With the movement to land the vertebral column became more important in locomotion and in maintaining posture.  Thus, amphibians have more completely ossified vertebrae than do fishes.  Overlapping zygopophysis, which can be seen in the Necturus skeleton, holds these vertebrae together.  The postzygopophysis of one vertebra lies on top of the prezygopophysis of the next.  The centrum shape of the Necturusâ vertebrae is amphicoelous, the centrum on both sides of the vertebrae is concave.  The arrangement of the zygopophysis and the centrum shape allow for lateral bending but prevent vertical bending.   The movement from water onto land was the primary factor in regional specialization of the axial skeleton over evolutionary time.  As the first terrestrial vertebrates lost their gills and the bony connection between the skull and pectoral girdle developed, the evolution of the neck began.  The development of the neck in early tetrapopds is due, in large part, to the introduction of the cervical vertebrae, which allowed for increased mobility of the head.  This allowed them to scan the environment for predators and prey more easily.  The regional specialization continues with the development of the one sacral vertebra, which is a modification to withstand the push against the earth.  The trunk vertebrae stretch between the cervical and sacral vertebrae and serve as attachment sites for the ribs.  The Necturus ribs are short, primitive ribs.  The ribs do have two heads, the caput and tuberculum, which continues to be seen in later vertebrates as well.  The last type of ribs seen on the Necturus is the caudal vertebra, which lack ribs and bear hemal arches as seen in Squalus.