Australia: The Land Where Time Began

A biography of the Australian continent 

Evolution of the Lungfish

The "fleurantiids" and the "phaneropoleurids" have a much reduced first dorsal fin and an enlarged 2nd dorsal fin, and have simplified cheek and skull roof patterns,  which makes them more advanced than the dipterids.

During the Devonian lungfish evolved towards the condition seen in the Queensland lungfish - reduction of the dorsal fins and merging of the anal and caudal fin. A transformation series demonstrates the progress  of this evolutionary trend. Uranolophus - Dipterus - Howidipterus - Scaumenacea - Phaneropleuron.

The Scottish genus Phaneropleuron demonstrates that the final body plan of lungfish was achieved by the end of the Devonian. This body plan was retained by lungfish ever since.

Howidipterus, a dipnoan from the Victorian Devonian, had tooth-plates, but with smooth crushing surfaces in the centre of the the plate and well-developed teeth along the margins. There were also many primitive features in the skull, such as the large "D" and "K" bones. Unlike the small diamond-shaped parasphenoid as in Dipterus, in Howidipterus the parasphenoid has a well-developed stalk.

2 of the lungfish found in the Mt Howitt site have identical body shapes, and similar plans of fin support bones and number of ribs. Even though they had superficially different dentitions - Howidipterus has tooth plates, Barwickia appears to be a denticle shedder - when studied in detail, they appear to have similar types of tooth- plates, Howidipterus  had tooth-plates dominated by rows of teeth with few denticles, Barwickia had the opposite arrangement, many denticles but few rows of teeth. Both have now been placed in the fleurantiid group. The dentitions of other fleurantiids, such as Andreyevichthys from Russia, demonstrate how both kinds can exist in 1 species, a result of growth variations. The 2 Mt Howitt dipnoans have very precise similarities of the post-cranial skeleton that are unknown in any other fossil lungfish, suggesting they evolved from a common ancestor, the 2 distinct forms had differing feeding strategies. They lived in a large lake.

In modern fish faunas in large lakes it is common for a number of species to have arisen from a common ancestor, each having a slightly differing feeding strategy. An example is the Cichlids of African lakes.

By the Carboniferous all dipnoans had a single median fin that merged with the tail fin. Some of the tooth plates from the Carboniferous were very specialised, often having many tooth rows with closely packed cusps, as in Ctenodus.

Delatitia breviceps is the only known Australian dipnoan from the Carboniferous. It was first assigned to the European genus Ctenodus, but after further study was re-assigned to a new genus in 1985. The majority of lungfish were ceratodontids by the start of the Mesozoic. The Queensland lungfish, Neoceratodus, belongs to this group, as does the living African and South American forms.

Gosfordia truncata from the Hawkesbury Sandstone, near Gosford, NSW, is one of the best preserved Australian Triassic lungfish. The body, about 50 cm long, of Gosfordia, was deep and plump, suggests that it was probably a strong swimmer, and unlike the African and South American forms, not adapted to aestivation. Most Australian Mesozoic lungfish, have been found only as isolated tooth-plates. An example is Ceratodus, tooth-plates of which have been found in Triassic Blina Shale of northern WA, the Rewan Formation in southern Queensland, and the Winamatta Group of NSW. 

Tooth-pates from the Queensland lungfish, Neoceratodus forsteri, have been found in Early Cretaceous deposits near Lightning Ridge. Tooth-plates of Ceratodus nargun show that it was living in rivers of the cold rift valley that formed as Australia separated from Antarctica.

Lungfish skull bones became simpler during the Mesozoic, and their tooth-plate morphology increased in variety, within genera as well as between genera. Based on large tooth plates and skull roof bones found in river and lake deposits from the Miocene of central South Australia, species such as Neoceratodus gregoryi could have reached lengths of about 3 m or more. In the lakes and rivers of central Australia there were at least 4 other species of Neoceratodus that were widely distributed in Queensland and Central Australia.

Semi-articulated skull and body remains of Neoceratodus denticulatus, of probable Eocene age, have been found in the Redbank Plains area of southern Queensland.

The Queensland lungfish is Australia's only primary freshwater fish, having evolved and not immigrated.

Sources & Further reading

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

Dipnoi