Northern Notback
The Northern Notback split from its ancestor and moved north. Though winter conditions are generally terrible for cold-blooded creatures dependent on the sun to warm up, the Northern Notback is able to get by without freezing through a combination of various adaptations. Its exoskeleton, though certainly not as effective as a fibrous coat, is able to hold quite a bit of heat in just by its protective nature and non-living outermost layer. Its body is also larger, generally thicker, and more rounded, without many thin easily-frostbitten extremities. It has lost its tail fin to prevent it from freezing off, and the end of its tail is a rounded ball. Finally, the Northern Notback has notably gained the ability to produce some of its own body heat through the motion of its own muscles, retaining that heat through the aforementioned features. This allows it to thrive in temperate winters and even extreme polar conditions, though especially far north subspecies must hibernate or even migrate south for some part of the year. Likewise, those residing in temperate habitats typically migrate north in the summer as well.
The Northern Notback retains its ancestor’s ability to burrow, which allows it to wait out unfavorable conditions such as frigid blizzards. It has developed slit-shaped pupils, which help it avoid damage to its eyesight from snow glare. Its lung shell partly covers its side nostrils, protecting the flesh there from freezing. Its face is also shorter to protect it from the cold, though its tentacles can stretch outwards up to three times their relaxed length when needed.
The Northern Notback is ovoviviparous. It developed this adaptation because, in regions with permafrost, its eggs could not be sheltered from the cold as effectively. The mother of the eggs is far better insulated as a result of various heat-retention adaptations, so retaining the eggs eliminates most risk of freezing. Its offspring are larval burrowers, resembling a Wormback even in color; they do not gain their brilliant coloration until adulthood.
Like its ancestor, the Northern Notback does not fight over burrows, but males will fight over the right to mate. Two males will interlock their tentacles, not actively trying to harm one another more than just trying to intimidate the other into leaving; however, the female may still choose the loser if the winner is overly aggressive or visibly unhealthy. To mate, the two Northern Notbacks will stand side-by-side, lift and twist their tails to position their respective cloacas, and perform a cloacal kiss. The cloaca is located between exoskeleton segments, but when exposed by this position the opening is actually fairly large—plenty sufficient to give birth to decently-sized babies. It does not engage in parental care, its wormback-like larvae being very capable of caring for themselves.