Skinback
The Skinback split from its ancestor and moved into wetter climates. Its habitat mainly consists of rainforest, where its ancestor's keratinous outer exoskeleton was easily soaked and damaged. As the inner layer of its exoskeleton was made of bone and never shed, it suffered no fatal consequence when a material replacement mutation simply replaced the keratinous layer with naked skin, instantly solving the problem at the cost of no longer being well-adapted to especially dry conditions. It has also lost its tail fin, as its exposed skin is far better at releasing heat, though it retains a noticeable kink in its tail.
To say that all exposed keratin has been replaced would be false, of course. The teeth remain the same, and very soon after the initial mutation a second, more selective mutation made the claws have keratinous sheathes once more. It is also very important to remember that the now fully internal skeleton is still functionally an exoskeleton, though with its new position it is inevitable that it will not remain a perfect copy of the ancestral skeleton for long.
The Skinback has returned to more omnivorous habits, its claw-like teeth also being very useful for uprooting small flora. A generalist, it feeds on a variety of small flora and fauna, including some sourced from the local rivers and watersheds. Though not exactly a specialized piscivore, the hydrostatic nature of its already-narrow tentacles is very useful for snatching small fauna from the water.
Like its ancestor, the Skinback is aggressive and combat between competing males can result in nasty scars. It is less aggressive towards other fauna, however, as in its more plentiful habitat it has less reason to scare other creatures away from its food. It lays soft-shelled eggs, which it buries to keep them safe, and its offspring hatch resembling miniature adults.