Australia: The Land Where Time Began

A biography of the Australian continent 

Microfossils of Devonian Vertebrates

Thelodonts

All major groups of fish are represented by the microfossils that have been found in Australian sites. These microfossils are aiding in the dating of the Australian rocks. One of the most commonly found fish in Australian rocks are the thelodonts (nipple tooth). These were the thelodonts, small agnathans of an average of about 150 mm, but they could reach up to about 1 m. They lived in marine and freshwater, and they were covered with small, thick dentine-covered scales, but no body armour.

The scales are from 0.1 to a bit more than 0.3 mm wide. The scales are very much like mammalian teeth. The shape of the scales can vary over the body of an individual fish. As a result of the study of the rarely found complete fish shows that accurate species identification can be made by examining isolated scales.

The only group known from Australia are the hybodontids, that range from the Late Silurian to the Late Devonian, and includes such genera as Turinia and Nikolivia. Australolepis, from the Late Devonian (Early Frasnian) Gneudna Formation in the Carnarvon Basin of Western Australia, is thought to be possibly the youngest known thelodont in the world.  By the end of the Early Devonian, thelodonts had almost completely disappeared from the rocks of Europe and America, though they still flourished around the shores of Gondwana, in Australia, as well as Iran and Thailand.

Sharks

Antarctilamna is one of the sharks, known from the Middle Devonian from the Bunga Beds in coastal south-eastern Australia. It also occurs in the rocks of Antarctica, Iran and Bolivia. Its spines and teeth identify it as a xenacanth shark, a group that invaded freshwater. Harpagodens (aka Thrinacodus ferox) was an unusual shark that had characteristic horn-like teeth. It is found in rocks from the latest Devonian and Early Carboniferous of Australia. It is also known from Thailand, China, Europe and North America.

Scales, spines and teeth from acanthodians, crossopterygians, dipnoans, actinopterygians and placoderms are also present in the rocks. Some of these can be associated with partial or whole fish, many of them have very restricted time ranges.

Sources & Further reading

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