Mesogean Grasslands

In the south of Mesogea is a large area of open, red coloured grassland. Forests are prevented from growing here not only because of the drier air further from the coasts, but also the planet’s strong winds. Forests do exist, but only in places especially conductive to the growth of plants; here, forests can more easily develop a protective outer layer of especially wind resistant trees. Open grassland is far more common a biome, however, especially far from the sub-stellar point.

Almost all of Mesogea is on the planet’s day side, and certainly the entirety of the area of grassland being discussed. The sun hangs low on the horizon, however, the region permanently stuck in late afternoon. The trees scattered across the area all face towards the near-motionless sun.

This region of grassland isn’t completely monotonous. There are areas of savanna as it transitions towards forest, and towards the continent’s central desert in the north it gradually gets drier, making way to shrubland. The savanna is where animals are most abundant, which consists of the usual red grass in addition to feathery trees. Terrestrial sponges are common too, and the trees aren’t large enough or densely packed enough to form a canopy overhead. Wind is less of an issue here since it’s obstructed by the trees and sponges over long distances, but further north the winds get much more intense. Winds become especially strong in flat plains, since even in open steps the wind can be obstructed by hills.

The savanna and grassland of southern Mesogea is regularly plagued by sandstorms from the large Akasara Desert to the north, blanketing the sky in the red sand of that region.  

Fin-backed Tarus

(Tilusa nusulu)

 

 
Size: Males; 1.4 – 1.8 meters up to their back (not including their dorsal fan), 1.8 – 2.5 meters in length. Females; 2.2 – 2.8 meters up to their back, 2.8 – 3.5 meters in length

Diet: Spiky grass (iculophytes), will supplement their diet with small amounts of fruit and other more nutrient-dense sessile food

Habitat: Savannahs and steppes

Reproduction: Sequential hermaphroditism; the fin-backed tarus is protandrous, with all individuals being born male and becoming female if they’re the largest member of a herd. Large, hard-shelled eggs are laid from which small larvae emerge, undergoing metamorphosis into their adult form over time. Young are cared for.
 
The herbivorous tariforms are common throughout the mainlands of Xenosulia, dominating their niche as large herbivores. Of particular note are the species of fin-backed tarus, of the genus Tilusu, which can be found in large numbers in the savannahs and open planes of Mesogea, Arunia and Occasia. As an order, tariforms are mainly characterised by the presence of an enlarged anterior stomach in their oral proboscis, and a horny protrusion at the end of their proboscis to aid in cutting vegetation. Perhaps more importantly, the gizzard, more commonly located in the proboscis, is pulled back into the head. Here, the muscles can push back against the skull to generate more force, allowing for the grinding of tougher vegetation.

The Mesogean fin-backed tarus, Tilusu nusulu, is fairly typical of the genus, and was the first to be extensively studied after the arrival of the initial colony ships. The name “tarus” itself comes from a contraction of the Gontanic term “tari us”, meaning “three horse”; early colonists often compared them to tripedal horses. This comparison isn’t far off, as tariforms do consist of a great number of cursorial herbivores, although the fin-backed tarus is less energetic than many other taruses, relying more on its greater size for defence. The term “tarus” typically only refers to larger species of tariform, although in a scientific context it is often used to describe the order as a whole.

In common with many other tariforms, they assume an unguligrade stance, supporting their weight on the tips of their toes. Each claw has developed into a protective hoof. When walking slowly, they only lift one leg off the ground at a time, but when running the two front legs act together while the powerful rear leg works to push them off. Their legs are made all the more powerful by a greatly enlarged hydraulic pump, giving their back a visible hump.

Reproduction and social structure
Characteristic of the genus is the presence of a dorsal fan which, along with the single nasal horn, is used for sexual display. Males use their horns to fight each other, and when they do so will often attempt to damage each other’s fans. Herds tend to consist of a single dominant female, far larger than the others, and numerous smaller males. All individuals start off as male, at least after their larval stage. Juvenile males will remain in the same herd as their mother, leaving to find another herd upon sexual maturity in the hopes of gaining mating rights from this herd’s female. The dominant female mates with a limited number of the herd’s fertile males, choosing those with the most impressive dorsal fans and horns; there is usually a single male the female mates with far more than any others.

If the female dies she will be replaced by the largest male; this is often, but not always, her preferred mate. Upon changing sex the new dominant female of the herd will shed her horn and dorsal fan in addition to drastically growing in size. Sometimes, more than one individual will change sex and compete for the position of dominant female, although this rarely lasts long; females tend to be hostile with each other and will even engage in infanticide if the competing female gets the chance to mate.

The female is provided with a great deal of food by the fertile males in addition to that which it obtains through grazing, aiding in her ability to grow as large as she does. Since she needs to lay enough eggs to sustain the herd, a large size is an invaluable advantage. Once the female’s eggs hatch, the males collectively care for the larvae, providing them with food and good soil to live in as well as protecting them from predators.

Taxonomic classification
Tree: Xenosulivitae
Domain: Rhytocaryota
Kingdom: Xenosulizoa
Phylum: Hydratozoa
Superclass: Tripoda
Clade: Sucodermata
Class: Spinoptilita
Superorder: Cerostomata
Order: Tariformes
Family: Cavidae
Genus: Tilusa
Species: T. nusulu
 

Red Mesogean Horseshoe

(Finusoma erythronoton)

 

Size: 30 – 50 cm in length

Diet: dead biomatter particles from algae and aeroplankton

Habitat: dry open steppes

Reproduction: protogynous, females capable of parthenogenesis, lay hundreds of eggs a year
 
With air algae abundant in the atmosphere, the ground inevitably receives detritus from the metabolic activities and death of these organisms. As such, many species have adapted to take advantage of this. This detritus is especially common in open plains, where countless species within the order pulusiformes can be found in abundance, grazing on it. 

Pulusiforms make common pets, kept in tanks in many people’s homes. Food for them, made from processed algae, can be bought at most stores, which is sprinkled onto the floor of their habitat.

Wind tolerance
In wide open grasslands they are subjected to strong winds, so to aid in their tolerance of this their bodies are flattened against the ground, reducing air resistance and allowing the wind to pass right over them with little trouble. Unlike many larger species, they’re unable to use their weight to stay grounded, and they tend to live in areas too windy for most other animals. Even plants taller than grass struggle in these places.

Their scales further protect them from the wind, preventing their skin from getting damaged by the abrasion of any particles caught up in the air. A muscular “foot” on their tail, similar to the underside of Earth gastropods, in addition to aiding on locomotion, also allows them to grasp firmly onto the ground if the winds get too intense.

The eastern winds, coming from the planet’s dark side, can be very cold, and are often the strongest and most prevalent of winds. This poses a challenge for animals living in open and windy areas, with the winds chilling them to the bone. As such, pulusiforms have a very good cold tolerance in spite of being cold blooded, and are even known to survive being frozen. They usually deal with cold weather by entering a state of torpor rather than putting too much effort into remaining warm.

Feeding
Unlike spinoptilites, the oral proboscis can be fully retracted into the head, with the relatively undeveloped digestive organs of the proboscis allowing for this. On the inside of the proboscis’s mouth is a radula, with the brush-like teeth specifically designed to pick up detritus off of the grass.

Although they are typically slow movers, they spend most of the day inching across the grasslands, covering a great distance over time. They stay in a single place until they’ve exhausted all usable detritus, before moving on to a different spot and scrapping the grass for more food. Although it’s not the most energy-rich food, it is plentiful, so little effort is spent searching for food.

Defence
One of the most recognisable features of pulusiforms, shared by most species, is the horseshoe shaped compound eye, providing the animal with good peripheral vision. Since they’re quite small, they’re a common target of predators, although their preferred defence tactic is keeping still in the hopes they aren’t noticed. Since they’re usually well camouflaged this often works, although they can be fast if they need to be, and many species possess poisons.

Although poisonous pulusiforms are common, Finusoma erythronoton lacks any such poisons. However, its red scales provide it with good camouflage against the red grass in its natural habitat. The animal can also move more quickly than many other pulusiforms, with reasonably long front limbs and claws that allow it to grip the ground. They’re quite common in the drier parts of the grasslands of Mesogea, and relatives of this species can be found in savannas and the outskirts of the deserts.

Reproduction
Pulusiforms are known for their fast breeding rate, Finusoma erythronoton being no exception. They’re able to lay hundreds of eggs in a year, each of which a hatchling resembling a miniature adult emerges from, very few of which survive to adulthood. They do very little to look after their young, instead relying on producing large numbers of offspring.

Most pulusiforms are capable of both sexual and asexual reproduction, which will vary depending on conditions. At low population densities, all individuals will remain female throughout their life and regularly produce countless offspring without the need for fertilisation. When populations are higher, or resources scarcer, they have comparatively fewer offspring, most of which are produced sexually. In these conditions, around half of all individuals become male.

Taxonomic classification
Tree: Xenosulivitae
Domain: Rhytocaryota
Kingdom: Xenosulizoa
Phylum: Hydratozoa
Superclass: Tripoda
Clade: Sucodermata
Class: Xenosquamita
Order: Pulusiformes
Family: Finusomidae
Genus: Finusoma
Species: F. erythronoton
 
 
 

Tackypod

(Kottos kottos)

Size: 70 – 110 cm up to the top of their hump

Diet: small animals

Habitat: savannah

Reproduction: protogynous; individuals are born female, and become male as they grow larger
 
On Xenosulia, bipedalism has evolved from initially tripedal ancestors numerous times. The predatory dromeiforms are one such group. As fast moving carnivores, they dominate the Xenosulian mainlands as apex predators, filling numerous different predatory niches. Most dromeiforms are pursuit predators, although some members of the clade do employ other strategies.

Dromeiforms
Characteristic of the group is the greatly reduced rear-leg, the splitting of the single compound eye into two separate eyes, greatly enlarged secondary eyes at the front of the face, an elongated – often tail-like – rear, and two lateral tentacles to aid in balance. Having flexible skin with a layer of sub-dermal hydraulic muscle underneath, tentacles evolve fairly easily, supported entirely by hydrostatic pressure. This is far from the only time such an innovation has occurred in sucoderms.

It seems likely that the spitting-in-two of the compound eye occurred in order to allow the dorsal ocelli to move forward; while the compound eyes are used primarily for peripheral vision, the simple eyes’ ability to detect faster movement makes them more useful than compound eyes for catching prey. These eyes are actually able to resolve images, unlike the secondary eyes of most other animals, although their colour vision is much more limited than that of the compound eyes.

Members of this group are commonly referred to as tackypods in Gontanic, referencing the resemblance of some species to birds (or at least their legs to those of birds). This term is usually only reserved for smaller species, however.

Kottos kottos
Although dromeiforms comprise the largest of land predators, there are many smaller species in this group too, such as Kottos kottos. They mainly prey on smaller animals, including entomopterite “bugbirds”, catching them with their rapid moving oral proboscises. Lacking any kind of teeth, much less jaws, they kill their prey by constriction, using their acidic saliva to dissolve their prey once they’re dead. Their clawed feet can also be used to assist in the cutting up of prey once it begins corroding, and are sometimes used to catch slower moving or unwary prey.

Taxonomic classification
Tree: Xenosulivitae
Domain: Rhytocaryota
Kingdom: Xenosulizoa
Phylum: Hydratozoa
Superclass: Tripoda
Clade: Sucodermata
Class: Spinoptilita
Order: Dromeiformes
Family: Kottidae
Genus: Kottos
Species: K. kottos
 
 

Onychodon

(Terraculi mesogensis)

 

Size: 1.2 – 1.6 meters in height, 2.3 – 3 meters in length (from nose to tail tip)

Diet: smaller taruses, bugbirds, small but fast-moving animals; will also hunt larger animals with the use of tools

Habitat: savannahs and grasslands

Reproduction: protogynous; individuals are born female, and become male as they grow larger
 
A much larger dromeiform species is Terraculi mesogensis, a successful pursuit predator and the main threat to fast moving herbivores. The animal’s speed provides it with an adequate means of catching such prey, which includes many of the more cursorial taruses, although because of its method of killing – constriction – it also hunts prey quite a bit smaller than itself. Constricting larger prey is difficult, and although they have been known to pierce them in vital areas with their claws, this requires a lot of precision. Terraculi mesogensis will also catch entomopterites, typically larger species than the smaller tackypods prefer.

The anatomy and behaviour of T. mesogensis is fairly typical of the family it belongs to, Dromeisauridae. Dromeisaurids are typically colloquially referred to as onychodons, which was originally the name of a now defunct dromeisaurid genus. The term onychodon may also refer to other large dromeiforms, and is often used interchangeably with the term “landshark”.

Intelligence
Perhaps because of the difficulties they face in catching larger prey, they are very social animals. They are known to hunt in packs, exhibiting cunning tactics, although solitary hunting has been observed too – especially when hunting bugbirds.

They are also known to fashion simple spears out of tree exoskeleton or animal bone, but based on the wealth of cognitive studies performed on them there is little evidence they’re much more intelligent than Earth’s reptiles. Or, perhaps, certain cephalopods. They show little innovation in their toolmaking, and efforts to teach them new techniques have proven unsuccessful; it seems they mostly rely on instinct, much like a bird building a nest, or bees building a hive.  

Still, they exhibit much greater cognitive abilities than most other animals native to Xenosulia. One should keep in mind that both reptiles and cephalopods have since been shown to be far more intelligent than assumed in the early 21^st Century, so the comparison isn’t to say they’re that unintelligent – just less so than mammals in many respects.

Physical adaptations
Terraculi mesogensis possesses many adaptations for running that smaller dromeiform species lack. Their bodies are elongated and aerodynamic, and to reduce drag their spines have been lost. Their balancing tentacles have been elongated, and are a bit thicker than in smaller species, improving their balance and allowing them to manoeuvre with more dexterity.

They have good senses, too, with their two largest secondary eyes greatly enlarged even for dromeiforms, almost looking like a second pair of compound eyes. The compound eyes are elongated for better peripheral vision, allowing the animal to scan its surroundings for potential prey. At the same time, the two enlarged simple eyes give good depth perception for successful grabbing of prey for constriction. Or, as is often the case, accurate spear strikes. The two lowest pair of dorsal eyes are sensitive to long-wave infrared radiation, which have lost their lenses; the cupitin their lenses are made from blocks such light. These heat pits are useful for locating well-hidden prey by detecting their body heat, although the sense organs are far more developed in related species that live closer to – or past – the day-night terminator line, where it provides an advantage in these darker environments.

Taxonomic classification

Tree: Xenosulivitae
Domain: Rhytocaryota
Kingdom: Xenosulizoa
Phylum: Hydratozoa
Superclass: Tripoda
Clade: Sucodermata
Class: Spinoptilita
Order: Dromeiformes
Family: Dromeisauridae
Genus: Terraculi
Species: T. mesogensis
 

Spotted Leopard-snake

(Leoserpens coccinus)

 

 
 
Size: 1.5 – 2.1 meters in length

Diet: Medium to large taruses, as well as hard-bodied animals. Also eats fruit, nuts and seeds

Habitat: savannah; can also be found in the outskirts of woodland areas

Reproduction: remain hermaphroditic throughout their life, lays eggs
 
Although tripods basally lack any kind of jaw, they have been developed independently in multiple different groups. Trignathites are one such clade to develop jaws. The jaws being repurposed from their front limb pair, trignathites are forced to move around on their belies like a snake, with the rear leg greatly reduced. This is a worthwhile trade off, as the biting strength this gives them allows them to excel as predators. Their lack of legs also provides an advantage in open spaces where animals are exposed to more winds, as this means their body is closer to the ground and has less wind resistance.

Since they’re unable to run great distances, most rely on ambushing their prey. Compound eyes aren’t well-suited for this purpose, so have been lost, with the simple eyes offering far more visual clarity than they do in other groups. In the case of the order Vermiformes, which Leoserpens coccinus belongs to, one pair of these eyes is situated on a pair of tentacles. This allows the animal to see above grass and other plant life as it hides in wait for prey.

As a member of the family leoserpentidae, Leoserpens coccinus is larger than other trignathites, and is partially omnivorous. They live in groups – often referred to as herds, in spite of the animals being predatory – both for protection from predators and for hunting, and spend a great deal of their time either resting or waiting for prey to fall into their traps.

Hunting and feeding
Herds of Leoserpens coccinus catch food by strategically positioning themselves as they hide in wait, so that if one individual misses their target this opens up the opportunity for another to catch it. As such they are one of the few ambush predators on the planet that hunt cooperatively. Whoever does end up catching the prey will always share it among the other members of the group, although the amount an individual is given depends on their position within the group hierarchy.

When not resting or hunting, L. coccinus can be found foraging for food, which largely consists of seeds and nut-like fruit and other easy to digest vegetation, usually from bushes and other plants that can be found in the savanna. Their jaws allow them to easily bite into such food, making them well suited to such a diet – especially when compared to the countless jawless animals they share their habitat with.

The jaws of L. coccinus are exceptionally strong even for trignathites, due to a certain adaptations shared with other members of their order. On either side of the body, close to the head (although technically inside the “skull”), are two large hollows – usually large enough to create bulges on either side of the body – which house muscle dedicated solely to jaw movement. Since their jaws are comparatively short, this gives an immense mechanical advantage, allowing them to bite down with virtually unrivalled strength. This makes them especially suited for hunting laminite prey, as well as the shelled relatives of pulusiforms. However, they do also take down larger animals by aiming for vital areas – often tearing out vital blood vessels.

Leaping
Another feature shared by other members of their order is the presence of long bladders stretched along their bodies, which extend under hydraulic pressure. Their spine is able to extend a bit, allowing vermiforms to use these hydraulic pistons to stretch their bodies forward suddenly and leap. Catching prey would be a lot more difficult without this adaptation, since their ordinary means of locomotion is much slower. They’re not able to leap like this outside of sudden bursts (hence their preference for sit-and-wait ambush tactics), as it takes time for them to build up the necessary pressure. Because of their dependence on hydraulics, the primary hydraulic pump of vermiforms is greatly enlarged in many species, creating a visible bulge in Leoserpens coccinus.

Taxonomic classification
Tree: Xenosulivitae
Domain: Rhytocaryota
Kingdom: Xenosulizoa
Phylum: Hydratozoa
Superclass: Tripoda
Clade: Sucodermata
Class: Trignathita
Order: Vyrmiformes
Family: Leoserpentidae
Genus: Leoserpens
Species: L. coccinus
 

Northern Tusk-dog

(Osteovorus savanna)

 

Size: 90 – 120 cm high, 100 – 140 cm long

Diet: carryon, especially tougher flesh and the contents of bones

Habitat: savannah, steppes, shrubland

Reproduction: become male or female upon reaching sexual maturity, remain that sex throughout their life. Lays eggs, from which larvae hatch, and they care for their young.
 
Culodonts are another clade to develop a jaw of sorts, although in the case of this order, despite the jaws originating from the front limbs as they do in trignathites, these limbs are still used for locomotion. They’re usually only used for shearing flesh once an animal is already taken down, so the inherent inefficiency doesn’t pose much of an issue.

Jaw anatomy
Basally, culodonts have four horns or “teeth” used for biting, which are actually highly developed dermal spines that have become ossified and gained attachments to the skeleton. There is one pair on either leg, just above the knee, and two under the head behind the oral proboscis. Most species also have a webbing of skin between the head and knee, and between both legs down to the knee, which function as cheeks. Such species also have hard growths on the upper legs and underside of the head that act as molars. This condition is now known to have evolved independently more than once, rather than representing a single clade.  

Feeding
While some culodont species have developed various means of taking down larger animals, such as using the upper pair of horn-teeth to pierce into their prey, Osteovorus savanna exists largely as a scavenger. They still do hunt, but will only catch smaller animals, using the horn on their proboscis to spear pulusiforms as well as other similarly sized prey. A larger portion of their diet consists of carryon, mostly large animals like taruses. The presence of their carnassial molars allows them to chew through tougher meat than most other scavengers would be able to, so that’s what they tend to focus on, and they’re even able to break open bones to consume the energy-dense starch stored within.

Social structure
While some individuals are solitary, many live in small groups of two to five to increase their odds of finding food. Tusk-dogs are often territorial, especially the males, and will mark their territory with infertile gametozoans. These gametozoans will defend the tusk-dog’s territory, stinging intruders, and although more of a nuisance than anything else they’re willing to fight to the death. Tusk-dogs will rarely tolerate another male in their group. Although all tusk-dogs are born hermaphroditic, they will become either male or female upon reaching sexual maturity based on their size, with larger individuals being male. Growth typically stops at adulthood, and males generally see more reproductive success at a larger size than they would if they were smaller.
 
Taxonomic classification
Tree: Xenosulivitae
Domain: Rhytocaryota
Kingdom: Xenosulizoa
Phylum: Hydratozoa
Superclass: Tripoda
Clade: Sucodermata
Class: Spinoptilita
Order: Culodontiformes
Family: Osteovoridae
Genus: Osteovorus
Species: O. savanna
 

Crested Screambird

(Acopti kibiatu)

 

 

Size: 60 – 90 cm wingspan, 30 – 45 cm from toe to head with rear leg outstretched (may be smaller in some urban areas)

Diet: seeds, tiny animals (spherozoans, cardozoans and various worm-like organisms), occasionally fish and other forms of food

Habitat: savannah

Reproduction: sequential hermaphrodites; all individuals are born female, and will either remain female or temporarily become male during breeding periods. Lays eggs, and cares for their young.
 
Acopti kibiatu belongs to the order Acoptiformes, a group of very proficient long-distance fliers. They are commonly known as the crested screambird; so named for its distinctive echolocative screech. To further aid in echolocation, they have an extra pair of spiracles specialised for that purpose, rather than the usual two in most entomopterites.

They are more generalist than many other acoptiforms, and can flourish in a wider range of environments. This is perhaps one of the reasons they’re so prevalent in human settlements, in addition to their already existing abundance. Their adaptability means they have little difficulty figuring out what waste is edible to them, and they’re intelligent and resourceful enough to find their way into buildings if they need to. The tendency for people to feed them certainly helps increase their numbers in cities.

Flight
As is common for acoptiforms, the crested screambirds migrates great distances towards and away from the sub-stellar point in response to changes in the sun’s temperature, often going to more tropical areas during solar winters. They depend heavily on soaring flight during their long migrations. The presence of magnetoreceptors in their beak, characteristic of their order, allows them to navigate very well; coupled with the fact the sun’s always in the same place, they’re able to determine both latitude and longitude most of the time. This magnetoreceptor is formed from the build-up of iron from the blood in the beak.

Because of ability to travel great distances, acoptiform species can be found in virtually every part of the world. Acopti kibiatu itself can be found all over Mesogea, Arunia, and Occasia. They are even present in Silidia and Zephyria, though their numbers aren’t as great here. Although there are some differences between the populations in each of these areas, they’re all classified as the same species, and the changes are so gradual that dividing them into subspecies is difficult.  

The hip processes of Acopti kibiatu is much longer than in many other entomopterites, giving them a much large range of control over the aspect ratio of their wings. This allows them to broaden their wings for more controlled flight or narrow them for soaring.

Reproduction
It is common for entomopterites to exhibit a kind of sequential hermaphroditism where the birds are born female and remain female most of the time, with some individuals temporarily becoming male during the breeding season. The crested screambird is no exception. Since Xenosulia has a year too short to have any significant annual seasonal cycle – with a year lasting about a week – they depend on the less predictable changes in stellar temperature instead. As a variable star, Zhimuchua 23 subjects Xenosulia to colder “solar winters” during periods when the star is covered with more sunspots than usual.

Screambirds will prepare to mate when the sun starts to get brighter during a solar winter, indicating it may be over soon. However, some winters can last particularly long, so screambirds will mate anyway after a certain amount of time has passed since the last mating season. Without doing so, they may risk dying before they get a chance to mate. While it is best to mate as a solar winter comes to an end to ensure eggs are laid at the start of the summer – which means their young will spend as much time as possible in warmer weather – if things seem unlikely to change soon it’s worth risking laying eggs in sub-optimal conditions. This waiting period is longer during winters than during summers.

The breeding season typically occurs at around the same time for all screambirds in a single area. This is despite the fact the exact thresholds for deciding to start mating can vary between individuals; chemical signifiers are used to trigger mating season behaviours in other birds. Whether an individual becomes male (known as “maleing”) or remains female will depend on which mating strategy is optimal. If an individual is able to consume enough high-calorie food to put on the weight needed to bear offspring, but hasn’t been able to consume many plants with the pigments males need for their vibrant colouration, then this individual is much more likely to have success mating as a female than a male. However, an individual with a fairly low body weight that has consumed a plentiful amount of plant pigments will see more success as a male. The conditions for maleing within Acopti kibiatu is fairly typical of entomopterites, although they do differ in some species.

When undergoing maleing, an individual’s skin changes to green, their crest becomes larger and purple, and they develop vibrant patterns on their wing membranes. This makes them a lot easier for predators to spot, but this is a worthwhile trade-off to attract a mate. As alluded to above, the pigments for this colouration are obtained from plants; screambirds are incapable of synthesizing the necessary pigments themselves.

The crested screambird after undergoing maleing

Taxonomic classification
Tree: Xenosulivitae
Domain: Rhytocaryota
Kingdom: Xenosulizoa
Phylum: Hydratozoa
Superclass: Tripoda
Clade: Sucodermata
Class: Entomopterita
Order: Acoptiformes
Family: Acoptidae
Genus: Acopti
Species: A. kibiatu
 

Green-breasted Lutoraptor

(Microfalcon sucu)

 

 

Size:  50 – 60 cm wingspan, 22 – 28 cm long (from toe to nose, with rear leg outstretched)

Diet: small animals, primarily other bugbirds, also eats carrion and fish

Habitat: savannahs and open plains

Reproduction: sequential hermaphrodites; all individuals are born female, and will either remain female or temporarily become male during breeding periods. Lays eggs, and cares for their young.
 
Specialised as a predator, Microfalcon sucu is a fairly typical lutoraptor, well adapted to its lifestyle as a bird of prey. They can be found all over the Mesogean plains, where the open landscape allows them to easily spot potential targets from a great distance away. Although they’re smaller than some of their larger relatives, the green-breasted lutoraptor is far more common, and has little difficulty taking down prey larger than itself. Its common name comes from the colour of their chest during maleing, although most of the time they are only shades of pale red.

Feeding
While they focus mainly on other bugbirds, they also eat terrestrial species and sometimes fish. Most of their food is obtained through hunting, but they will also consume carrion when it’s available.

Prey is killed primarily by spearing them with their sharp beak. Although their rear-leg claw is capable of grasping, with only one digit this is hardly effective and is only used to grab smaller prey.

The proboscis curls differently to other entomopterites, allowing the beak to spring straight forward rather than taking a curved path as it does when other bugbirds uncurl their proboscis. Hydraulic pressure is used to shoot the beak out more rapidly, and they have good enough aim to consistently hit vital areas.

Once prey is killed, it is ripped up with the beak, a process that takes much longer than the rapid act of hunting itself. They focus mainly on the softer tissues, leaving behind the bones and tougher flesh for animals better suited for scavenging; among them certain related lutoraptors.

Anatomy
In addition to the atypical way the proboscis curls, lutoraptors have a number of other features that distinguish them as a group. While their compound eyes are outstretched and on the sides of their head to give them a wide field of view, they have two enlarged simple eyes, similar to those of dromeiformes, that provide them with good depth perception. This is essential in making precise strikes with their beak, and without this adaptation they’d frequently miss. They also have a pair of simple eyes adapted for heat detection, which can be useful for locating well-hidden prey.

The hip spurs are more typical of entomopterites than those of Acopti kibiatu, much smaller in size, although they do possess a joint at the base as well as muscles specialising for the repositioning of the appendage. This gives them slightly more control in flight than they would have otherwise.

Taxonomic classification
Tree: Xenosulivitae
Domain: Rhytocaryota
Kingdom: Xenosulizoa
Phylum: Hydratozoa
Superclass: Tripoda
Clade: Sucodermata
Class: Entomopterita
Order: Lutoraptoriformes
Family: Lutoraptoridae
Genus: Microfalcon
Species: M. sucu
 

Sufi

(Sufi vulgaris)

 

 
Size: 1.0 – 1.4 m high, 1.3 – 1.9 m long

Diet: aeroplankton, supplements their diet with vegetation 

Habitat: open steppes, shrubland

Reproduction: start off as male, but become hermaphroditic as they grow larger, gaining the ability to lay eggs
 
With the planet’s strong winds and abundant aeroplankton and airborne algae, filter feeding is common on the surface. Although the most widespread terrestrial filter feeders are the large sedentary members of the phylum Xenospongozoa, more active filter feeders do also exist. One such group to specialise in terrestrial filter feeding are the sitostomes, characterised primarily by their grossly enlarged oral proboscis, which can sometimes take up more volume than the body itself. Sitostomes are especially common in the planet’s open plains, where winds are strong.

Sufi vulgaris is a fairly small sitostome that also supliments its diet with plants. Like most sitostomes it fairly slow moving, spending a lot of its time standing still and facing the wind. For protection, they live in herds, just like many of the plain’s herbivores. Specialised hairs are used to detect the wind direction, and Sufi vulgaris has holes at the side of its proboscis to allow air to pass back out.

Taxonomic classification
Tree: Xenosulivitae
Domain: Rhytocaryota
Kingdom: Xenosulizoa
Phylum: Hydratozoa
Superclass: Tripoda
Clade: Sucodermata
Class: Spinoptilita
Order: Sitostomatiformes
Family: Sufidae
Genus: Sufi
Species: S. vulgaris
 

Southern Mesogean Mac

(Macrocephalus australis)

 

 Size: 1.8 – 2.5 meters high, 2.6 – 3.6 meters long

Diet: aeroplankton

Habitat: open steppes, shrubland

Reproduction: start off as male, but become hermaphroditic as they grow larger, gaining the ability to lay eggs. Mating occurs by allowing gametozoans to be blown in the wind; their sensory hairs are greatly elongated to allow for this
 
Macrocephalus australis is a much larger sitostome, and is far more specialised towards filter feeding than Sufi vulgaris. However, they lack the side holes of the smaller relatives, instead employing a different method of filter feeding; facing towards the wind, they hold their mouth wide open, allowing air as well as smaller particles to enter. Then, they close their mouth, leaving it open just enough that only the numerous spines and hairs that line their lips block the passage. Forcing air out, small particles that can’t fit through the gaps between the hairs get stuck.

As a larger animal that depends more on filter feeding, Macrocephalus is even more slow moving, and their proboscis is far more enlarged, almost unnaturally so.

Taxonomic classification
Tree: Xenosulivitae
Domain: Rhytocaryota
Kingdom: Xenosulizoa
Phylum: Hydratozoa
Superclass: Tripoda
Clade: Sucodermata
Class: Spinoptilita
Order: Sitostomatiformes
Family: Macrocephalidae
Genus: Macrocephalus
Species: M. australis

 

Common Field Oc

(Acanthonota maximus)

 Size: 20 – 30 cm long, 10 – 15 cm high (up to the highest point of the back)

Diet: seeds, fruit, grass

Habitat: grassland and savannah

Reproduction: remains hermaphroditic throughout their life, breeds at a rapid rate.
 
Acanthonota maximus belongs to a group of spinoptilites called polylutiforms, so named for their tendency to shed through multiple “tongues”. In actuality, it’s just the hard outer layer that’s shed, rather than the radula itself. Members of the family Ancanthonotidae are colloquially referred to as “ocs” or “oclins”, a Gontanic term originally referring to the hedgehogs of Earth in certain dialects.

The main defining characteristic of the group is the hardened radula; rather than consisting of soft tissue with cupitinous teeth, the entire radula has become covered in a layer of hard cupitin, the whole organ acting as a single spiny tooth. This is shed and replaced by the layer beneath at a rate rapid enough to compensate for the constant erosion of the tooth. This tooth tends to be enlarged compared to the radulae of other groups, and while they’re often inflexible, in the case of the field oc the cupitin layer is thin enough that it can be flexed to an extent.

Defence
The oclin is a small animal, although its small size is typical of polylutiforms. While far larger species have existed in the past, they have since been outcompeted by more modern herbivore groups like taruses. As such, it depends largely on its small size and ability to hide to avoid predators.

While their small size allows them to hide well, and they have sharp enough senses to be quickly alerted to danger, Acanthonota maximus is able to use its spines for defence if this fails. With the spines it shares with other spinoptilites concentrated on its back, the animal is very uncomfortable to attempt to catch. This is made worse when these spikes are made to stand erect. What’s less obvious at a glance is the fact these spines broaden at the base and are wide enough that they interlink, forming a thin shell under the spines. This shell offers a degree of further protection, and it’s flexible enough not to interfere too much with movement.  
Another feature that allows them to avoid predation is the elongated compound eye band, which stretches all the way around the head from one shoulder to the other. This offers them a wide field of view, and in addition to the compound eye, two of their simple eyes are enlarged and specialised for detecting predators.

They don’t depend solely on sight, though, and have well developed hearing too. Unlike many other spinoptilites, the lower pair of ears have been retained in polylutiforms; both pairs possess an external pinna to filter sound, developed independently from the pinnae of taruses. Those of Acanthonota in particular are greatly enlarged. These aren’t their only hearing organs; they are also able to detect vibrations in the ground via sensory organs in their feet, which is much more useful whenever it’s too windy for the air to transmit sound very well.

Although their limbs aren’t specialised for digging, they do have a limited ability to dig and prefer to sleep in burrows for safety. Often they will take up residence in abandoned burrows dug by more proficient burrowers. If they are close enough to their burrow when a predator is near, they prefer hiding underground to anything else.

Wind avoidance
Unlike pulusiforms, who have adapted to be able to tolerate strong winds, Acanthonota maximus tends to avoid it entirely. They have well developed sensory hairs, similar to those of sitostomes, that allow them to detect changes in the wind. The animal seems to be able to predict when the wind will increase, hiding in a burrow until the weather is tolerable again.

Reproduction
While not nearly to the same extent as pulusiforms, these animals are fast breeders, allowing them to spread rapidly. Unlike pulusiforms, they do care for their young, although they’re not very selective in who they mate with. All individuals are hermaphroditic, and if they’re unable to find other individuals to mate with soon after they enter heat, they will release male gametozoans into the grass which will go looking for an individual to inseminate.

Interaction with humans

Acanthonota maximus and other oclins have quite adaptable diets, and as such are one of the groups to take advantage of the recent arrival of humans on the planet. Their fast breeding rate only makes things worse, to the extent that they’re considered a pest by many. They have a tendency to dig small burrows underneath people’s houses, only coming up when people are sleeping or away to steal whatever food is edible to them. Although they’re unused to daily cycles as inhabitants of a tidally locked planet, they seem to have quickly learned the cyclic patterns of human behaviour. In spite of this, they are also common pets, perhaps because their small size means they require little space to look after.

Taxonomic classification
Tree: Xenosulivitae
Domain: Rhytocaryota
Kingdom: Xenosulizoa
Phylum: Hydratozoa
Superclass: Tripoda
Clade: Sucodermata
Class: Spinoptilita
Order: Polylutiformes
Family: Acanthonotidae
Genus: Acanthonota
Species: A. maximus

Other Phyla

In addition to hydratozoans, there are numerous other animal phyla that inhabit Xenosulia. Most animals have a similar optic nervous system as hydratozoans, in addition to similar pneumatic muscle tissue, which they likely inherited from a common ancestor (in fact, in most cases genetic evidence confirms this). However, hydraulic muscles are rare in non-hydratozoans, and those on land tend to be much smaller, lacking good skeletal support.

 
This is far from a comprehensive list; there are countless animal phyla on the planet, far too many to cover here. This includes the various worm-like phyla, in addition to a great number of aquatic animals.  

Mycozoans

These organisms resemble the fungi of Earth, and are often referred to as such. Despite belonging to Xenosulia’s animal kingdom, they lack any muscles or a nervous system; evidence shows they branched off before these were developed, rather than having lost these features. 
Mycozoans are detrivores, feeding off decaying organic matter, and are especially common on the planet’s night side. On the night side, they are able to feed off of dead air-algae that has been blown over from the day side and fallen to the ground, giving them an ample supply of food. 
Like hydratozoans, mycozoans have separate diploid and haploid generations; this is believed to have been an ancestral trait of animal life and has been retained by almost all groups. The diploid generation is the primary generation, with the haploid generation only consisting of microscopic – but still multicellular – spores that are spread by wind dispersal. 

Xenospongozoa

These large sponge-like organisms are similar to the sponges of Earth, sedentary filter-feeders, but unlike the sponges of Earth there are many terrestrial linages. Wind passing through their pores can leave various particles behind, including the nutrient rich air algae. They can grow to large sizes, and are a common sight in the open savannas of Xenosulia, where they can often be almost as common as trees. Xenospongozoans are abundant on both the day and night sides of the planet. 

Like mycozoans, they reproduce by wind dispersal, releasing tiny haploid spores into the air. 

Spherozoans

Spherozoans are tiny animals who basally have spherical symmetry, but some groups have become radially symmetrical. The most “primitive” spherozoans have a body plan consisting of a central sphere with numerous equally-spaced legs sticking out in all dimensions for locomotion and the gathering of food. Their legs are typically arranged in a polyhedral formation. 

This body plan has been retained by many slow-moving aquatic groups, where distinction between directions isn’t really relevant. However, most terrestrial lineages have developed a distinct top and bottom, with the lower legs adapted for locomotion and the upper legs specialised for food gathering. This has happened in multiple different orders independently, most notably in hexahedrites and octohedrites. Sight has also developed in these groups, with eyes at the ends of the feeding limbs. 
Food is eaten through mouths in the arms, which enters the main body through an oesophagus running the length of the appendage. In most radial terrestrial lineages, only the upper limbs have retained this function, although some groups graze with their feet. 

They are covered by a flexible outer cuticle, protecting them from desiccation. There is an air filled layer beneath this cuticle, allowing oxygen to be absorbed directly into the more permeable skin underneath; holes in the cuticle – usually three per segment – allow for passage of air in and out of this layer. 

Air absorbed by the inner skin enters the circulatory system; this is an open circulatory system, in which the body cavity is filled with hemolymph rather than contained entirely in blood vessels. Suspended in the hemolymph is the oxygen carrier hemoflavin, a protein unknown on Earth that, like hemerythrin, is immune to carbon monoxide poisoning. However, it is susceptible to hydrogen poisoning. Hemoflavin is yellow in colour in both its oxygenated and deoxygenated state, and although its oxygen carrier is iron the protein’s colour comes from its structure and not the metal. 

Without a hard skeleton, the body of a spherozoan is supported entirely from the hydrostatic pressure of the hemolymph. There is a layer of muscle under the inner skin which can increase internal pressure by contracting against the hemolymph, making the animal much more rigid. 

There are countless different species of spherozoan, greatly exceeding the total number of hydratozoan species, and they occupy a niche similar to the arthropods of Earth. However, unlike many insects, no spherozoans are capable of flight. 

Unlike hydratozoans, although spherozoans have both a diploid and haploid generation, it is only the haploid generation that are motile. The haploid generation is also far more longer-lived than the relatively short diploid generation. The diploid generation are fungus-like in appearance, spawning numerous spherozoans from tiny holes before dying, and in some spherozoan taxa the haploids may produce numerous diploid “fungus” in their lifetime. 

Cardozoans

These organisms consist of a rigid external shell that bends in the middle, the two halves of the body joined by a “hinge”. The subphylum, Cardoptera, is an example of one of the handful of times in which flight has evolved. All cardozoans outside of this subphylum are aquatic, and it seems likely that cardopterans developed flight straight out of the water, with an intermediary surfacing and gliding stage. The flight of cardopterans is similar to the swimming of aquatic cardozoans, involving the flapping of both halves of the body. 

Cardozoans are known as “hingeflies”, due to the appearance of their entire body consisting of just two wings joined together at a hinge. However, this is just their outer shell, and their anatomy is actually more complicated than this. Inside the shell is an – often tentacled – worm-like organism, parts of which emerge for the purposes of feeding and mating. The eyes themselves are often attached to eye-holes in the shell itself, right at the hinge, although in the class Maculocardita there are also eyes distributed across the wings. They may have additional sensory organs that can be retracted, such as tentacles covered in taste receptors or a stalk eye, but all the senses needed for flight are available to the hingefly while its safely retracted inside its shell. The shell itself consists of a woody material, made of a substance similar to cellulose or chitin. 

Terrestrial cardozoans are usually small in size, due to limitations of their respiratory system, and show an even greater range of diversity than spherozoans. They fill a similar niche as flying insects do on Earth. 

Like spherozoans, only the haploid generation of cardozoans are motile, and the diploid generation is typically shorter lived. Diploid cardozoans appear like a woody branching fungus, often red in colour, on which growing cardozoans are often seen hanging from. Usually cardozoans are folded up at this stage of development, and may resemble seeds. They can typically fly immediately upon detaching from this branching structure. 

Hydratozoans

The planet’s largest animals, especially on land, belong to the phylum Hydratozoa. While most higher groups have become bilateral, they basal body plan is radially symmetric, with the most obvious trait shared by such primitive hydratozoans being the compound eye ring surrounding the mouth. They usually also have multiple togues for catching food, and their mouth is positioned at the top of the body with an anus underneath. They have a water-vascular system somewhat similar to those of the echinoderms of Earth, which functions as both a respiratory and circulatory system as well as aiding in movement. They’re boneless, supported entirely by hydrostatic pressure, but more advanced groups have developed skeletal supports. 

The first bilaterally symmetric hydratozoans looked something like a slug or sea cucumber, with their bilateral symmetry coming from the fact that one half of their body is adapted to sitting on the ground. Many such hydratozoans still exist today. A specific clade of these slug-like hydratozoans with an endoskeleton – mainly used for protection – eventually gave rise to more fish-like organisms capable of swimming, which later moved onto land. 

The majority of terrestrial hydratozoans belong to the clade Tripoda. Although some of the more slug-like hydratozoans also moved onto land, most are very small and unable to compete for the same niches as tripods. Tripods descended from “fish” with only two paired fins, and as such there are no tripod species with four or more limbs. However, the tail has developed into a third limb, allowing them to have up to three legs – although there are many bipedal, legless, and even monopodal taxa. The first members of the clade are believed to have been tripedal, hence the name, although the line between a third leg and a heavily specialised tail can be very blurred.  

Spinoptilite anatomy

Spinoptilita is a very successful group of tripod, with an active lifestyle, erect limb posture, and a type of warm-bloodedness that allows them to keep different parts of their body at different temperatures. Many of the features of spinoptilites are shared by other tripods, so this should give a good idea of general tripod biology. 

Spinoptilites belong to a large clade called Sucodermata, characterised by thick skin protecting them from desiccation. This allows them to live further from bodies of water, although more basal groups still depend on water for reproduction. Most tripods outside of this group dry out very easily, and will shrivel up and die in contact with salt similarly to a slug. The vast majority of fully terrestrial hydratozoans belong to this clade. 

Internal anatomy of a spinoptilite. The species pictured is the Occasian fin-backed tarus (Tilusa nusulu).

Skin

The most obvious defining trait of spinoptilites is the presence of spiny integument on their bodies. These spines can vary in length, density, and distribution, and serve a number of functions. Defence was likely the original reason these spines evolved, but they serve a very limited defensive purpose in most groups. They also break up an animal’s silhouette, helping them escape the notice of predators or prey more effectively; this is especially true of species with longer spines. The least obvious but perhaps most common function they serve is in temperature regulation; each spine has a large blood vessel, and increasing the flow of blood into the spine can help cool the animal by allowing heat to pass out. Spines are usually thinner on one side to allow more heat out, which actually limits their defensive ability. 

As sucoderms, the skin of spinoptilites is thick and very good at holding in moisture. It is also very flexible, and contains a layer of muscle underneath that allows it to change shape slightly or tense up. These muscles aren’t attached to the skeleton, and only seem to serve a role in changing skin texture or shape for better camouflage, as well as protecting the animal. It can also help an animal balance by changing the distribution of muscular fluids throughout the body, modifying the centre of gravity. 
This layer of muscle means that developing tentacles isn’t that hard along fairly short evolutionary timescales, and they have evolved many times among tripods as a whole. 

The skin of all tripods consists primarily of a protein that has been labelled by human researchers as cupitin. The properties of cupitin are similar to that of keratin, but it’s quite different in structure. The skin consists of outer layers of cells whose cell walls consist of the protein, underneath which is tissue consisting of wall-less cells. 

Skeletal system

Skeleton of the fin-backed tarus

Tripod bones consist largely of silica, rather than the calcium phosphate of vertebrate bones, and tend to be much stronger. Rather than possessing any bone marrow equivalent, that function is instead taken up by a specific organ, with the hollow interior of bones being used as a long-term energy store in the form of starch. Other than these differences, the bone tissue of tripods is very similar to that of vertebrates in terms of structure and function. 

Their skeletal structure differs from a vertebrate’s in a number of ways. The first hydratozoan skeletons consisted of a internal “shell” with a number of interlocking plates, which was very effective at protecting the internal organs. However, in larger and more active animals such a skeleton can weigh them down, so many groups have developed a lighter skeleton with more gaps and holes. 

In most sucoderms the skeletal plates of the torso have developed into thinner “ribs”, but have maintained triangular processes that interlock with the rib behind it to provide the skeleton with more strength than it would have otherwise. These processes have roughly triangular holes in them to reduce weight without sacrificing strength too much. The fossil record shows that rather than developing from projections growing from a more vertebrate-like rib, the processes are actually what remained of the broader skeletal plate as it lost bone. The ribs attach to a spinal column running underneath the body, rather than the back as in vertebrates. 

The bones of what can be considered the skull have retained their ancestral shape, however, offering the brain a greater degree of protection. The front legs attach to the skull onto a specialised “shoulder blade” plate. The skull lacks any kind of jaw, with a mere opening for the oral proboscis. 
The rear leg is quite different anatomically from the front pair. While the muscle is on the outside of the front legs, the rear leg has much broader hollow bones with space inside for digestive organs, muscle, and other tissues typically found in the torso. This is due to its development from the rear of the animal, with each rear-leg bone being homologous to the skeletal plates or ribs of the torso and skull. 

The skeleton pictured is of a fin-backed tarus, which bears a number of distinctions from other spinoptilites. Two of the dermal spines have become ossified, and have attachment points on the skeleton, which can be seen in the image. They also have a horn at the tip of the skull, and the processes of the rear ribs have been greatly extended to protect and support the enlarged primary hydraulic pump.

 

Nervous system

The nervous system of hydratozoans, and most animals on Xenosulia in fact, is bioluminescent rather than electrochemical. This is far from uncommon on other planets; however, whereas most bioluminescent nervous systems encountered elsewhere involve the production of light at one end of the nerve, which is transferred across a kind of biological optical fibre, the nervous system of hydratozoans works differently. Instead, there is a kind of relay system where light is sequentially produced across the nerve. 

Hydratozoan nerves are filled with photosensitive light releasing chemicals. When light is produced in one part of a nerve, it stimulates the release of the same chemical compounds further down the nerve; this bioluminescence only lasts for a brief period, which is followed by that part of the nerve becoming fatigued, unable to light up again until it’s recovered. This way, the part of the nerve that’s lit up is prevented from moving in reverse. Instead, it will continue to trigger the next portion of the nerve fibre to light up until the signal has run across the whole length. 

More primitive animals have to rely on the diffusion of chemical signals throughout the body; these chemicals are similar to the bioluminescent chemicals in hydratozoan nerves, which they’re likely ancestral to. 

The central nervous system consists of a brain at the front of the body and a nerve cord running through the middle. Since it’s located deeper within the body than the spinal cords of Earth’s vertebrates, it doesn’t need as much protection, with a flexible sheath covering it. Nerves branch from the primary nerve cord to the rest of the body, although the oral proboscis has its own distinct nerve cord. At the end of the oral proboscis is the oral ganglion, used for processing sensory information from the proboscis. 

Muscular system 

On a basic level, the muscles of hydratozoans work in a superficially similar way, contracting to generate force. However, at a closer look the way this is accomplished can be seen to be quite different. Rather than fibres of myosin and actin sliding together as in Earth animals, the muscle cells of hydratozoans and many other Xenosulian animal phyla are pneumatic. They work by increasing internal pressure, forcing the stretched-out cells to become more spherical and contract. On larger scales this looks similar to vertebrate muscles, with contracting muscle tissue becoming shorter along one axis but swelling in others. 

The increase in pressure is caused by a release of gasses, mainly oxygen, from a protein contained within specialised organelles found in muscle cells, which forms bubbles. This realise is triggered by the same chemicals used in conveying nerve signals, and muscle cells are in fact homologous to nerve fibres and able to transmit light in the same way. The gas molecules bind back to the protein soon afterwards, reducing cell pressure again and allowing muscles to stretch. 

While the above is true of the majority of motile animal life on Xenosulia, the muscular system of advanced hydratozoans has a number of features that sets it apart. Most notably is a system of hydraulic pumps adapted from the ancestral water-vascular system, which is linked to the cardiovascular system. The muscles themselves have both parallel and traverse fibres, allowing them to both contract and elongate. The pumps increase the fluid pressure in the muscles, which allows them to elongate even more forcefully. There is a hydraulic pump for each leg, as well as a primary hydraulic pump larger in size than the rest; this primary hydraulic pump is supplied with deoxygenated blood from the veins. This hydraulic muscular system gives tripods more strength than would otherwise be possible, but it can’t be used as dependably over long periods as normal contraction because of the need to build up pressure. 

Circulatory and respiratory systems

The circulatory system of all “higher” hydratozoans is derived from the ancestral water-vascular system, as is the respiratory system. They have a closed circulatory system, with the blood contained in veins and arteries that branch and deliver blood to all the body’s tissues. The lung, as well as the hearts, are derived from the water-vascular system’s muscular pumps, just as the hydraulic pumps are. Unlike the water-vascular system, the circulatory system is closed off from the surrounding water (or air) in the outside environment, and contains fluid quite different in composition rather than just consisting of the surrounding seawater. 

The single lung is derived from the gill pump of the more fish-like ancestors of tripods, and is located in the animal’s back. Numerous paired spiracles on the animals back are used for breathing, and while breathing in is generally passive (although some derived groups employ active inhalation), exhaling air is an active process. This is accomplished by the contraction of a wall of muscle that surrounds the lungs, forcing air out. 

Each trachea has a sphincter at the base that allows for the control of airflow, and many groups have an additional set of sphincters at the spiracles, especially aquatic groups that might want to prevent the trachea from filling with water. In spinoptilites, there are also vocal folds present in the trachea, allowing for the production of sound. Many other groups have developed similar mechanisms of vocalisation independently. 

There are numerous hearts, most often small, located throughout the body, but they fit into three main groups. One group, the arterial hearts, are located ventrally, along the animal’s chest and abdomen, and encased within the spinal column for protection. The arterial hearts are responsible for pumping oxygenated blood from the lungs. These are usually the smallest hearts, and are the reason why some vertebrae are larger than the others (since those are the vertebrae which contain a heart). The arterial hearts can number anywhere from seven to twenty, occasionally even more, and there are usually twice as many vertebrae as there are such hearts. 

To pump blood, the arterial hearts successively contract in waves, from front to back. Each arterial heart is single chambered, so they depend on this sequential worming motion to work effectively. The arterial hearts of tripods and other “advanced” hydratozoans are derived from the major ventral artery of the more primitive slug-like hydratozoans, which pumped blood through similar waves of contraction. This blood vessel developed areas where muscles were more concentrated which over time became a long chain of hearts.

A second group of hearts, the lung hearts, pump deoxygenated blood through the blood vessels that branch through the lung, increasing the efficiency in which oxygen is taken up by the blood. The two lung hearts are located just behind the lung and act together as a single two-chambered heart. They are often, but not always, larger in size than the arterial hearts, likely because there are less of them – although in terms of total muscle mass they are collectively smaller. 

Unique to spinoptilites and entomopterites are the cranial hearts, which are responsible for pumping oxygenated blood directly to the brain. These are located just below and behind the brain, and like the lung hearts they consist of a pair of single chambered hearts that act together to pump blood. 
The blood itself contains pellet-shaped blood cells which use the protein hemerythrin to transport oxygen. It’s likely that hemerythrin outcompeted other oxygen binding proteins, such as the hemoglobin used by vertibrates, due to its immunity to carbon monoxide poisoning. Not only are levels of carbon monoxide higher on Xenosulia than on Earth, but the geological record indicates it was far higher at certain points in the past. Hemerythrin isn’t the only oxygen binding protein used, however; chlorocruorin is also suspended directly in the blood, with most tripods having a dual system of both proteins, although many groups have lost either one or the other. Blood cells are produced in a specialised organ, rather than from bone marrow, which also plays a role in the immune system. 

Hemerythrin gives the blood of most tripods a purplish colour when oxygenated, although blood colour can vary depending on the varying levels of hemerythrin and chlorocruorin. Those with low or no hemerythrin typically have green blood, and the brightest purples are seen in blood lacking chlorocruorin. 

Digestive system

The digestive system begins at the oral probiscis, present in almost all tripods, used for the gathering of food. In spinoptilites, much digestion takes place within the proboscis itself, but in many other groups these digestive organs are greatly reduced and the proboscis can be retracted into the head. The skin of the proboscis is generally more elastic than in other parts of the body, allowing it to stretch in length for more efficient food gathering. The presence of such a proboscis is likely why so few tripods have developed necks, in addition to the wide field of view the compound eye band offers. 

In side the mouth of the proboscis is a tooth covered radula, although this is lost in one of the two main spinoptilite branches. This tongue-like radula, present in almost all non-spinoptilites, is used to scrape up food matter through an abrasive “licking” motion. There is also a salivary gland, which is capable of producing very acidic secretions; this secretion is especially concentrated in the spinoptilite clade lacking a radula. In such spinoptilites, feeding is accomplished by first spitting acid onto their food, and then consuming the partially liquified biomatter. A dependence on this method of feeding is likely what led to the loss of the radula. Tripods basally lack any kind of jaw, spinoptilites included, and liquivory was developed to make up for this – however, jaws were developed independently in multiple tripod taxa. 

Once food has left the mouth it enters the oesophagus, which leads to a muscular gizzard. This organ is lined with tough cupitinous teeth and grinds up food through a series of contractions, although there are certain groups that lack such teeth and need to swallow stones. The gizzard is usually located near the mouth of the proboscis, but in the animal pictured above, the fin-backed tarus, it has been pulled back into the skull. 

At the base of the proboscis is an anterior stomach, which releases enzymes involved in the breakdown of food and plays a role in the absorption of water into the blood. After it has been processed by this stomach food enters the crop which stores food; it is also involved in fermentation in many herbivorous species. 

The crops leads to the posterior stomach, where sulphuric acid is released for the breakdown of food. There are also enzymes involved, but many of the enzymes found in the anterior stomach can only survive in the presence of the weaker acid present there. 

Beyond the stomach is the intestines, which absorb nutrients as well as playing a role in the further breaking-down of food, which leads to a tube known as the cnemic passage which makes its way through the rear leg. In most groups the cnemic passage is straight, but in many herbivorous groups like that fin backed tarus pictured above, this has developed into a secondary intestinal tract for the further breakdown and absorption of food. The cnemic passage leads to an anus at the back of the foot, where waste is excreted. This waste tends to be quite dry, with most excess water excreted out of the oral proboscis. 

Nitrogenous waste is extracted from the blood by a kidney-like organ, where it is collected and stored in a specialised sac. When an individual is low on nitrogen, this storage organ releases some of its contents back into the blood. If this organ exceeds its capacity, these nitrogenous compounds enter a passage to the intestines where they’re excreted as solid yellowish crystals. 

Senses

The majority of sense organs are distributed towards the front of the body, as well as the tip of the proboscis. These sense organs provide many of the same senses as those seen in the vertibrates of Earth, including sight, sound, olfaction, and taste. In addition to the specialised sense organs described below, pressure, pain, and temperature receptors and distributed throughout the skin. Pressure receptors are especially densely concentrated on the oral proboscis. 

Compound eye
At the front of the body is a large compound eye band; in spite of most spinoptilites having just one such eye, the fact it’s a compound eye in addition to its shape offers it a great degree of peripheral vision. 

Like the compound eyes found in the arthropods of Earth, this eye consists of numerous small lenses that focus light on photoreceptor cells, with each unit unable to resolve images on their own. However, together they are able to form a mosaic of the animal’s surroundings. Compound eyes typically have a poorer resolution than the eyes of many vertebrates; however, spinoptilites and other tripods have numerous means of improving their vision. The most significant is their ability to quickly vibrate their eyes, which is accomplished by quick sub-dermal muscular movements. These vibrations are actually fast paced scans that allow the eye to pick up more visual data; visual data from each position in the scan is put together to make an even more detailed mosaic than would otherwise be possible. 

Some of the other means by which vision is improved also depend on subtle movements of the eye. The compound eye is able to bend, so sub-dermal muscle can flatten the eye in certain areas so a specific object the animal is focusing on comes into better focus. The greater number of ommatidia directed towards the object allows it to be viewed with far greater clarity. 

The compound eye has very good colour vision, especially the compound eyes of spinoptilites, with most spinoptilites being hexachromatic. However, these colours tend to be shifted to the red end of the spectrum since the planet orbits a red dwarf. It’s rare for an animal to be able to detect blue light, but they’re very sensitive to red light and most species can see well into the near-infrared. Although Xenosulia seems slightly dimmer than Earth to most humans, it actually receives the same amount of light because of its close distance, with a large part of that light being in the infrared and therefore invisible to humans. To an animal native to the planet, it looks much brighter, at least on the day side. 
Simple eyes

In addition to the compound eye, there are numerous smaller single-lensed ocelli surrounding it. The ocelli of the head are divided into two groups; the lateral ocelli and the dorsal ocelli. These eyes evolved separately from the compound eyes, and are present in most tripods, although the ocelli of the first tripods lacked lenses. The ocelli of spinoptilites, however, do have lenses, which are primarily composed of hard and transparent cupitin. They lack the ability to resolve images, but have advantages over the compound eye such as a greater ability to sense changes in light and dark. They’re also much faster acting than the compound eye, since the neurons at the base of the compound eye photoreceptors have an inefficient mechanism for transmitting information, something that’s difficult for evolution to work around. Also, the compound eyes, while able to distinguish colour, have a comparatively poor ability to distinguish light intensity.  

Sound
Sense of sound is complicated on Xenosulia by the strong winds. Sound travels more dependably through the ground, however, so more basal tripods with a sprawling posture have developed a pair of ears at the bottom of their body to pick up such sounds. There are periods of stiller weather, however, and areas more protected from the winds, so sensitivity to airborne sound isn’t useless. The ears under the body aren’t in an optimal position for picking up airborne sound; because of this, there is a second pair of ears, higher up, that serve this purpose. 

Standing with a more erect posture, most spinoptilites have lost this lower ear pair, retaining only the upper ears. Raised off the ground, the lower ears serve little purpose, although in certain groups they have been repurposed for the detection of airborne sound. Because the need to detect vibrations in the ground still exists, spinoptilites have developed a secondary set of hearing organs in the feet, characteristic of the group. 

Smell
While the oral proboscis, mentioned below, is able smell and taste at short distances, spinoptilites can pick up smells at much further distances using other sensory organs. Namely, olfactory receptors located inside the respiratory spiracles are used, this being an ideal location due to the rapid movement of air into and out of this part of the body. These sense organs are especially developed in certain predatory groups, where they can be used to aid in the location of prey. 

Sensory organs of the proboscis

At the tip of the proboscis there are a number of sense organs too, assisting in the location of food. Numerous sensory hairs project from the tip, which not only help tactile detection but are also lined with taste receptors. The proboscis can smell, too, with olfactory slits near the mouth that are able to draw in air with the help of small internal bladders. These slits aren’t able to smell very well beyond short distances, but it is only used for finding food at a close range anyway. There are taste receptors on the inside of proboscis, too, right near the opening of the mouth. 

The oral proboscis also has eyes near the end, similar in structure as the dorsal and lateral ocelli, but specialised for short range vision. Although the oral eyes of many spinoptilites lack lenses, in those where they are present these lenses are made of hardened cupitin. In spite of these lenses these oral eyes have a similar inability to resolve clear images as the ocelli do, although they’re able to see colour and in many species can make out rough shapes. 

Reproductive system 

Reproduction is accomplished via specialised organs in the oral proboscis. There are numerous glands near the mouth, capable of producing either male or female gametozoans; a Xenosulian term for the animal equivalent of gametophytes. These gametozoans are distinct organisms, representing the haploid generation. What is usually thought of as the actual animal is the diploid generation; in fact, all the anatomy described above applies only to them. The much larger diploid animals have cells with paired chromosomes, whereas the chromosomes of gametozoans are unpaired. 

The gametozoans of most tripods are motile and radially symmetrical, with six legs covered in sensory hairs. Although tiny, they are usually macroscopic in size. However, female gamatozoans of many taxa have lost their legs and ability to move, instead remaining within the diploid parent. This is the case in spinoptilites, where mating usually consists of simply linking the oral proboscises together and allowing the male gametozoans to crawl in to the other animal and mate with it. 

Soon after a male and female gametozoan mate, they die, the materials of their body being used in the initial growth of a new diploid organism with genetic material from both parents. In spinoptilites and most other sucoderms, the developing embryos migrate to a uterus in the skull. 

The diploid generation of most tripods are hermaphroditic, and sequential hermaphroditism is common, especially in spinoptilites. The gametozoan producing organs in the mouth can easily change over time to be female producing or male producing, and there is nothing stopping an individual from having some produce male gametozoans and others produce females. 

One feature that distinguishes spinoptilites from most other tripods is the phenomenon of eusociality with regards to the gametozoans. In addition to fertile males and females, spinoptilites also prouduce infertile males gametozoans; these infertile males play no role in reproduction, instead assisting the animal in various ways like worker ants in an ant colony. These infertile males are produced by separate specialised gametozoan glands, which are present in both male and female diploids. 

Infertile gametozoans assist in numerous ways, such as cleaning the animal, distributing various oils across the body, and even serve a sensory function by probing the ground or objects. Gametozoans can also be used for defensive purposes, attacking other animals with toxins. Many live inside the parent animal as well as crawling along the skin, and early explorers initially believed spinoptilites were covered in unrelated bug-like organisms, presumed to be parasites of some kind. 

Spinoptilites are oviparous, laying large hard-shelled eggs. These eggs are grown in the uterus and laid through the oral proboscis, which acts as an ovipositor. Each egg contains multiple larvae, and is lined with groves along the surface; these gaps in the shell allow air to reach the membrane below, where there are numerous stomata that are able to open and close depending on humidity. This provides the developing larvae with a steady source of oxygen. The egg’s groves gives the shell a cracked appearance, and coupled with its round shape this can lead it to resemble certain icy planets or moons like Europa or Oculia. 

Spinoptilite larvae are tiny and less developed than the young of other sucoderms, lacking bones or the legs that will appear later in life. However, they do have numerous paired tube legs, supported entirely by hydrostatic pressure; these aren’t homologous to the legs of adults. Most spinoptilites care for their young. 

Entomopterites

The entomopterite pictured is the field daymoth (Dimutili occasiensis), a fairly small fruit-eating animal - on the left is the top-down view, on the right is the underside

Powered flight has evolved multiple times on Xenosulia, with the planet’s strong winds and high oxygen content making for ideal conditions for the development of flying animals, in spite of the thinner atmosphere. The most diverse clade of large flyers are the entomopterites – colloquially called bugbirds, batbugs, or sometimes just birds – exceeded only by the much smaller cardozoans in number of species. 

This group is closely related to spinoptilites, likely a sister clade, and like spinoptilites they produce infertile gametozoans that aid in cleaning as well as other purposes. Where they differ, apart from the lack of spines, is mainly due to the presence of adaptations for flight.  

Entomopterite wings

The most obvious characteristic of entomopterites is their wings, formed by the presence of a membrane of skin between each front limb and the side of the torso, with the finger of each wing greatly elongated to provide the animal with a larger wingspan. There are pads just at the base of the finger, used for walking on, with most entomopterites assuming a tripedal stance when walking on land. The wing membranes are supported by enlarged and partially stiffened blood vessels, often forming complex cross-linking patterns that divide the wing into various cells. These blood vessels can be made to become more rigid by increase their blood pressure, allowing bugbirds to stiffen their wings while in flight, and then make them more pliable to fold them up.  

In most species there are horny processes on each hip supporting the wings. This allow the bugbirds to have wings with a greater variety of aspect ratios than they would if the membranes attached directly to the end of the rear leg. There are species lacking this feature that have narrower wings, but this results in the loss of the tail membrane, a trade-off that need not apply with the hip processes present to support more complex wing and tail membrane structures. Although they were initially believed to have originated as extensions of the hip, fossil evidence suggests they’re actually ossified wing veins. In many groups, development of the dermal muscle surrounding the hip spurs as well as more flexible attachments of the spurs to the skeleton allows them to actually move position, something that provides them with a greater ability to modify their wing shape. 

Entomopterite senses

Their compound eye has split in two, both of which are usually fairly round, and often large in proportion to their body. This eye splitting likely occurred to allow their dorsal simple eyes to move further to the front of the head, similar to certain spinoptilites such as the dromeiforms. The secondary eyes are mainly used for stability, working in tandem with their balancing organs to correct for any changes in orientation. While their vision is well developed, they are also able to use echolocation, making vocalisations or clicks with their two frontmost spiracles which have become specialised for this purpose. This is used far more on the planet’s dark side, but is still a valuable sense for purely dayside birds as it can provide them with sensory information they might not get from their compound eyes; while their sonar is more limited in range, it is better at picking up fine details, as well as sensing the texture and density of objects. 

Entomopterite mouth parts

The oral proboscis is small, and curls up when not in use to reduce drag. Because of its reduced size, it plays less significant a role in digestion than in spinoptilites, with the organs inside the main body doing more to compensate. The radula of bugbirds has hardened and developed a bend in the middle, giving them a kind of beak at the end of their proboscis. 

Roosting

With only one rear leg, and most entomopterites lacking wing claws, perching on a branch is impractical. Instead, entomopterites hang upside-down by their rear leg, roosting like Earth’s bats. Their foot has a locking mechanism that allows them to do this without exerting much strength, so they prefer resting this way than landing to rest – they’re much safer from predators than when resting on the ground.