Tribbets of the Late Thermocene: Meet the Hoppers

Tribbets, the terrestrial descendants of the mudwicket fishes, have continued to thrive in the tropical climate Serina has offered them since the start of the Thermocene, building upon their adaptations and becoming increasingly at home on the land. They have made themselves at home across the globe, broadly filling niches which on Earth would be taken by reptiles and amphibians. They are predominately cold-blooded creatures dependent on the environment to keep themselves either warm or cool. They scurry along forest floors, pulling themselves along with long claws derived from their fin rays or bounding inchworm-style through the trees, supported by a long prehensile tail. One group of tribbet in particular, however, is worth mentioning on our latest stop to the late Thermocene era; the hoppers.

Already fully terrestrial fifty million years ago, another fifty million years of highly favorable warm and wet conditions has seen in this particularly successful lineage the appearance of many new adaptations and specializations which are not found in the rest of kin. Among these have been the development of a proper forearm, not simply a mobile lobe with claws, but complete with a bony skeletal support and shoulder, elbow, and wrist joints, together allowing greatly increased mobility. The tail in this group has developed joints as well, coming to resemble an additional leg more than a prehensile tail. The two forelimbs are semi-sprawling, projecting slightly outwards, but the tail, twisted 90 degrees directly underneath the body so that it flexes back and forth relative to the spine, now functions as an erect limb. It is tipped in three to four grasping digits also edged in keratinized claws and capable of grasping in some species, providing not only a leg, but a hand. Some "hoppers" thus become climbers. The beginning of a neck apparent in their ancestors has become longer and can now rotate as far as ninety degrees to either side, allowing the animals to look around for food or danger without having to totally rotate their bodies.

above: two species of hopper from the late Thermocene; a semi-arboreal omnivore (top) and a terrestrial carnivore (below), shown both at rest and with jaw extended in pursuit of prey.

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Hoppers are small animals, rarely much larger than a rat, but they are still able to compete successfully with many of their kin, sometimes even moreso. This is because hopper metabolism is particularly advanced compared to that of other species and indeed they are especially active among their kin, an attribute that has likely evolved simultaneously along with their adaptations to move more rapidly and efficiently over land. This is because they have become capable of maintaining their body temperature to a few degrees above that of ambient conditions and thus no longer as dependent on the sun to warm themselves at the start of the day. Because of this, hoppers can also continue to be active and feed even on fairly cool or cloudy days when other tribbets will be forced to retire to the safety of a den or burrow and lie inactive until conditions improve. No longer needing their dorsal fins for thermoregulation, they've lost these structures entirely.

Hoppers are the first tribbet to develop a solid palate in their upper jaw which effectively separates the mouth from the nasal chambers, allowing them to breathe through their nostrils while feeding. The respiration of hoppers is more efficient than that of their ancestors and they are capable of breathing in and out simultaneously, increasing the amount of oxygen they can get out of every breath. This puts the hopper on par with birds so far as respiratory efficiency and past mammals, such as ourselves, which must exhale before taking in a new breath, wasting a considerable percentage of the oxygen in the process. Even the lungs of hoppers have been improved from their ancestors, becoming chambered; the increase of space for gas exchange further improves the amount of oxygen they can get from each breath, and the more oxygen they can dissolve into their tissues, the more active they can be without tiring. In warm conditions, hoppers may be as active as fully warm-blooded birds, only slowing down and retiring to a sheltered place to wait out the cold if temperatures fall below approximately fifty degrees Fahrenheit (10 degrees Celsius). More oxygen also allows for the evolution of larger brains and social behaviors; some hoppers develop parental care and watch over the offspring, carrying them or leading them to food and away from predators rather than simply abandoning them to their fate at birth.

Though some tribbets share some convergent traits to tetrapods, particularly in the design of the hopper forearm, the tribbet jaw is considerably distinct from the tetrapod jaw and retains many features from their teleost fish ancestors; in particular, the jaws of many, including the hoppers, are still extensible, elongating when the animal opens its gape and retracting when the mouth is closed as a result of a mobile maxilla attached via a mobile joint to the front and side of the skull. Attached on its lower end to the upper (the premaxilla) and lower jaw (formed in the tribbets by a fusion of the mandible to the opurculum), the jaw is thus opened by sliding the maxilla from a nearly horizontal position to a nearly vertical one, projecting both jaws forward in the process. By elongating their jaws in this fashion, predatory hoppers can improve their chances of grabbing small fleeing prey items before they have a chance to escape, and herbivores can improve their reach to get to higher leaves or branches, if only by a little bit. There is one way in which hoppers haven't yet been able to improve upon the ancestral tribbet bodyplan, however; they, like, all tribbets, can still only very crudely chew their food by gnashing their jaws together, relying on their stomachs to do most of the work.

The ears of all tribbets are bone-supported structures attached to the operculum. A remnant of fleshy gill covers, the ear of many tribbets continues to become a more effective sound-amplifying device which in the hoppers can now be rotated to exactly pinpoint the noises of predators or prey, transferring sound down into ossified gill arches hidden under the braincase which now function as the tribbet's ear canals, transferring sound to the brain. As it originates from the operculum bone, a hopper's ears move forward when it opens its jaws, coming to rest directly behind the eye when the jaw is opened to its full extent and sliding towards the back of the braincase when the jaw is closed.

above: a diagram showing the major bones involved in opening and closing a tribbet's jaw and how they move in relation to eachother.