MOSASAURINAE, Gervais, 1853

Caldwell, M. W., 2007, Ontogeny, anatomy and attachment of the dentition in mosasaurs (Mosasauridae: Squamata), Zoological Journal of the Linnean Society 149 (4), pp. 687-700 : 695

( MOSASAURINAE )

The illustrated specimen ( Fig. 5A, B ) is a fragment of the right dentary of the one of the geologically youngest known mosasaurs [there are some potentially younger mosasaur fossils ( Jagt, 2005) but they are nowhere as complete as this specimen], a large prognathodontid found less than 2 m below the Cretaceous–Tertiary boundary along the Waipara River, New Zealand ( Welles & Gregg, 1971). The crosssectional view ( Fig. 5A, B ) presents the distinct tissue types described by Caldwell et al. (2003): fibrolamellar bone of the TBE, woven-fibre bone of the alveolar process lining the alveolus, the root cement, the cementcovered dentine root, the pulp cavity, the crown and a resorption pit with a developing tooth crown deep within the resorption crypt.

This specimen (CM ZFR 108) presents important ontogenetic information. First, the diameter of the developing tooth crown is substantially larger than the diameter of the resorption pit at its upper limit. In other words, the crown was substantially smaller when resorption was initiated but has been developing within the resorption crypt as resorption has proceeded. Secondly, it appears that at this stage of development, at least in this species of Prognathodon , only the enamel portion of the crown is present (while all teeth develop in the manner, i.e. enamel cap first, the unusual aspect in this case is that the cap is deeply set down into the resorption crypt). The dentine tissues are not yet present as an ossified matrix, indicating that prior to its descent into the resorption crypt, the only mineralized portion of the tooth is the enamel crown. Thirdly, resorption appears to affect only alveolar bone and the root cement; the bone of the dentary appears not to be undergoing resorption. Additionally, resorption has created such a substantial erosional surface into the root cementum that an overhang or crypt is formed.

Figures

Figure 5. Cross-sectional views of mosasaur tooth-bearing elements. A, right dentary of Prognathodon waiparensis (CM ZFR 108) (Upper Cretaceous, Maastrichtian) from New Zealand. B, close-up of ‘A’. C, right dentary of Platecarpus sp. (FMNH P 495) (Upper Cretaceous, Santonian) from Kansas, USA. D, close-up of ‘C’. Abbreviations: pc, pulp cavity; rt, replacement teeth.