The Truly Terrestrial Fishes: The Tribbets
100 Million Years Post-Establishment
100 million years have now passed on Serina, a currently tropical celestial body. Since the start of the Thermocene, life has continued to adopt new strategies to win in the game of life, a game whose rules never stay static for long. The longer life stays this race, often the stranger it becomes. On Serina in the Thermocene, perhaps the strangest of all are the terrestrial fishes which have experienced a boom in diversity and entered into their heyday since the warming of the climate at the end of the Cryocene.
Fish Of the Forest
The warm, wet hothouse climate of Thermocenic Serina had been ideal for organisms once completely restricted to watery habitats to try their hand at moving inland. The last twenty five million years have seen the appearance of a wide variety of new diversity, particularly of the eelsnakes and the mudwickets. Both groups have expanded their ranges toward the poles, following the tropical forests. Eelsnakes, which early on developed a lung to take in oxygen from their stomach tissues, have evolved further and further from their dependence on water, with some groups being completely independent of it and living underground or even in trees, kept from dehydrated by smooth, scaly body coverings that prevent the loss of water. Burrowing eelsnakes often lose their eyes completely and rely on their strong noses to find food, resembling large, ravenous earthworms that prey on whatever they can catch, from molebirds to crickets to other eelsnakes, while species that hunt above ground develop large eyes and excellent vision, often hunting at night. Prey is usually taken whole in all circumstances, for eelsnakes cannot chew, though if an animal is too large to swallow alive, it may be constricted, and venomous forms have evolved which need do little more than strike and wait.
The more that members of this group cut their ties to water, the fewer superficially piscine traits they carry; most eelsnakes no longer sport fins or tail flukes, though primitive species similar to the common ancestor surely persist in wet and boggy environs throughout the world, some filling niches akin to the Earth's crocodiles.
From the mudwickets, a particularly alien new group of terrestrial fish known as the Tripodichthyes, or tribbets, has evolved, which are equally adapted to a life on land. Through the tribbets, the mudwicket group is today experiencing its heyday - thousands of species have arisen and spread across the global jungle, moving into new habitats and niches and places that fish have never before gone. Like many eelsnakes, they have redeveloped an armor of small, beaded scales to prevent water loss and have also developed a set of lungs to gather atmospheric oxygen to replace skin-driven respiration, which is also situated off the stomach. The tribbets are thus now as wholly and completely independent of water for their survival as any bird, needing only relatively small quantities to drink, allowing them to survive not only in wet forests but even dry grasslands and new desert habitats. They have specialized their tails into a proper foot, evolving opposing lobes at its tip to function like fingers and provide stability, plus to get a better grip on the ground when hopping or walking. From here it wasn't a great step to lengthen the lobes into fingers, and in some cases the whole tail into a third arm of sorts, to hold onto the branches of trees to reach new food sources. In conjunction with an increasingly mobile set of clawed, fingered forearms - derived from the rays of their ancestor's pectoral fins - these new land-lubbers are now quite well-suited to scurrying, jumping, and climbing over all the land has to offer. They often have powerful jaws and sharp teeth, feeding on a wide variety of invertebrate prey as well as smaller fishes and birds, as well as fruit and even foliage in some forms which have started to experiment in omnivory. Most species are fairly slow, particularly the lethargic arboreal forms that ambush their prey, but others can move at fairly rapid speeds for short bursts to pursue prey or escape predation, bounding with a strange inchworm-like locomotion.
A variety of tribbets - and one eelsnake - showcasing the diversity of Serinan terrestrial fishes in the Thermocenic.
Top branch: an arboreal insectivore which catches insects with a long sticky tongue postures defensively against an arboreal eelsnake, making an effort to appear too large to eat.
Lower branch: an active omnivore pauses alertly to sight for danger while nearby a lethargic predator with a particularly stout, muscular "tail hand" well-suited to holding onto the branches basks with its sail extended into the sun to warm up.
Ground: An omnivorous agamid-like tribbet suns itself on a rock, demonstrating a similarly muscular tail to the arboreal predator which is likely a close relative. Hidden in the leaf litter, a little frog-like tribbet - a relatively basal member of the group - spends its days searching for worms and small insects that it can stuff into its wide mouth with its stout forearms and swallow whole.
Tribbets, as well as eelsnakes and all other land-living fish on Serina by this time, still lack the ability to control their own body temperature, but in a tropical world this is hardly a handicap as the fishes can simply bask in the sun if cool or retire to shady glades when overheated, much like Earth's lizards. Some groups have adapted their dorsal fins into large sails for purposes of thermoregulation, which can be raised or lowered at will depending on environmental conditions; in some species the large gill covers, already mobile and used as display structures, are used in the same vein. Tribbets additionally have begun to adapt their gill covers into something far more novel, however - the beginnings of mobile ear pinnae. The gill arches in these land-lubbers have all but closed up and become vestigial, save for a small canal which leads into a hollow chamber just below the skull where a number of small bones transmit soundwaves up to the brain, allowing the tribbets to hear. Though at rest their gill covers are positioned flush against the skull and directed backward, when hunting or otherwise listening intently, many species can rotate them at least to some extent so that they catch noise situated ahead of the animal. As might be expected, with the ability to hear also comes vocalization. For the tribbets, this is done with a frog-like air sac on the underside of the throat, which can be filled with air and resonated to produce loud chirps and calls, which are particularly utilized in males to call potential partners.
Both the tribbets and the eelsnakes practice internal fertilization - the retractable gonopodium fin of the male serves to transfer his sex cells directly to the female's ovum - and the young are born live, just a few at a time (relative to most fishes), but well developed and able to care for themselves on dry land immediately.