Australia: The Land Where Time Began
Fish Photo Gallery
The earliest fish found in Australia so far are Arandaspis and Porophoraspis.
The record of jawed fish in Australia begins in the Late Silurian and Early Devonian. The Devonian is called the 'Age if Fishes'. It was a time of explosive radiation of fish types, the jawed fish expanded, while some of the more successful jawless fish persisted. At this time Australia was much closer to the equator than at present and a Palaeozoic great barrier reef (Devonian Barrier Ref) existed at what is now the Gogo formation in Western Australia. At Buchan in Victoria there were reef-like structures.
Fish faunas of Australia during the Early Devonian were closely allied with those of other parts of Gondwana, Antarctica in particular, and were distinct from the European and North American faunas of the time. There were some faunal similarities between those of Australia and Southern China, and Amorica, another continental block, between Europe and the North American landmass and Australia. Plate tectonics moved the Gondwanan and Laurasian landmasses close together in the Late Devonian allowing a major faunal interchange between the 2 main continental masses. The placoderms were among the fish involved in this exchange. Many of the Gondwanan species were successful in this merger, in particular the placoderms, displacing northern species.
During the Middle Devonian, the combination of climate, Australia was on the Equator, and it was a time of globally maximum flooding of continental areas. It proved to be ideal conditions for the fish to diversify widely. There was a rich diversity of fish and invertebrates in the warm shallow seas that covered much of Australia, and the remains of the Devonian great Barrier Reef on Gogo Station contains some of the best preserved vertebrate fossils from the Devonian anywhere in the world.
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 possibly 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.
By the end of the Devonian Gondwana was being moved towards the south pole.
- John A Long The Rise of Fishes - 500 Million years of Evolution, University of New South Wales Press, 1995
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| Author: M. H. Monroe |
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| Last updated 07/10/2014 |
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