Australia: The Land Where Time Began

A biography of the Australian continent 

Gnathostomes - Jawed Fish

It was at this time that the first jawed fish with paired fins arose, diversifying and competing with the dominant agnaths. The stock from which these gnathostomes arose, and the interrelationships between members of the group, are unknown, and are still hotly debated. In the Late Silurian several different groups suddenly appear in the fossil record. As they are already well along in the diversification process, they must have arisen some time before from an unknown stock that is still to be discovered. The poor record of this time is the main problem with sorting out where they came from. 2 modern groups are represented among these fish, the sharks and bony fish, and 2 archaic groups, the acanthodians and placoderms.

The placoderms had a number of unique features that separates them from other fish types, so it seems unlikely that they gave rise to other fish types. The acanthodians, or spiny sharks, on the other hand, share a number of features with sharks and bony fish, making it possible that they might have given rise to the other 2 groups of fish.

These early Gnathostomes were the first of a line that eventually gave rise to all the later vertebrates, including humans. The movable jaws allowed these fish to diversify into herbivores and carnivores, filling niches the agnaths weren't able to occupy because of the limitations imposed by their jaws. The herbivores could now diversify into more specialised and more complex patterns of feeding, and the carnivores were no longer restricted to prey much smaller than themselves.

Another advantage the gnathostomes had was the paired fins with internal support possessed by most of the early forms which allowed greater manoeuvrability. The evolutionary advantages of these new anatomical features led to an explosive radiation of the vertebrates during the Devonian.

In Australia, the vertebrate fossil record in the Silurian is very poor, unlike the Devonian in Australia, when there is a rich fossil record. Most of the Australian Silurian fossils are of Acanthodians, represented by scales, found in Late Silurian deposits from eastern Australia. From these known deposits it seems the  acanthodians appeared in the Australian record at about the same time they appeared in other parts of the world.

See Origin of Jaws.

Sources & Further reading

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