Clade Profile: Thriae
Assuming the aliens who built the space station did not have a time machine, it is a miracle they sampled Earth’s biota at such a pivotal period. Insects are by far the most successful clade of animals on Earth, their prosperity owing in no small part to their wings. The first evidence of flighted insects we have found was Delitzschala, a six-winged oddity from the early Carboniferous. Though some still defend early Devonian fossils, such as Rhyniognatha, as pterygote precursors, Rhynian biota states the contrary; no winged insects have been found in the decks or any of the domes. In the absence of flight, Rhynian insects could not replicate their success on Earth, remaining obscure and morphologically conservative to tremendous ecological consequences. Instead of beetles, bugs, or grasshoppers, the microfauna of Dome 4 is populated by basal isopod-relatives, shortened millipedes, four-eyed harvestmen, terrestrial horseshoe-crabs, and even lobopodian leeches, none of which have evolved flight either. Yet when the first expedition teams set off into Dome 4, they encountered flying animals at first glance unmistakable as flies, moths, or wasps. Upon dissection though, they were shocked to find jaws and vertebrae. These were not insects, but flying fish playing their part.
Anatomy and Relationships
Thriae, as they were named, were an evolutionary enigma from the onset. Though tetrapedal when landed, they arranged their limbs in a diamond shape, with one at the front, two at the sides, and one at the back. Their median limbs doubled as powerful wings in flight, complimenting their second wing pair with a radically different structure comparable to a folding fan. All four wings tugged against a tall but thin keel at the animal’s dorsal end, providing attachment points for their flight muscles. Right under the keel were openings that led to a pair of book lungs which had air actively pumped in and out of them by sympatric action of their flight. Most extraordinary were their heads. The eyes and nostrils were placed at the animal’s ventral side. What seemed to be the lower jaw was completely immobile, while the upper jaw could open up to 90 degrees. Though fine examination of their internal anatomy found a cartilaginous skeleton somewhat similar to modern selachians and the recently extinct holocephali, the animal looked flipped. The vertebrae were on the ventral side while the eye sockets were on the ‘mandible’.
Despite this, all other internal features flawlessly lined up with a group of extinct fish known as acanthodians. These were a grade of basal fish, thought to be ancestral to chondrichthyans - the cartilaginous fish. Acanthodians reached the peak of their diversity in the Devonian, but were unable to recuperate from the Kellwasser event mass extinction, declining throughout the late palaeozoic and going extinct at the permo-triassic boundary. The most striking anatomical feature of Devonian acanthodians were their broad and heavy fin spines which give them their name. These were composed of three layers of dentine, just like the bony support of thriae wings and legs. Less obviously, Devonian acanthodians had both tooth rows and tooth whorls, the latter theorized to have given rise to the ever-replacing teeth families of elasmobranchs. The jaws of thriae similarly possess both dental conditions, with a set of grinding plates at the back of the jaw and regrowing teeth at the front. Using Occam’s razor, it seems the most logical explanation for the thriae’s anatomy is that they are genuinely upside-down, but this raises more questions than it answers.
On Earth, swimming upside-down has only been documented in two clades of fish, the whipnose anglerfish (Gigantactinidae) and the upside-down catfish (Mochokidae), though only the latter are relevant to thriae evolution on account of being a neritic freshwater clade. Upside-down catfish swim inverted for several reasons. Not only does it help them access the abundant upper water plankton and insects trapped on the surface, the oxygen-rich waters at the top also aid in respiration. Though the Rhynia escaped the Kellwasser event in the Frasnian, the Hangenberg event, caused by a nearby supernova explosion, hit the station just as hard. Stratigraphic evidence from both Dome 4 and Dome 5 suggest freshwater anoxic episodes from this time, possibly implying that deoxygenation was a station-wide phenomenon. The ancestors of thriae may have already evolved to swim upside-down at this time and would be less affected, thus capitalizing from the decline of many competitors.
Following this, leaping from the water would not have been hard to evolve. Though incapable of true powered flight unlike previously thought, freshwater hatchetfish (Gasteropelecidae) can take off and jump great distances using their pectoral fins. With powerful muscles attached to an enlarged keel, they generate a powerful downwards stroke to fly into the air. Freshwater hatchetfish employ this technique both to escape from predators, but also to capture waterside prey, both applicable to the hypothetical thria ancestor.
Once flight evolved, thriae likely then modified their dorsal fins, which were on the ventral side on account of flying upside-down, into landing gears, while using their front wings as further support. Their book lungs might have evolved from operculum-covered gills in conjunction with their growing terrestriality. The second pair of wings, however, still remain mysterious. The possession of 5 spiny supports per wing suggests it was not derived from a single pelvic fin, but a conglomeration of 5 pairs of post-pectoral finlets. Only one species of acanthodian is known to have that many paired fins, the early Devonian Brochoadmoanes milesi, a taxonomically uncertain benthic predator. Though B. milesi did not make it into the Givetian, at least one relative evidently did.
The one catch to this explanation is the fossil record. In both Dome 4 and Dome 5, thriae appear abruptly about 260 million years ago in forms already comparable to modern taxa. Just as frustrating, the deposits of the other three explored domes are either below fathoms of ocean or have been ground to dust by daily rainstorms. Though hook-shaped fin spines attached to a wing were found in a Dome 1 deposit recently, further tests determined they were made of chitin, an obvious red herring. We may not ever know where thriae came from.
Just like their counterparts on Earth, thriae are restricted to small sizes, usually with wingspans not exceeding 15 centimeters. Most thriae are considerably smaller, and many more difficult to clearly see. This limitation in thria size may owe to their general lack of wing articulation, an adaptation instrumental to providing agility in flight, or perhaps the lack of pneumaticity in their fin spines.
Curiously, thriae seem to disregard the vertebrate size limit on the other end - tetrapods rarely go shorter than 2cm, but the majority of thriae by species are below that size. Vertebrate cochlea no longer function after they reach a small enough size, causing animals to lose balance. Thriae deal with this in a simple but effective way, upsizing their internal ears, which erupt from the back of the skull and loop around the ribs. Some clades further augment this by using visual detection to orientate themselves, much like insects. In these clades, the eyes have two pupils, one facing upwards and extremely sensitive to polarized light. In most domes, this lets them navigate by the sun’s trail in the sky and see colors beyond human comprehension.
Thriae are broadly placed in two groups by Dome 4 explorers from head morphology.
Beaked Thriae
The first and most typical group are the beaked thriae, characterized by a keratinous sheath which surrounds their mandible. Despite this, teeth are still present on their cranium, displaying remarkable heterodonty. Teeth near the front of the jaw are usually sharp and grow in whorls while those at the back are flat and rounded. Most beaked thriae have filamentous integument, though the purpose of this is unclear - no Dome 4 thriae from this group are known to thermoregulate. Beaked thriae are viviparous, but in a very peculiar way - much like Caenorhabditis elegans, they practice worm bagging. Beaked thriae hatch inside the parent and eat their way through the flesh, ultimately emerging from their mother’s corpse.
Most typical of the beaked thriae is the aptly named common thria (Thria vulgaris). Common thriae are small animals with wingspans of about 74mm. Simultaneously detritivorous and frugivorous, they hang around carrion and the fruiting bodies of horsetails, using their strong beaks to break apart spore capsules. Common thriae and allies have rudders on their back legs used to steer, a trait often used to tell different species apart. Thria is by far the most speciose genus of thriae, with over 45 species as of writing, though former paleodipterist on Earth, Professor Iosua Dżąsziw, believes that it is over-packed.
Ichneumon thriae (order Kleodoriformes) either exhibit an ancestral or derived form of worm bagging. Comparable to their namesakes on Earth, they inject their hapless prey with eggs that eat their hosts from the inside-out. Though thriae lack mobile abdomens, ichneumon thriae compensate with enlarged and muscular ovipositors derived from external genitalia. To locate their hosts, they use a pair of nares which extrude from their crania. Mwopionflies (Kleodora silvestris) are the most commonly encountered ichneumon thriae at 36mm in wingspan, mainly parasitizing arboreal horseshoe crabs. However, there have been reports of ichneumon thriae attempting to inject their eggs into team members when they took off their protective suits, pointing to potential opportunistic megafaunal parasitism.
Harpoonists (order Cardoharpaga) are one of two tripedal thria clades, turning their front limb into a deadly weapon. Harpoonists, such as the nocturnal 40mm long stardial harpoonist (Melaina dzasziwi), lock their front limbs in place like a trap jaw ant, fly over their prey, and spring their limbs loose to impale them. M. dzasziwi usually targets aerial prey such as other thriae or votemis, but some of its larger relatives are known to attack large pauropods and even small placoderms.
Beakless Thriae
Beakless thriae, as the name suggests, lack beaks, and have jaw structures far more typical of aquatic acanthodians. They also generally lack filament, their integument comprising either scales or bare skin. Unlike the worm-bagging viviparity of the beaked thriae, beakless thriae have far more straightforward life cycles, with the female laying leathery eggs in carefully constructed dens.
The presumed most basal group of thriae are the true beakless thriae (order Euarhynchophora), a clade of animals so far only found at the foot of the Flaming Mountains. True beakless thriae lack the keratinous sheath of beaked thriae and their advanced integument. They also lack the outlandish adaptations of their more derived relatives. Exemplary of them is the dramatically named wreathbearer (Corycia yanhuoensis), an unremarkable and small (30mm wingspan) animal only known for their elaborate mating rituals in mid Spring: males carry circular helenophyte helens while performing acrobatics.
Flingers (order Pallouriformes) are the second group of tripedal thriae, though unlike the harpoonists, they have instead derived their back limb into a specialized take-off organ. Owing to this, they appear to sit on the ground when landed. Pultflies (Palloura ingens) are the largest flingers so far discovered, with an impressive wingspan of 90mm. Though sap-eating, they will indiscriminately attack other animals that get near their food source.
Most enigmatic of all thriae are the keelmaws (order Karinagnatha), omnivorous to predaceous creatures which appear to have been let out of hell itself. Not content with just two jaws, keelmaws have evolved a third out of a keel protrusion. When capturing and butchering prey, they usually swivel their mandibles upwards towards the keel-jaw. Only when masticating and ingesting do they act the mandible against the cranium. Adorning both the keel-jaw and the mandibles are sharp teeth, likely derived from denticle precursors. Keelmaws are also among the largest thriae, with the largest described Dome 4 representative, the eyeglass keelmaw (Parakarinagnatha specula), averaging a wingspan of over 140mm. They hunt any animals smaller or at the same size as them, even including small shantaks. However, the eyeglass keelmaw might not even be the largest of its ilk. An eyewitness from before my assignment alleged seeing a giant keelmaw carrying a small quadrupedal tieheform into the dusk. Sadly, no such giant keelmaw has been recorded any time thereafter, leaving the team to dismiss it as an urban legend.
One group among the keelmaws stands out for their particularly horrific jaw arrangement, even for thria standards. Once thought to be a highly aberrant species, the tonguepike (Gladiognatha wongi), in fact belongs to a diverse clade of horrific fliers. Tonguepikes act their mandibles against both the keel-jaw and the cranium, using it more like a radulus than a jaw. As such, fleshy sheathes have extended from both its cranium and keel-jaw to keep food from falling out of the mouth when chewing. As its encased mandible is less effective for prey capture, the tonguepike has opted for turning the terminus of its mandible into a sharp rostrum. It swings this structure up and down to cut aerial prey, analogous to the extinct swordfish of Earth. Though limited in both size and number in Dome 4, team Schoedsack reports far larger social relatives that have taken residence in Dome 5 - another reason why I dare not go there.
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