Rivet
Splitting off from its ancestor, the Rivet resides in Darwin. Through neoteny, it has lost its back fin, regained a proper hand-like hand on its anal arm, and shifted its organs forward into its neck and pelvic area, the latter better placing its center of gravity over its legs. This causes it to look somewhat like the larvae of its relative, the Kugard, but this is only coincidental. It retains the ability to eat crunchy foods, but with the abundance of softer food in the rivers, it happily eats that more often. Adults lack gills with the upper pair being replaced with a tympanic ear; as it is not yet able to resolve the problem of caring for its offspring on land, rivet larvae are deaf and retain gills. To reduce the amount of material that has to be reabsorbed in adulthood, juvenile gills lack covers and are no longer connected to the nostrils at all. The gill material is exposed, mobile, and frilly, absorbing oxygen as it flows over them. Both juveniles and adults retain the 4 ancestral lungs, which serve for both respiration and flotation. Without mobile gills to force air into the lungs, the rivet inhales by expanding its sides.
Though it has better balance, the Rivet is not capable of running, still relying on short hops to move about and escape predators living in the riparian biomes. Still, its legs have better leverage due to the lengthening and shortening of specific segments, so it is generally better at escaping than its ancestor. It can be sluggish on a cold day, though with no endothermic predators in its environment at the time it evolved this isn’t a severe disadvantage. It is notably more aquatic than its ancestor, but this is partly because there is so much food in the riparian zones of its habitat. It can use its lateral mandibles to grab at potential prey, such as crestgills and knightworms, to bring them closer to its mouth. The Rivet’s skin contains even more keratin than before, making it tough and better able to survive attacks from predators.
Due to its anatomical changes due to partial neoteny, the Rivet has an elongated cloaca and therefore birth canal reaching all the way to its tail-like anal arm. Like a spotted hyena, it must give birth through this long narrow passage, though with the streamlined shape of its offspring and an overall stretchiness of the passage itself this is not nearly as difficult a task as it is for a hyena. Still, it is not unheard of for first-time mothers to tear their cloacas and bleed to death after giving birth, leaving the father alone to raise his offspring. The womb is located under the "tail”, and so this region bulges when a female is pregnant. Despite the long passage the Rivet has to work with, its mating method of “holding hands” is hardly changed, though males produce considerably more sperm to ensure reproductive success.