The pattern of tooth plate formation in the Australian lungfish, Neoceratodus forsteri Krefft

Abstract

Tooth plate formation in the Queensland lungfish, Neoceratodus forsteri, Krefft begins with simple groups of isolated cusps, three in each tooth plate. The cusps fuse in ridges radiating from a point situated posterolingually. During growth, cusps are added to the labial ends of the ridges, and more ridges are added posteriorly, giving a total of seven in each tooth plate. Each tooth grows in thickness by the addition of layers of material, in line with the new cusps, beneath the tooth plate. The tooth plate grows outwards and is resorbed from the inner angle at the same time. The crushing surface is formed by the growth of cusps between the ridges. Angles between the ridges become progressively smaller, and angles between more posterior ridges are consistently less than between more anterior ridges. Similar but less pronounced changes in angles between ridges occur in a fossil genus, Sagenodus inaequalis, examined for comparison. Vomerine teeth grow in the same way, by fusion of isolated cusps and the addition of new cusps to one end (labial) of the tooth plate. Layers of material are also added beneath the tooth plate. The vomerine tooth plates are initially low-based with long cusps but develop into high-based low cusped incisiform tooth plates in fully grown adults. The labial dentition of the lower jaw starts to develop like the vomerine teeth, but degenerates by stage (vi) of tooth development. The single medial tooth is resorbed even earlier. The pattern of tooth plate formation described in this paper is consistent with illustrations published by Semon (1901) and Greil (1908, 1913) but the inferred developmental processes are different. Implications of the results for the Zahnreihe hypothesis of Edmund and for the phylogeny of Dipnoi are discussed.