Flopett
The Flopett split from its ancestor and moved into freshwater environments. This creature prefers to lie flat, unlike its ancestor which sits, and as such its raptorial arm is greatly reduced. It has lost its taste for shells, instead using its snout and mandibles to catch softer meals or on occasion pull shelled food out of its shell. It is also amphibious, able to pull itself onto land for short periods of time to escape predators or snack on terrestrial prey.
The Flopett’s lateral mandibles have become segmented due to a malfunctioning hox gene. This allows them to be used to grip additional food to bring to their babies, and it may be used in combat over food disputes on occasion, but overall it doesn't offer a significant advantage. The mutation was retained as a result of it not being dependent on its mandibles to crush food anymore. Its back fin has shrunk somewhat, being more effective at a smaller size.
The Flopett has advanced its reproductive system. When fertilized, it develops a temporary, superficially placenta-like womb which carries its babies. This is shed shortly after giving birth and regrows at the next pregnancy. Like its ancestor, it mates by “holding hands”—pulling back the membrane of its tail fin reveals a 3-fingered hand used to help it perform a cloacal kiss—and gives live birth to radially symmetric babies. It actively feeds its babies and defends them from predators, but unlike its ancestor it may weigh the odds and choose to abandon them instead if the situation is dire.
The Flopett is otherwise much like its ancestor. It has four lungs in addition to its four gills, four nostrils to match, a bony endoskeleton, camera eyes, nictitating membranes, a horizontal tail fin, and a pair of hearts near its lateral limbs.