Australia: The Land Where Time Began

A biography of the Australian continent 

Heterostracans

This was a diverse assemblage of agnathans. In these, several bony plates formed the shield, a dorsal and a ventral shield, with a single plate covering each of the single pair of brachial (gill) chamber openings along the side of the armour, the brachial plates or plate, where they are fused into 1 bone. In some cases there were smaller separate plates around the eyes (such as orbital, suborbital and lateral plates. There could also be unusual oral plates lining the mouth opening. Intersecting sensory lines cross the plates as linear or curved grooves in the bone, or as lineations outlining the surface features. They often had elaborate bone surface patterns.

The major radiation occurred early in the Silurian, being common in Euramerica and Siberia during the Devonian. Most were small, 10-15 cm, but the largest were the flattened psammosteids at a metre or more.

Some of the small forms were traquairaspids and cyathaspids from the Canadian Arctic and the UK. The surface ornament that was very elaborate on their shields that were relatively simply shaped makes them easy to distinguish. Among the heterostracans these were primitive forms lacking the elaborate spines that were present in later lineages such as the pteraspidiforms, and there were only a few large scales on their tails. Athenaegis (Athena's shield) from the Silurian age Delorme Group in Canada's Northwest Territories is very well preserved, whole fish being present. It was a small fish about 5 cm long with a lower lip of the mouth that had a V-shaped leading edge that has been suggested to have been used for feeding on plankton or detritus.

Other cyathaspids such as Tarquairaspis, Corvaspis, Tolypelepis, and Lepidaspis had surface ornaments that were very elaborate bony ridges. Flourishing during the latter half of the Silurian, they had disappeared from the fossil record by sometime in the Early Devonian.

The pteraspidiforms (wing shield) was one of the most successful heterostracan groups in the Devonian, they had winglike pointed spines, cornua, at the sides of their armour. There was a more complex shield on Pteraspidiforms than covering the cyathaspids, the upper part of the armour being formed of separate rostral, pineal, and dorsal discs. Bizarre pointed processes or protuberances at the front of the armour were present on some forms such as Doryaspis from Spitsbergen in Norway. In Doryaspis the rostrum, with laterally flared wide wings, is ventral to the mouth. In forms such as the Canadian form Unarkaspis had high dorsal spines and wide lateral spines on the armour. A French palaeontologist, Alain Blieck, has now subdivided Pteraspis, originally regarded as a single genus, into several distinct genera. The Pteraspidiforms have now become useful as age markers in Devonian rocks of Norway, as well as other parts of Europe, western Russia and North America as a result of Blieck's work. A detailed stratigraphic zonation was first established by Errol White, a British palaeontologist. Among the pteraspidiforms some of the better-known ones are Errivaspis from Britain and France Rhinopteraspis (with a long elongated rostrum) from Europe and North America, and Drepanaspis, a large flattened form found in the Hunsrück Shales of the Rhineland, Germany.

Strange armour also occurs in the amphiaspids, a group of heterostracans that are unique to Russian terranes. Their armour took the form of a single bone piece that was wide and rounded. Some of these bone shields have been compared to flying saucers. The shields on most amphiaspids were about 10-18 cm long, the largest being about 40 cm; There was a bony feeding scoop or tube at the front of the heads of some species such as Lecaniaspis and Elgonaspis that Long suggests may have been used as a pump to suck up small organisms from the mud. This group had very small eyes, and in some cases they were entirely lacking. He also suggests that the apparent unimportance of eyes in these fish was because they lived on the mud surfaces and relied on burying themselves in the mud of the seafloor to evade predators. They also had what Long describes as "exquisite lateral line systems" to help avoid becoming prey. As some of the amphiaspids have been found with bite marks on their remains that had healed it indicates that they often escaped after being attacked by gnathostomes they shared their habitat with.

Sources & Further reading

1. Long, John A., 2011, The Rise of Fishes - 500 Million years of Evolution, University of New South Wales Press, 2011

Fish