Osteopulmas of the Early Hothouse

280 MPE

The osteopulmas are the smallest birds, a branch of the verminfan subgroup of the metamorph bird lineage that diverged in the late Pangeacene. These birds are generally so small that the most efficient way for them to breathe is through passive respiration, and their common ancestor evolved to breathe through spiracles on their backs where their hollow spinal vertebrate, connecting to their spinal vertebrate and their system of respiratory air sacs. These birds survived the mid-ultimocene ice age with a handful of tiny, fly-like species, and are now widespread and more diverse in form than ever before in the early hothouse age. Some have now increased in size, while others are smaller than ever. 

~~~

Descendants of the zebra tweezle, the knife-flies are common predatory osteopulmas found worldwide in the hothouse. They exhibit comparative gigantism from their housefly-like ancestor as a result of the warm and oxygen-rich atmosphere of the newly warmed world, but because they no longer have functional lungs and rely on passive respiration through spiracles in their spinal vertebrae, they are now limited in how big they can ever again become.

Knife flies are all carnivorous, and are adapted to prey on large animals relative to themselves. Scissorjaw knife-flies, widespread across Serinarcta's soglands and open plain regions, are nocturnal hunters that feed heavily on blood-drinking parasitic animals which bite thorngrazers, both true insects and other verminfan birds like squicks. Their diet of vampiric pests makes them beneficial to large herding animals, which they may live on full-time. They strongly favor thorngrazers over birds, likely because these animals lack swatting tails, long flexible necks, or paired hind legs, and as such are less capable of scratching their bodies or otherwise dislodging the knife-fly as it perches. They are territorial over their hosts, often zipping through the air and striking their beaks together in tests of strength to determine which can lay claim to the largest and most productive feeding grounds. Females are the larger sex and habitually cannibalistic, usually eating her mate after copulation.

Males live as winged adults for around 40 days, which is how long it takes them be reach full maturity after emerging from their pupae, and even the female's adult life expectancy of only around 70 days and ends after laying two to four clutches of eggs. The larva still looks similar to that of their ancestor, and in this species hatches from small, numerous eggs laid in carrion. A single female can lay five-hundred eggs in one clutch, and repeat this weekly for up to four weeks before she is exhausted and ultimately expires. Most larvae do not reach adulthood, most dying when carrion is scavenged and consumed by various larger animals, but their reproductive rate is so high that some always manage to survive and perpetuate their lineage. Larvae feed for only two weeks before reaching 3/4 of their adult weight and then drop into the soil and spin mucous cocoons in which to complete their development. Subadults dig out of the ground 3.5 weeks later; they have full sized wings by then, but undersized bodies, and take another 40 days of active feeding to reach their full adult mass and be ready to reproduce.  

Hammerhead knife-flies are one of several species with the odd trait of widely-spaced stalked eyes which provide 360 degree vision. Though the stalks cannot flex, the eyeballs can rotate independent of each other to view every direction, making them among the most keen-sighted of all predators and able to detect small movements indicating potential prey from anywhere in their vicinity. The inner hind toe on each foot is extravagantly elongated and together form a paired tail that streams behind the body in flight, serving like halteres to improve its balance and being used to facilitate rapid turns while in the air. The hammerhead knife-flies are the most derived of this clade and include the biggest of its species. They are quick, highly athletic, long-distance endurance fliers and very widespread as a result of a tendency to fly ahead of powerful storms and ride their winds across the world. They feed in flight, catching anything which they can overpower, and in the largest hammerhead species this will sometimes include small sparrowgulls and even seraph chicks, in addition to other verminfans and all types of flying arthropods. Though the jaws of these species do not overlap, they are designed to crush and tear flesh and to close together with significant force to kill prey; though much smaller, a hammerhead knife-fly's beak can bite down with the same force as a rat, and can crack the skulls or sever the spinal cords of birds even larger than itself. This is made possible by a tendon in each jaw that tightens when the jaws are held open, which is the default resting position for these birds. When the tendons suddenly release all of their stored energy, the beak clamps shut extremely quickly and with an audible snap. A hammerhead knife-fly's initial bite after unlocking its jaws is as quick and as deadly as a mousetrap and earns their clade an alternate name: the snapjaw tweezles. Not all members of the hammerhead clade actually exhibit pronounced stalked eyes (and the trait may have arisen twice independently), but all share this rapid-fire closing jaw. Thus this alternate term may accurately be used to refer to the broader group.

Snapjaw tweezles, including the hammerhead knife-fly pictured above, reproduce by laying eggs near, but never directly on large carrion. Their larvae are born bigger and more mobile than those of the scissorjaw knife-fly and will move between food sources, making them more independent. They are more aware of their surroundings and are better able to drop off the carcass and escape larger scavengers, which improves their survival rates. This goes hand in hand with fewer offspring being born at a time; females lay only around 40 relatively large eggs in a clutch and may do so only every 3 to 4 weeks, but this species is also longer lived than other groups of knife-jaws and may survive as an adult for as long as fourteen months. Cannibalism in most hammerheads is uncommon and many species are highly dimorphic, with brightly colored males engaging in dizzying aerial courtship displays to attract the attention of subtler colored females. Both males and females have similar lifespans, but like their relatives they do not emerge from pupation at fully adult sizes. In this group it takes up to 5 months to reach full adulthood and a subadult will increase in size by up to ten times its starting weight. This requires more significant developmental changes between pupation and sexual maturity, and hammerhead knife-flies will molt in additional plumage, including new sets of barb-like wing feathers, two or three times to fill out gaps in their wings as they outgrow the small set of feathers they initially emerge with. This also allows males to change color dramatically when fully grown, with their final set of plumage differing in color from their earlier ones.

~~~

Waterwings are a group of flightless, aquatic osteopulmas which have adapted their wings into fins. These animals are not direct descendants of zebra tweezles like the knife-flies are, but their last common ancestor of some 25 million years ago already exhibited all of the defining osteopulma physical traits including extremely small size and spiracle breathing. They differed in their life history; adults fed on decomposing material in damp areas, while their larvae fed on algae and had adaptations for swimming, including bristle-lined bodies that formed a fin-like row on their top and bottom edge. These traits persist in the larvae of waterwings, which swim sideways and hunt independently for food in freshwater, still mainly algae and detritus, occasionally smaller animals. The evolution of the waterwing lineage was rapid following the great thaw as a single survivor species of their ancestral clade diversified in the warm, wet climate that suddenly turned the entire world into their ideal habitat. It took less than 1 million years for them to become aquatic as adults, which occurred as they began to forage for food for short duration beneath the water instead of only feeding on shore. Waterwings take advantage of physics at very small body sizes to respire with air that remains trapped against their skin under their feathers. More than just a portable diving bell that needs to be refreshed at the surface, osteopulmas have low enough metabolic needs that they can get all the oxygen they need through the diffusion of gases between this air pocket and the water around them through the process of plastron respiration, which is also used by certain arthropods of similar size. Their wings, already very simplified before, are now nothing more than bristle-lined rods that flick in sync to push their torpedo-shaped bodies through the water.

Dipdarters are omnivorous scavengers with relatively large heads that tuck their chins against their breasts habitually, making their true shape hard to parse when they turn upside down to feed at the surface and that beak suddenly resembles an odd horn on the "back" of their head. Some species do this posture only occasionally, but the inverted dipdarter swims this way constantly and is reverse countershaded as a result, with a dorsal surface paler than its belly. Ranging from 1-2 cm in length, It has very large eyes that see above, below, and ahead at all times and elongated toes, two of which reach out around its body in a curious rotating pattern in search of small food particles. When it finds something - a piece of algae, a small insect larvae, or an adult insect drowned on the surface, it will then quickly hone in on and snatch it up in its lengthy reaching talons and bring it into the beak to be torn up and swallowed. Fast-breeding and living just a few months, dipdarters are a common prey for almost any sort of waterbird and are easily captured due to their surface-skimming habit rendering them conspicuous, though they will dive when threatened. Though mostly at home in freshwater, some dipdarters will also feed in fully marine environments near river outlets, something similar insects do not readily do. Unable to survive submersion in seawater as larvae, female oceanic dipdarters must migrate upriver to lay their eggs and will do so in large synchronous spawning events. Hundreds of eggs are broadcast into the water at once and will hatch in a few days into free-swimming larvae. Very few will reach adulthood, which occurs after the fully grown larva burrows into mud on the shore and pupates for about 21 days.

Waterslaughts are carnivorous waterwings which swim only infrequently and prefer to crawl around weedy vegetation in search of animal prey. One toe on each foot is specialized into an antenna-like feeler as in their relative, but in this species it is sensitive to slight electrical impulses, such as those given off by prey animals, letting it successfully approach and catch food even in the dark of night or in highly turbid water with no visibility; some such species are almost blind. The hooknosed waterslaught, though, retains good vision. It is a creature of shallow, flowing rivers and streams, though it avoids rapids, and it will perch high on a leaf or twig and seek to snatch passing fish in the especially large, mantis-like first toe on each foot. Captured prey is held tight and skewered through the brain with its characteristic down-turned upper mandible to kill it; the lower jaw is highly reduced and functionally vestigial, and this bird feeds by spitting saliva full of digestive enzymes into the tissues of its meal, then drinking the liquefied components of its victim with a tongue that folds into a tube-like shape against the remaining beak. They can also bite enemies in this way, their saliva mildly venomous like a bee's sting, discouraging predation by larger animals. Strictly freshwater in their distribution, waterslaughts are well-hidden, well-defended, and not heavily predated, and so they can afford to have a very different way of reproducing than dipdarters. Females carry extremely small clutches of just two to four eggs adhered to their own legs for several weeks to protect them from predators, not feeding for that duration. But while these eggs are few, they are very large, up to 1.5 centimeters across. This great size and the long incubation period all make sense once they hatch into young that skip the larval stage entirely, emerging as miniature, fully-formed versions of the adult.

Almost all waterwings will, at least occasionally, use large flying animals as transport from one water body to another and in this way reach isolated water bodies quickly, often even before fish do so. The drive to seek out a ride is strongest during times of high population density, when some individuals will cling onto the legs and feathers of birds such as pteese and drop off again at their next landing, though it is unlikely they know that doing so will fly them to new habitats, and more likely a result of a simpler instinct to crawl out of the water when there are too many of your kind in your pond for you all to find enough to eat. The tiny, shell-less eggs of broadcast-spawning species can also be transported this way in a less intentional version of the process.

~~~

Extremely small and exceedingly strange, the backward-skipping osteopulmas called backpedalers spend their entire lives in water and have lost most of the traits normally associated with even other metamorph birds of their clade, including the ability to respire through their spinal vertebrae (instead, they are so small that they can breathe passively simply by existing in water and letting oxygen diffuse into their tissues, and carbon dioxide diffuse out.) These tiniest of this group of already diminutive birds, which may be as small as 5 millimeters when fully grown and which never exceed 1.5 centimeters, live at such a small scale that to "swim" is more akin to burrowing through water, and hydrodynamic shapes become almost unnecessary. These birds are thus awkwardly shaped, with many odd, elongated appendages that dangle from their rotund bodies. They move through occasional, synchronized jerks of their hind legs which are covered with fringed hair-like scales, and they can only move backwards or upwards relative to the direction their beaks face on their immobile necks, having to rotate their entire bodies to descend lower into the water. To improve their ability to detect either food or predators behind them, their eyes are set very far back on their heads - now mostly fused to their rounded torsos - and they can see equally well both ahead and behind them.

Backpedalers use their bristle-lined wings, now long since incapable of flight, to capture food particles which vary from single-celled algae in the extremely abundant puddlejumpers (Tremulala sp.) to other backpedalers, arthropods and even fish fry in the comparatively huge, 1 centimeter long bristly butcherback, whose wing feathers function like spikes to impale its helpless prey. The former's beaks are also comb like, used to collect particles captured in the wing feathers with preening motions, while that of the latter is serrated and blade-like, quickly dismembering its still-living victims into manageable pieces to be swallowed. Puddlejumpers are so small as to be semi-transparent, their long intestines and even their beating hearts often visible through their abdomens, but butcherbacks are often camouflaged to avoid their own enemies. While puddlejumpers are short lived, persisting for only a few weeks and having rapid generational turn over, butcherbacks may live a month or more, just long enough to need defenses beyond sheer numbers, and so they are armored with sharp spines on their bodies that make them hard to swallow.

All members of this group reproduce by mating face to face, grasping each other with their feet, and females then carry their eggs adhered to their underbellies for the several days it takes them to hatch. Their larvae are free-swimming and vermiform, lacking differentiated appendages but covered in small hairs with which they collect detritus as they swim side to side in the water column, and they eat any sort of organic matter that they can nibble out of their own hair. Though the larvae lack fully formed eyes, they have light-sensitive tissues where their eyes will later develop, and can perceive shadows and naturally avoid them in order to remain in sunlight near-surface areas where food is more abundant. A major component of freshwater zooplankton at this life stage, the larvae of puddlejumpers pupate around nine days of age by forming a mucous cyst around their curled-up bodies and emerge as a fully mature adult seven days later, living only around 14-21 days afterwards; butcherbacks take roughly twice as long to complete every life stage. 

While most of these birds are freshwater distributed, especially in calm waters rich in green algae, some species of puddlejumpers are marine and tolerant of saline water and can reproduce in oceanic ecosystems, though they are rare in open water where they may be less competitive than crustacean zooplankton. Some 200 species of puddlejumper exist worldwide, around one in ten of them being at least sometimes found in saltwater; most species tolerant of seawater may only end up there incidentally if washed out to sea in rivers, but around 8 species are obligately marine including species endemic to the floating forest, trilliontree islands, and meridian seamount. There are no saltwater butcherbacks, perhaps because they are just large enough that they are more vulnerable to predation in habitats without substantial cover, and just big enough that they may struggle to swim in currents to chase prey, whereas puddlejumpers can simply go with the flow and always be surrounded by food an arm's length away.