New alternate explanations of the origin of the Devonian dipnoan tooth-plates

Abstract

Analysis of Early Devonian dipnoan tooth-plates has been discussed extensively over the last twenty years. A recent discussion by AHLBERG et al. (2006) used cladistic methods with a range of characters, and reached the conclusion that the dentitions underwent extensive and non-parsimonious evolution in their early history. CAMPBELL & BARWtC~ (2000) had proposed that such morphological designs of dentition formed the basis for phylogenetic development, but the cladistic analysis indicated that this was not so. On the other hand we consider that the basic dentition types developed independently by genomic changes (gene regulation) in the Early Devonian. Further development of these designs took place in the later Devonian and subsequently, by Neo-Darwinian micro-evolutionary processes. As well as the distinguishing development ofpalatal biting, new evidence indicates that the Dipnoi were separated from other sarcopterygians by modification of the gill system and the long transverse anodeithrum that lay across the roof of the gill chamber. Further evolution took place within this framework of the Dipnoi so defined. New discoveries within the Early Devonian group are described. It is possible that some genera that have been difficult to place taxonomically (e.g. Diabolepis), were examples of new designs developed by genetic regulation, and they were short lived in competition from the few successful groups. The new lineages based on dental types continued into the later Devonian carrying a large number of older genetic units. These would have evolved in parallel by Neo-Darwinian principles, producing similarities that would have been recognised by cladists as synapomorphies.

A Lungfish (Sarcopterygii, Dipnomorpha) Tooth Plate from the Lower Devonian of Vietnam and the Onset of Modern Dipnoan Dental Organization

“…Devonian taxa show a substantial amount of morphological diversity, particularly with respect to the dental plates. Dental morphology has formed the basis for previous attempts on lungfish phylogenies (e.g., Campbell and Barwick 1983, 2008, in which preference was given to functional feeding scenarios, dividing lungfish into tooth-plated, dentine-plated and denticulated lineages with each of the dental types evolving as separate radiations. This unparsimonious view has been challenged by numerous subsequent studies (e.g., Schultze and !…”