylosuchians are best imagined as para-amniotes. Unlike in the cancrisaurs, the head and shoulder-girdle are separate, giving the animals a true neck, and the members reproduce by laying hard-shelled eggs with an amniotic sack or by giving live birth. While basilisks are a relatively new order, they likely represent the general bodyplan that the earliest cotylosuchians would have had, just like how lizards on Earth still broadly resemble the earliest stem-reptiles. 

The same can probably not be said about Arachnosaurus, as it evolved some peculiar adaptations to live as an arboreal ambush-predator. The prehensile tail makes up over half the body length and is used to lower the animal head-down into the air like a lantern. The six legs are relatively short and clustered close together around the squat, round body, but the fingers are disturbingly long and splayed. Cryptically camouflaged among the progynmosperm-canopy, Arachnosaurus hangs in wait for hours with its hands stretched out waiting for prey. This may include chimaera flies or even smaller gargoyles and celaenosaurs that happen to fly through the jungle, though the predator also seems to be content with cancrisaurs and isopods that happen to waddle a few tree-branches below. Once in reach, Arachnosaurus envelops its prey in a deadly hug it cannot escape from and uses its pointy mouth to feed on it while it is still alive. After the first few bites the prey-animals seem like they are paralyzed, suggesting the use of some sort of venom, but autopsies have so far been unable to find venom glands in Arachnosaurus, making this rather mysterious. It is of course still not recommended to closely handle living arachnosaurs without safety precautions.

Arachnosaurus are surprisingly gregarious, often hanging from the trees in medium-sized groups like bats do, though they do not cooperate when it comes to hunting or fending off predators. They make no audible sounds and instead seem to communicate by changing the color of their headcrest. Mating usually entails the male presenting the female with a gift in the form of food. After mating, the female builds a nest in a hollowed-out tree-hole and lays its eggs there. The mother guards the nest and even leaves some food behind shortly before the eggs hatch, but once the young emerge they are left to themselves.

Wednesday, 8 September 2021

Species Profile: Beetle-Fish

If there were such a thing as an "evolutionary ladder", animals such as the beetle-fish (Coleopterichthys) would be only a few steps above the sixfin. Beetle-fish are a species of true krossosaurs inside the larger clade Cancrisauria. Cancrisaurs are the sister-group to all other krossosaurs and, for simplicity's sake, if the krossosaurs are comparable to tetrapods, then the cancrisaurs are the group's equivalent to lissamphibians on Earth. While the beetle-fish lives on land, it needs to constantly keep its skin moist and also lays its eggs in the water, where they hatch into tadpoles that have to undergo metamorphosis. Unlike Earth-amphibians or any other tetrapod, cancrisaurs have no neck, instead the shoulder-girdle directly attaches to the skull like in more basal bony fish. Cancrisaurs, like the majority of krossosaurs, also do not walk with a serpentine motion of the spine like reptiles or salamanders do, instead their six-legged gait closely resembles the walking motion of insects with its alternating double-tripod stance.

The beetle-fish has the size of an average toad and bright colour patches along the back. As could be expected, the colours warn of the animal's toxic skin, though the effects are thankfully quite harmless to humans. Though it is still recommended to wash one's hands after handling a beetle-fish without gloves on. The animal lives in the underbrush of dense jungles, usually hidden among leaf-litter. With its short but stout, pointy teeth it ambushes arthropods of all kinds, from trilobites to millipedes. It is itself eaten by other krossosaurs. Instead of croaking like a frog, the beetle-fish attracts mates through a loud clacking-sound, which it produces by clapping two tooth-plates at the back of its jaws together. Mating occurs like in Earth-amphibians through external fertilization of waterborne eggs. The tadpoles take about three months to develop into fully grown beetle-fish.

Beetle-fish are typical members of the so-called Holospondyli, but there is growing suspicion that this is a waste-basket taxon of various terrestrial cancrisaurs or alternatively a paraphyletic group. The other two main groups of cancrisaurs, the Thalassospondyli and Chelichthyes, are likely monophyletic but probably evolved out of beetle-fish-like ancestors. Thalassospondyls are, sometimes huge, cancrisaurs which vaguely resemble the temnospondyls of Earth, except that the majority of them are marine and neotenically retain their gills into adulthood. An ecological catastrophe somewhen deep in the past of Dome 6 seems to have extinguished most lineages of bony fish, allowing the thalassospondyls to take over their roles and evolve into various fully aquatic forms. Chelichthyes are cancrisaurs which compensated for the clade's lack of a neck by instead evolving their first limb-pair into grabbers or crab-like pincers. These come in both terrestrial, freshwater and marine forms.

Monday, 6 September 2021

Species Profile: Old Six Leg

Out of all the habitats on the Rhynia, Dome 6 presents a megafauna that is both the most familiar and most alien to us. Many of the terrestrial vertebrates here do have characteristics which reminds one of amphibians, reptiles and sometimes even mammals on Earth, as they all descend from lobe-finned fish like we tetrapods did. They all however share two features which put them in the uncanniest of valleys. Most obviously, nearly all these terrestrial vertebrates have six limbs, instead of the four that we inherited. Nearly all of them also have a noticeably enlarged pineal eye. Pineal eyes are a very ancient trait that can already be found in jawless fish. It is a small, immobile proto-eye at the top of the skull, which in early bottom-dwelling fish helped warn of larger shadows swimming over their heads. Many tetrapods, mainly amphibians and squamates, still have this trait as it helps them determine the weather conditions for their cold-blooded metabolism. In archosaurs and mammals this was eventually lost. In the animals on Dome 6 on the other hand it has expanded in size and function to a degree never seen in any Earth-vertebrate. While still immobile and structurally different from true eyes, looking like a big, opaque disk stuck on the skull, the pineal eye in these animals has become complex enough that it can be called a genuine third eye. If behavioural tests are anything to go by, it is even able of producing an image in the owner’s brain. Coming up with a name for this new clade of animals has been difficult so far. The most obvious choice, “Hexapoda” to contrast with Earth’s Tetrapoda, is already occupied by the clade that contains true insects. Something along the lines of “Triopida” is also too close to home to the Triopsidae, which are notostracan crustaceans. Alternative proposals have therefore been Sexapoda, Fenestrocephalia or Stracastegalia. I personally prefer the name “Quastensaurier” or Krossosauria, which I coined myself for the reasons mentioned below.

How or why exactly the two defining traits of the krossosaurs evolved is still a great matter of debate, as there is no true equivalent among vertebrates on Earth and fossil deposits in Dome 6 have yet to be accessed. An important clue might however be a recently discovered fish that is sometimes found in the dome’s lagoons and estuaries. Hexopterygius buccaphagus, nicknamed the sixfin or Old Six Leg, is a nearly two meter long fish which outwardly resembles Earth’s coelacanth and yet also shares traits with the krossosaurs: Six limbs and an enlarged pineal eye. Our genetic tests show that the sixfin is indeed the closest relative of the true krossosaurs without directly belonging to the group. Given our current (lack of) knowledge, this stem-krossosaur seems to be an evolutionary holdover and therefore our closest approximation of what the ancestor of the clade may have looked and behaved like. As can be gleaned from this fish's anatomy, the krossosaurs' extra pair of limbs evidently evolved out of a paired anal-fin. A paired anal-fin is certainly extraordinary, as in all other living fish on Earth it only exists as a single fin along the midline, though it is actually not unique. From Earth we know at least one example of this occuring, namely in the fossil fish Euphanerops from the Devonian. Unfortunately, Euphanerops was a jawless fish of the Anaspida order. While anaspids were actually not that distant from the origin of jawed fish, it remains hard to estimate if these fins were homologous to the anal fins of later fish and therefore if the genetic mechanisms that led to them could have been repeated. At least lab-experiments on goldfish have shown that bony fish are also capable of mutations that produce paired anal- and even tail-fins, though examples of this have so far never been seen occurring in the wild or fossil record. Some of the more radical thinkers among our teams have therefore argued against the leading hypothesis that the Rhynia was purely built for conservation. Instead it may have been an extraterrestrial Isle of Dr. Moreau, with the sixfin, its relatives and perhaps various of the other biological wonders in the domes being the products of genetic experimentation by the aliens who built the station. Evidence for this is lacking however and claimed signs of artificial insertions in the animals' genomes may just as well have occured naturally through viruses, as regularly happens on Earth. Observing Old Six Leg it is not too hard to see how an accidental mutation like this could have suddenly been beneficial at the right moment and then passed on instead of being selected against. Male sixfins are observed tenderly stroking females with their anal fins to initiate mating and when the two get at it they use these fins to grasp onto each other in a missionary position. It is likely that the paired anal-fin therefore evolved as an analogous structure to the claspers seen in sharks or placoderms, but otherwise lacking in bony fish. Outside of mating, the fins are also observed helping the animal finely control its direction in the water, the same way the median anal fin is used in Earth's coelacanth.

In this light, the grossly enlarged pineal eye may actually be harder to explain as there really is nothing comparable to Earth's vertebrates. Here again the behaviour of the sixfin may still shed some light. Half of its prey consists of smaller fish and invertebrates it catches in the water like any other aquatic predator does. Often however, the sixfin, which lives in shallow lagoons and estuaries, searches itself a sandbank that sits close to the water's surface and just lies there in wait, observing what lies above. Its usual prey are small flocks of gull-fish, a sea-going type of chimaera fly unique to Dome 6, sitting on the water like ducks. At the right moment the sixfin does a push-up with all its fins (including the anal ones) and with its powerful tail shoots out of the water like a crocodile to catch one or two of the hapless animals before they can fly away. I propose that the functionally three-eyed condition may have evolved as a way to flexibly switch between these two hunting styles. When in the open water hunting other fish, the sixfin can use its regular eyes to do so, while in ambush-mode its third eye helps observe surface activity. While certainly awkward, this condition may have at some odd point in evolution been easier to evolve and even less limiting than to significantly restructure the skull so that the two eyes on the side look upward, like in, say, capitosaurian temnospondyls. What is further interesting is that the ambush hunting style (and in tandem the predatory use of the pineal eye) may have been what led to the terrestriality of the krossosaurs, as it brings the animal to actively use all of its six limbs on the ground and also forces it to go dangerously close to the water's edge. That said, nobody has observed so far what would actually happen should the sixfin accidentally beach itself. Perhaps it is capable of hauling itself short distances over land, perhaps it is not. It is notable that autopsies show fairly well-developed lungs typical of early sarcopterygians. While not as developed as those of dipnoi (lungfish) they seem to be significantly more useful than the very vestigial ones of the coelacanth.

One of the most surprising things about the sixfin is its genetic placement in the tree of life and therefore also that of the Krossosauria. Due to their various similarities to true tetrapods, such as the use of true fingers instead of lepidotrichs, it was initially thought that like us they evolved in parallel out of tetrapodmorphs, such as the Middle Devonian Platycephalichthys, which may well have been indexed by the aliens when the Rhynia was built. Or at the very least they could have come from other rhipidistians like famous Eusthenopteron or maybe even lungfish. Our genetic tests have instead shown that the outward similarity of Hexopterygius to the coelacanth is more than just skin-deep. Latimeria chalumnae and L. menadoensis are indeed the closest Earth-relatives to the sixfin, meaning that the ancestors of the Krossosauria directly evolved out of the actinistian coelacanthiforms. I therefore chose the name Quastensaurier, as Quastenflosser is the German word for coelacanth, while Krossosauria is a nod to the outdated name Crossopterygii.