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 : 692-695

( MOSASAURINAE )

The illustrated specimen ( Fig. 4A–C ) is the right dentary from a very large Mosasaurus collected at Maastricht in the Netherlands. The fragmentary condition of the jaw reveals extremely important details of mosasaur dental ontogeny beginning at the point where vertically positioned tooth crowns are resting in the resorption crypt to their upward eruption just prior to the formation of the periodontal ligament and implantation.

Beginning with the smallest replacement crown tips, crowns 3 (c3) and 12 (c12) ( Fig. 4A ), both of which are resting in shallow replacement crypts formed labially by the cement of the root, and lingually within the alveolar bone tissues of the interdental plate, there are a number of important features to note. The first is that both of the crown tips are positioned vertically in small crypts. The second is that there is no cement portion fixed to the base as both enamel and dentine formation is incomplete. The third, and perhaps most important feature is the relative size of the crown tips as compared with a completely formed crown (c8) ( Fig. 4A ). Quantitative comparison of c12 (∼ 3 cm) to c8 (∼ 9.5 cm) indicates that the crown of c12 is only about one-third of the way through its development; by comparison, c7 and c10 are two-thirds and four-fifths developed, respectively, as compared with the crown of c8. Although an absolute metric for the timing of mosasaur crown development is not available, it is possible to state reliably that two-thirds of the time required for mosasaur crown development occurs in the expanding resorption pit/crypt produced by resorption of cement and alveolar tissues.

Comparison of crowns c5 and c8 reveals that once the crown has been fully formed, and presumably the dentine root portion as well, cement begins to form distally from the crown margin. As cement is added to the expanding and elongating root, the tooth begins to erupt out of the alveolus, rising to its attachment position as a functioning replacement tooth. Evidence for the timing of formation of the dentine portion of root (see Fig. 1B ) is derived from a comparison of the size of the cement-covered portion of c5, which has a broken crown, with the dentine root as presented in Figure 1B , which is approximately one-third of the total crown–root length, with the crown height and total tooth length of c8. The amount of cement on the root of c5 is approximately one-third of the total crown–root length for c8. The position of c 5 in the dental crypt, assuming it has a developed dentine root, now surrounded by cement, is appropriate as compared with c7, which is about two-thirds developed, is sitting quite low in the alveolus/crypt, but has developed neither the dentine root nor cementum components.

Tooth crown c8 is clearly in the process of erupting out of the alveolus and into it final attachment position. However, it is not yet in that final position. A comparison of c8 with the two teeth illustrated in Figure 1A and B indicates that the latter two teeth have significantly more cement added to the base of the root system ( Fig. 4A–C ). For Mosasaurus sp. the complete tooth–root system divides the tooth into fifths: two-fifths crown, three-fifths root + cement ( Fig. 1A, B ). For c8, the proportions are evenly split such that the crown is half the erupting tooth’s length. Eruption will be complete for c8 when this remaining fifth of the total tooth length is extended by the basal addition of new cement. Attachment of the tooth in the alveolus will only be accomplished by the development of new alveolar tissues and the development of new periodontal ligament fibres following the cessation of cementum deposition.

The progression of tooth crown development presented by this specimen, from c12/c3 to c8, shows very clearly that as much as 99% of the development of a mosasaur tooth occurs within the alveolus. Comparison with Figure 2A and B indicates that a small percentage of tooth crown tip development occurs within the dental lamina. Specimen NHMM 6698 further verifies Caldwell et al.’s (2003) conclusions that cement tissues cannot be considered in the traditional sense as bone of attachment. This is because when cement is forming, it develops around the dentine root and then grows ventrally to assist the tooth in erupting, not in attaching. Tooth c8 is loose within the alveolus, and more importantly, mobile. It is still erupting and does not yet bear the mature complement of cement. When the tooth has reached its mature eruption position, formation of the alveolar process begins followed by the formation of attachment tissues identified by Caldwell et al. (2003) as Sharpey’s fibres associated with a periodontal ligament.

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 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.