The anatomy, evolution and interrelationships of Devonian Dipnoans, with insights from the extant Australian lungfish, Neoceratodus forsteri

Abstract

The modem Australian fauna includes one of the most enduring extant vertebrate lineages, the Dipnoi, and this body of work examines their anatomy, evolution and relationships, with insights from the Australian lungfish, Neoceratodus forsteri. Through application of modern technology and techniques, this thesis provides comparative, ontogenetic, phylogenetic and functional insights into both fossil and extant lungfishes. Three recently discovered fossil taxa are discussed. These include the first Gondwanan species of a European genus (Rhinodipterus kimberleyensis, Clement 2012), and a primitive genus belonging to the holodontid family (Xeradipterus hatcheri, Clement and Long 2010b), both from the Late Devonian Gogo Formation of Western Australia. Also, a Middle Devonian genus from central Australia (Harajicadipterus youngi, Clement 2009) and a detailed examination of the postcranial anatomy of two lungfishes from Victoria (Howidipterus donnae and Barwickia downunda, Long 1992a) is included. Key findings include the first unequivocal marine lungfish with air-breathing adaptations, possible examples ofsympatric speciation driven by competition for trophic resources, and some of the earliest dipnoans to make the transition to freshwater in Australia. Cladistic analysis was employed to examine dipnoan interrelationships, with special emphasis on early Australian forms. The analysis suggests there were significant adaptive radiations of lungfishes in Devonian reefs, with three locally-evolving clades identified; the chirodipterids, holodontids and dipnorhynchids. The monophyly ofa number ofgenera found in Australian deposits, namely Chirodipterus and Griphognathus is not supported, whereas, the monophyly of Rhinodipterus is confirmed. The analysis suggests that air-breathing probably evolved only once within the Dipnoi. Partition homogeneity tests and Farris' successive weighting provide support for the hypothesis that anatomical characters relating to feeding, such as those of the dentition, jaw and palate, may be less reliable than some other, less convergence-prone traits. Basic jaw lever mechanics in conjunction with 3D bite-modelling software were used to estimate bite force and velocity in extant and fossil genera. Neoceratodus was shown to have a mid level mechanical advantage for its bite compared to earlier fossil forms, implying it has neither a particularly strong nor fast bite. The two main adductor mandibulae muscle portions, the temporalis and masseter, contribute similar forces, and effective mechanical advantage remains constant throughout ontogeny despite an observable shift in diet. Analysis of functional morphology illustrates a large variety of feeding ecomorphologies, and thus, inferred behaviours of Devonian lungfishes. Early members of this lineage were evidently capable of eating diverse prey types, and likely filled a wide range of ecological roles. An inherent ability to remodel plastic dental morphologies is supported as a mechanism by which the early lungfishes were able to evolve, adapt and exploit new niches throughout their long history.