The microstructure of the dentition and dermal ornament of three dipnoans from the Devonian of Western Australia: a contribution towards dipnoan interrelations, and morphogenesis, growth and adaption of the skeletal tissues

Abstract

Detailed descriptions of the microstructure, which includes both histology and ultrastructure, are given for each type of tissue in each of the three species of Australian dipnoan. Most of the tissues have not previously been recorded in this way even for other species within the same genera. The microstructure is interpreted from examination of ground sections in transmitted and polarized light, together with examination of functional, forming and fracture surfaces in the scanning electron microscope. No previous studies of dipnoan dental tissues have been published in which the histology is related to surface morphology as seen in the scanning electron microscope. The histology, microstructure, growth and distribution of buccal denticles are described in Griphognathus whitei and Holodipterus gogoensis , and their morphogenesis and adaptation to function is discussed. It is concluded that the phyletic trend towards macromerism, as shown in chondricthyans and primitive teleostomes, is illustrated in the three species of dipnoan. The continuous layer of dentine of the tooth ridges, lips and covering to the dermal snout is described and found to be similar in the three species and equivalent to the tissue described previously in other dipnoans, sometimes as a form of cosmine. A term not in current usage is suggested for this dentine, namely syndentine. Dentine terminology is reviewed and the relevance to dipnoan dental tissues is discussed with the conclusion that an older term should be reintroduced to eliminate the current anomalies in terminology. Chirodipterus australis is the only species with typical tooth plates. The histology is compared with previous accounts of tooth plates in dipnoans and found to have some similarity with those of Neoceratodus forsteri . The microstructure is reported from examination of the worn functional surfaces and acid-etched functional surfaces with the scanning electron microscope, this information has not previously been reported for dipnoan tooth plates and is presented here as a basis for comparison with other species. New features of cosmine structure are described from scanning electron micrographs of the surfaces and these are related to the probable mode of formation and the involvement of the epithelial cells. The structure is found to compare in some ways with the cosmine of osteolepids. The loss of cosmine and replacement by tubercles in Griphognathus whitei and Holodipterus gogoensis is explained as retention of an ontogenetic potential which is comparable with Ørvig’s theory of cosmine regression in the porolepids. Superpositional growth in the dermal skull bones is described for the first time in dipnoans. It is postulated that the ancestors of dipnoans had superimposed denticles beneath a cosmine covering. The information obtained from the microstructure is used to examine the hypothesis proposed by Miles (1977) in a paper on the phyletic relations within the dipnoans. This confirms that Griphognathus white i has retained a primitive dentition with separate buccal denticles and tooth ridges; Holodipterus gogoensis has retained these features, denticles and lip ridges, together with an advanced feature of tooth cusps which are a divergent specialization phyletically preceding tooth plates; Chirodipterus australis has not retained buccal denticles but has lip ridges on the snout and anterior part of the lower jaw, and the specialized tooth plates. Pleromic dentine is recorded in two of these species and discussed with other observed methods of adaptation to wear.