Tetrapodomorphs

Characteristics

Among the characteristics defining tetrapodomorphs are modifications to the fins, notably a humerus with convex head articulating with the glenoid fossa (the socket of the shoulder joint). Another key trait is the internal nostril or choana. Most fish have two pairs of nostrils, one on either side of the head for incoming water (incurrent nostrils) and another pair for outgoing water (excurrent nostrils). In early tetrapodomorphs like Kenichthys, the excurrent nostrils had shifted towards the mouth's perimeter. In later tetrapodomorphs, including tetrapods, the excurrent nostril is positioned inside the mouth, where it is known as the choana. The nearly-equivalent clade Choanata often refers to these later forms specifically.