Australia: The Land Where Time Began

A biography of the Australian continent 

Fish to Tetrapod - Postcranial Skeleton

In all fishes and tetrapods the postcranial skeleton, like the skull, consists of elements which can be divided into 2 different regions, as can be seen in the skeleton of Eusthenopteron.

Axial skeleton

The vertebrae and associated structures, such as the ribs and the midline fins, in the case of fishes, comprise the axial skeleton. A vertebra consists of the notochord, the supporting embryonic bar around which the centrum forms occluding the notochord, and the neural arch over the spinal or nerve cord rests on the centrum. The centrum can be formed from a single element, or they can be a compound of single and paired elements, and the neural arches may or may not be joined to one another. Ribs may be present, articulating with a neural arch and/or a centrum. Haemal arches may be present under the centra in the tail, which allows passage for the blood vessels serving the posterior part of the body. There may be differentiation of different regions of the vertebral column, such as the cervical region being at the front, with the thoracic in the middle, the sacral near the pelvis, though in fishes the differences are minimal, and the terms may not really be applicable, and all of these elements are endochondral.

Appendicular skeleton

The limbs and girdles, and at the front are the shoulder or pectoral set, and at the rear the hip or pelvic set, together comprising the appendicular skeleton. A mixture of elements, including the dermal bones, such as the cleithrum, clavicle, and interclavicle and the scapulocorocoid, which is of endochondral origin, comprise the pectoral girdle. The articulatory socket, or glenoid, is provided by the scapulocorocoid, for the pectoral appendages, fitting within and behind the sheathing series of dermal bones. The pectoral girdle is attached indirectly to the vertebral column by soft tissue or by other linking bones. The pelvic girdle, which is composed entirely of bones of endoskeletal origin, carries the acetabulum or the socket, for the pelvic appendages.

Dermal skeleton

An additional part of the skeleton in fishes includes the bony scales and the bony fin rays, or lepidotrichia, which support the web of the fin, all being of dermal origin. The tail was heterocercal in most of the primitive fishes, i.e., the tail was asymmetrical, the body lobe extending up the leading edge of the fin with the main part of the webbing of the fin being suspended below like a flag from a pole, and a lesser fringe was present above the body lobe in some of these early fishes. The primitive condition of the scales, cosmoid scales, was to have a layer of tissue, cosmine, that was enamel-like which imparts a glossy look to these fossilised remains.  

The features Clack¹ mentioned in this section of her book are common to all lobe-finned fishes, tetrapods and the ray-finned fishes; they are the shared inheritance of all the bony fishes, the osteichthyans.

Sources & Further reading

1. Clack, Jennifer A., 2012, Gaining Ground: The Origin and Evolution of Tetrapods, 2nd. Edition, Indiana University Press