Platecarpus undetermined

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 : 691

NMC 40914 PLATECARPUS SP. (RUSSELLOSAURINA)

The specimen as illustrated ( Fig. 2A, B ) is a medial view of the left maxilla at about the midpoint of that element. Four tooth positions are shown, one of which, tooth position number three (III), has already shed the previous tooth generation, and has an incipient replacement tooth ( Fig. 2A, B ) identified as III-1 at the anterolabial margin of the alveolus; there are three other replacement teeth, not in the alveolus, but identified here as part of that tooth position’s replacement series. Each of those crowns, III-2, III-3, III-4, is successively smaller than III-1 and aligned posterolingual to the third tooth position.

The remaining tooth positions, identified as I, II and IV, still have mature teeth anchored within the alveolus (crowns are all broken away), and all possess small resorption pits with replacement crowns (I-1, II- 1, IV-1) in the pits, but ‘tipped’ over and pointing posteriorly with the exception of I-2. Of importance to note in relation to the direction in which the tooth tips are pointing is that there is no such uniformity to the tooth tip direction of other replacement teeth associated with the replacement series for a tooth position. For example, for tooth position III: III-1 is vertical, III- 2 is horizontal and posteriorly directed, III-3 is horizontal and anteriorly directed, and III-4 is horizontal and anterolaterally directed. Similarly chaotic directions of preservation exist for all other tooth crown tips preserved along the dental groove of this specimen.

There are several important characteristics of mosasaur tooth development typified by NMC 40914 and worthy of repetition here: (1) tooth crowns develop apex to base and appear to include both enamel and dentine tissues in even the smallest crown tip (nonhistological observation); (2) multiple replacement teeth are continuously in position, within the dental lamina and along the dental groove, and for any one tooth position; (3) cement and other tissues of attachment are not present on any of the small tooth crowns in NMC 40914, including III-1 which is already vertical within the alveolus of tooth position III (therefore, cement and attachment tissues are late-stage features in the ontogeny of a replacement tooth); (4) there is no evidence to suggest that the tooth crowns develop in a recumbent position; rather, the tooth crown positions as preserved are a random effect of taphonomy; (5) in life, the dental laminae appear to have been continuous along the length of the dental groove in all TBEs [not discontinous as in crocodilians and located at the bottom of the alveolus ( Edmund, 1960)]; the presence of teeth along the dental groove is one line of evidence while the second is the posterolingual and exposed position of the resorption pit. If the dental lamina was discontinous and positioned within the socket, then resorption would be expected to be initiated at the tooth base, and not at the level of the gumline ( Figs 1–5 ) .

Figures

Figure 1. External and internal views of shed mosasaur teeth. A, medial view of shed maxillary/dentary tooth of Mosasaurus sp. (NHMM 002459-1) (Upper Cretaceous, Maastrichtian). B, cross-sectional view of maxillary/dentary tooth of Mosasaurus sp. (NHMM 002459-1) (Upper Cretaceous, Maastrichtian). C, distal view of ‘A’, showing pulp cavity penetrating base of tooth.

Figure 2. Medial view of maxillary tooth row of Platecarpus sp. (NMC 40914). A, photograph; B, line drawing with all visible tooth crowns highlighted with light grey. Note: Roman numerals refer to the tooth position counting from left to right, numbers refer to the replacement teeth as arranged along the dental margin and, presumably, as emplaced within the dental lamina (based on size of tooth crown).

Figure 3. A, palatal view of right and left maxillae and vomeropalatines of Mosasaurus hoffmanni (IRScNB R12). B, close up of ‘A’ showing vertical position of all replacement teeth sitting in posterolingually positioned resorption pits. C, lateral view of right pterygoid of holotype specimen of Mosasaurus hoffmanni (MNHM 9648).

Figure 4. Medial view of right dentary of Mosasaurus hoffmanni (NHMM 6698) (Upper Cretaceous, Maastrichtian). A, photograph of complete specimen. B, detail of dentary, emplaced and replacement teeth, and alveoli. C, illustration of ‘B’ with all tooth crowns highlighted in light grey. Abbreviations: C1–C12, tooth crowns 1–12.

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.