FROM FISH TO TETRAPOD
The development of ancestral tetrapods from the fish involves physical changes.
1.The loss of several bones from the skull. This isolated the head from the shoulders and produced a recognizable neck.
2.The loss of bones that support internal gills. The gills themselves may have been retained in some genera.
3.The development of a backbone of strong vertebrae as the main organ of support, rather than a notochord – a rigid rod of gristle – and the subsequently reduced notochord not reaching into the skull.
4.The development of muscular limbs with separate fingers and toes.
5.The formation of a bony connection between the backbone and the hips (the sacrum). This is a weightbearing adaptation.
6.The loss of the fin rays – structures that would get in the way on land.
7.The first feet did not have the standard five-toed pattern.