Australia: The Land Where Time Began

A biography of the Australian continent 

Bone Structure in Early Fish

The evolution of bone begins in fish, as they were the first vertebrates. The first bone to evolve in the early fish is different from the bone found in later vertebrates. Bone is defined as the mineralised tissue that supports the skeleton, internal or external, of vertebrates. Components of bone are the mineralised portion, hydroxylapetite, collagen fibres that support the formation of mineralised bone, and some vascular tissue that supplies blood to the living cell component of bone.

The bone of early vertebrates is of 2 main types, cellular and acellular. The structure of these bone types are based on the same basic components, the only difference is that there are spaces in cellular bone for the osteocytes, the bone-forming cells that occur throughout the bone.

The earliest forms of acellular bone have a tendency to be laminated, forming in successive layers that were deposited by the dermis. Some early agnathans have acellular bone that is called aspidin in heterostracans. Acellular bone is also found in layered bone of more advanced fish. In placoderms acellular bone is found in the basal bone layer of their plates.

In the earliest fish most of the skeleton is formed of bone produced by the dermis, hence its name dermal bone. Dermal bone composed all the externally visible bones of the head and trunk and the scales and biting surfaces inside the mouth. The dermal bones and scales of crossopterygians and dipnoans were primitively covered with a shiny enameloid layer covering an interconnecting network of flask-shaped cavities in the uppermost bone layer. The complex layer is called cosmine.

There other types of specialised bone that developed in various early osteichthyans. These bone types are characterised by the complexity, nature and composition of the external layers of the dermal bones, as well as their growth processes and whether or not resorption was involved in their development.

Among other bone types found in early vertebrates are perichondral bone (a thinly laminated acellular bone) and endochondral bone. Perichondral bone is often found surrounding soft tissue that passes through cartilage, as can be found in placoderms with cartilage braincases where the nerves and arteries passing through the braincase wall can have perichondral bone surrounding them. These delicate tubes preserve the outline of the soft tissue in fossils. Such remains can produce very intricate fossils when prepared with weak acetic acid. The endogirdles that support the pelvic and pectoral fins also has perichondral bone surrounding it.

Endochondral bone forms around a cartilage precursor. These bones make up much of the internal skeleton in reptiles and mammals, and forms the arm and leg bones. Endochondral bone first evolved as a specialised feature in some groups of gnathostomes (osteichthyans and acanthodians).

As well as these basic tissue types, there are many types of bone involved in dental tissues that evolved in fish and continued on to the higher land animals. Chondrichthyans have a mostly cartilaginous skeleton, but in the dorsal fin spines they have a type of dentine with a thin enameloid layer. Nearly all the fossil parts of sharks are a similar type of dental tissue, including the placoid scales covering the body that are similar to small teeth. In many early agnathans there are layers of dentine covering the dermal bone, as in thelodont scales.

Sources & Further reading

1. John A Long The Rise of Fishes - 500 Million years of Evolution, University of New South Wales Press, 1995