Australia: The Land Where Time Began

A biography of the Australian continent 

Lungfish Origins

Lungfish have been a highly characteristic group from their earliest appearance in the fossil record. They are easily recognised by a number of features of the skull and their dentition. The relationships between the lungfish and other bony fish are uncertain. Consensus now seems to be that they are most closely related to the Crossopterygii. A similarity between the 2 groups is that the both had cosmine and thick rhombic scales, and they have well-developed internal skeleton in their fins.

An enigmatic fish from Yunnan in China, a proto-lungfish Diabolepis (devil scale) has been suggested as possibly being a link to the Crossopterygii. In 1984 it was originally called Diabolichthys. This fish had a skull roof pattern with a B bone in front of the contacting I bones, as occurred in crossopterygians, and the 2 sides of the lower jaw meet in an extensive, strong symphysis. The anterior external nostril is situated on the upper margin of the mouth, as in dipnoans, and broad crushing tooth-plates are present.

There is still uncertainty, but most agree that it is a close link between the first true dipnoans and the crossopterygians. Together with 2 other genera, they have been suggested as possibly ancestral to the porolepiforms Youngolepsis and Powichthys. Some believe the 3 genera are closely related to porolepiforms, lungfish being a sister group of the porolepiforms. Still others regard the lungfish as a distinct group, completely separate from the crossopterygians, which they believe are a natural monophyletic group, their shared features excluding lungfish. Among these features are the presence of a large number of submandibular bones, and teeth containing plicidentine tissue. It must await the discovery of more complete specimens of these 3 genera before the actual relationships can be elucidated.

Of the 3 genera, Diabolepsis is believed to be the closest to the lungfish. Based on the possession of an enameloid layer that continues into the cosmine pores in Youngolepis and Powichthys, they are regarded to be porolepiforms crossopterygians.

Uranolophus, from North America, is the oldest known true dipnoan. But the Early Devonian limestone deposits in southeast Australia have the greatest diversity of primitive dipnoans, and possibly are the key to understanding the early evolution of the group.

Sources & Further reading

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

 

Author: M. H. Monroe
Email:  admin@austhrutime.com
Last Updated 03/01/2011 

 

Dipnoi

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading