Handfishes

Handfishes

In the middle Pangeacene, 228 million years since colonization, the fishes have established themselves firmly alongside the birds upon the land. The fully terrestrial descendants of live-bearing poeciliid fishes, descended from early amphibious mudwickets, have radiated into numerous forms including the warm-blooded tribbetheres, but these are but one branch of an even more diverse clade - the tribbets.

The first tribbets evolved from the mudwickets roughly 150 million years ago, though their group as a whole which includes all mudwickets including tribbets, appeared more than 160 million years ago. While the early mudwickets had already developed a mobile forelimb and finger-like fin rays to crawl over land, the tribbets were further distinguished by a fully impermeable skin, paired lungs (developed from an offshoot of the stomach), and a tail adapted to move in an up and down arc, rather than a strictly side to side one, which greatly improved the efficiency of their movement over the ground. Though it is difficult to distinguish exactly when the first tribbet evolved, as evolution is a gradual process of change, forms that were undeniably early tribbets had evolved by 65 million years PE, though did not speciate spectacularly until the warming climate at the start of the Thermocene ten million years later.

Tribbets evolved to be tripedal, adapting their pectoral fins and their tails simultaneously into walking limbs. Despite very different origins, natural selection in some groups - notably the tribbetheres - modified both structures into similarly jointed legs quite similar to those evolved by the tetrapod animals much earlier. In animals such as the hoppers and in particular their later descendants the molodonts, the leg structures are so similar that superficially, they are virtually identical. The tail has developed a series of joints allowing it to bend and flex similarly to the hind leg of a mammal or reptile, with the tip developing a fleshy weight-bearing pad frequently bearing ossified extensions of the spine which function as digits. The muscular "wrist" of the early mudwickets, meanwhile, has then elongated into a jointed arm with the fin rays forming a hand or foot on which the weight is supported, giving an overall impression that all three limbs share a common origin when in fact their underlying structures were once very different.

However, this is not the case for all tribbets. In the Thermocene, the tribbets diversified considerably. From a starting point of two grasping, clawed wrists and a tail that allowed for a jerky inchworm-like locomotion, several groups experimented with a number of different variations on their ancestral bodyplan that included the upright-legged ancestors of tribbetheres, sprawling reptilian forms, entirely inchworm-like species and even animals that walked not on longer legs, but on long spider-like fingers. Though most of these aberrant forms were killed off during the Thermocene-Pangeacene extinction, leaving only a few primitive survivor species and a single species of tribbethere, the resulting evolutionary vacuum gave these hardy forms a second opportunity to diversify. While sprawling tribtiles and mammal-like tribbetheres with erect legs remained the dominant forms, however, they were not the only ones. Multiple tribbet groups followed in the footsteps of their more unusual lost relatives and followed similar paths to some of these earlier forms, including the legless bodyplan which was re-adopted shortly after the extinction event, but surely the most surprising form to reappear were the hand-fishes, clade Cheirichythes.

Instead of elongating their arms, as had the tribbetheres, the handfishes - descended from some small, lizard-like arboreal form to come through the end of the Thermocene - began to evolve very long fingers, with which it could more easily clamber up into the newly returning forests and capture prey. While the arm itself stayed proportionally small, sprawling and primitive, the first three of these extremely long digits which were suited to climbing and holding onto branches became longer and sturdier and eventually became like functional arms in their own right, evolving their own small protrusions that eventually became fractal fingers - digits branching off of other digits. The same process occurred in the tail; as the spine stayed short, the side-projected digits instead elongating into a grasping pincer, perfectly adapted to attain a sturdy grip in the treetops. Over time this structure enlarged into what effectively amounted to a pair of legs - really a large grasping hand that originated from a very short tail - and which also developed smaller extensions of tissue to improve its grip and eventually grasping "fingers" of its own. Known informally as spiderfrogs, their moniker is apt; their general appearance is amphibian, with a wide mouth, large eyes, and smooth - though not permeable - scaleless skin, coupled with four extremely long fingers on their forearms used to climb and capture food. Most species still hunt insects, using their fingers to grab prey they come across while climbing through the branches and stuff it into their toothy jaws. Some forms have become ambush predators of birds and other tribbets, hanging by their tails on the underside of branches with their fingers spread, ready to grasp any unfortunate passers-by in a deadly grip. Known as dropdowns, these have developed elongated, almost eel-like bodies able to curl upwards along the bottom of a tree branch. Camouflaged like the bark, they lie here in wait, held up by a powerful tail pincer, until some small animal passes beneath. They lie motionless, moving only their large eyes, until the prey is within range and strike with great speed. The splayed fingers are closed in a cage around the hapless victim and retract upward toward the predator's powerful jaws, where it is quickly dispatched. With slow metabolic rates, they can wait long periods between meals and move only rarely, under the cover of darkness, with an ungainly inchworm motion over the tops of the branches.

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Some spiderfrogs are more active predators, which leap and chase after their food. The pinnacles of this evolutionary pathway - and the tribbets with the most specialized fingers and tail of all - are the Gibbets, a clade of extremely aberrant spiderfrog that appeared roughly 15 million years ago.

Gibbets are spiderfrogs with two hypertrophied fin rays on each stumpy forearm which have become so large as to function as individual arms in their own right, making the gibbet a truly hexapedal animal and the first terrestrial vertebrate to fit this description. Though they retain four digits on their wrists, only the first two on each are functional. The tail is extremely short, just three vertebrae in length, from which project two enormous jointed limbs that were originally derived from the small fingers that allowed the tail to grasp. Each hypertrophied finger is now proportionally longer than the limb that bears it and work together as a pair of legs, giving the gibbets exceptional dexterity in the treetops versus other tribbets which have only a single tail limb to work with. The two hind legs and four front legs of a gibbet give it an edge in climbing, as it can always be holding on with at least one or two legs while it moves through the branches and thus very rarely slips. They are predators of insects, though some species are omnivorous, and are lively, energetic creatures with higher metabolisms than their relatives and indeed most other tribbets. While not as fully warm-blooded as the tribbetheres, they are truly mesothermic - able to raise their body temperatures several degrees above their ambient environment - and thus can remain active in a wider range of temperatures. Even so, they - like most spiderfrogs - are restricted to warmer tropical and subtropical forest habitats, where they thrive in the high canopy.

above: three notable handfishes of the late Pangeacene; a primitive insectoid predator (top), a brachiating, hexapedal gibbet (middle), and a dropdown (bottom.)

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