Australia: The Land Where Time Began

A biography of the Australian continent 

Coelacanths - Actinistia Crossopterygii - Lobe-finned fish (tassel-finned fish)

These appear in the Middle Devonian. The coelacanths probably reached a peak in their diversity in the Carboniferous, many forms inhabited the shallow seas and rivers throughout the world. The most characteristic features of the actinistians is that they don't have an upper jaw bone (maxilla), their cheek bone arrangement is loose, many paired snout bones with large pores in each-the snout has a special rostral organ, and there is a special double-tandem articulation of the lower jaw. They have a long median lobe on the tail with a rear tuft, the paired fins and some of the median fins are all strongly lobed. Some genera have strong spines supporting the fin web on those fins that are not lobed. Compared with other crossopterygians the shoulder girdle bones are elongated, most genera have a bone, the extracleithrum,  attached to the cleithrum, that is unique to coelacanths

The earliest known coelacanths are found in Germany, Canada and Australia. Features typical of coelacanths are found in such forms as Diplocercides, while more advanced features are missing, braincase specialisations, equal number of tail fin rays as supporting bones in the tail. An as yet undescribed Australian species from the Late Devonian of Mt Howitt, is know only from parts of the body and tail and fragments of the skull. The parts that have been found appear to be much like Miguashaia, a better known Canadian form from the Escuminiac Formation of Quebec, Canada, that had many primitive features, and like other crossopterygians, a heterocercal tail.

The coelacanths became conservative in body form by the Mesozoic. Forms such as Mawsonia gigas from the South American and African Cretaceous, the largest known predatory lobe-finned fish of the Gondwanan seas, grew to probably more than 3 m.

Sources & Further reading

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

Links

Devonian macrovertebrate assemblages and biogeography of East Gondwana (Australasia, Antarctica)