Arguments for interpreting the vertebrate functional neck as an exaptation for terrestriality

The initial stages leading to the vertebrate neck are aquatic; this has been long known. These initial stages include the disconnection between skull and pectoral girdle, and the loss of several opercular bones. The late Devonian (early Frasnian stage, 385.3-374.5 mya) Tiktaalik is currently known as the sister group of Tetrapoda and already presented these features. A fully functional vertebrate neck is only complete after the notochord is withdrawn from the skull. The pertinent literature indicates that the progressive shortening of the rear region of the skull in tetrapodomorph vertebrates reduces available space for the notochord within the skull. We suggest that the withdrawal of the notochord is a byproduct (a ‘spandrel’) of this evolutionary trend. This hypothesis is congruent with the usefulness of the functional vertebrate neck in water. Both hypotheses support a third view, the emergence of a functional neck in the ‘original aquatic environment’, instead of considering this feature to be an adaptation for land. This novel, three-stage proposition is more parsimonious than the previous interpretation, in which the character was considered to represent an adaptation for dry land.