Australia: The Land Where Time Began

A biography of the Australian continent 

Basic Structure - Primitive Agnathans

The early agnathans are mostly known from cranial shields, isolated bone fragments and scales. The occasional find of more complete remains shows that the early forms were heavily armoured and their tails were covered by thick bony scales. They often had either a dorsal fin or an anal fin, only 2 groups, Osteostraci and Pituriaspida, are known to have had paired pectoral fins.

The bone of the shield is made up of an outer ornamental dentine layer which is often covered with a shiny enameloid layer over the dentine that is perforated by many pores. The next layer is a middle vascular layer with spaces that are occupied by collagen fibres and in some cases osteocytes (bone cells). The base is composed of laminated, non-cellular bone - aspidin.

Each armoured agnathan group has a different form of bony shield. The Osteostraci ("bony shield") and the Galeaspida ("helmet shield") had a shield composed of a single unit of bone with holes for the eyes, nostrils and other sensory organs, and open on the ventral side for the mouth. The armour of the Heterostraci ("different shield") is formed from several different sized plates. Large overlapping scales cover the body.

There is no enlarged bony covering in the Anaspida ("no shield") and the Thelodonti ("scale tooth"), the head and body being covered by scales. Some Anaspids had large, flat scales, others such as Jamoytius from Scotland seem to have had naked bodies similar to that found in the lampreys.

In the few fossil agnathan brain cases there are 2 semicircular canals in the inner ear, and the brain is divided into separate divisions, as in higher vertebrates. There was a large vein draining blood from the head and most fossil forms had complex system of lateral lines or sensory fields.

The soft tissue of the braincase is preserved as delicate shells of perichondral (laminar) bone in the Galeaspida the Osteostraci. These 2 groups are also the only ones known to have had the head vein in a dorsal position below the top of the armour.

Sources & Further reading

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