Australia: The Land Where Time Began

A biography of the Australian continent 

Global Faunal Similarities in the Devonian

During the Early Devonian Gondwana and Euramerica were separated by a big stretch of ocean. In addition to the main continental blocks comprising Gondwana, the were a number of smaller landmasses. In the southern hemisphere, closely allied with Gondwana, were Shan-Tai and southern China. Some other smaller blocks situated in the Northern Hemisphere, and thought to be associated with Gondwana, were Kazakhstan and Siberia. It is believed there was possibly another microcontinent, named Armorica, that was somewhere between Euramerica and Gondwana. It comprised parts of France, Germany, Bohemia and Spain. During the Early Devonian the fauna of Armorica seemed to be much more closely related to that of Gondwana than of Euramerica.

In the early Devonian there were several distinctive faunal provinces: Euramerica, Siberia, South China, East Gondwana. At the time the Euramerican Province was defined by the agnathan cephalaspids, the Siberian Province by the amphiaspids, the South China Province by the agnathan galeaspids and yunannolepid placoderms, the East Gondwana Province by the wuttagoonaspid and phyllolepid placoderms.

This latter province is most characterised by Wuttagoonaspis fletcheri. This was a unique arthrodire that had a long head shield with small openings for the eyes and a tall crest along the dorsal surface of the trunk armour. On the head shield and body armour was a meandering ridge ornamentation similar to that of phyllolepids.

Palaeogeographical reconstructions indicate the existence of Armorica, the fauna of it and Gondwana share a number of genera or closely related genera, and these genera are missing from the Euramerican province to the north during the Early Devonian. Speonesydrion, a dipnoan,  is found in south-eastern Australia but close relatives have been found in the Hunsruckschiefer sediments of Germany. An acanthodian, Machaeracanthus, is known to have been widespread in Gondwana, including Australia and Antarctica and is also found in marine rocks from the Early Devonian in the Rhineland of Germany. The placoderm, Buchanosteus has been found in eastern Australia, China and the Middle East, and a near relative has been found in Brittany. A Petalichthyid placoderm, Lunaspis, known from the Rheinische Schiefergberg of Germany is also found in Gondwanan Australia and South China. A close relative Wijdeaspis, found in Australia is also known from Elburz Mountains of Iran. A similarity of the Early Devonian faunas of Spain and Gondwana has also been shown by microfossils of  acanthodians and sharks, as well as several osteichthyan groups.

This all leads to the conclusion that in the Early Devonian a major faunal province included Gondwana and parts of modern Europe. In the Late Devonian this province was brought close to Euramerica by tectonic plate movements, allowing an exchanges of faunas. This close proximity allowed at least some of the Gondwanan  placoderms to spread to Euramerica, forms such as  Bothriolepis, Phyllolepis, Groenlandaspis and Remigolepis, first appeared in the fossil record of Euramerica, so are assumed to have crossed from Gondwana. The arrival of the Gondwanan fauna had a big effect on the Euramerican endemic fauna. One example known from this time is the agnathan heterostracan psammosteids, bottom-dwelling forms, that appear to have been outcompeted by Phyllolepis from Gondwana.

Sources & Further reading

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