Abstract
Mosasaur researchers have used varieties of tooth crown ornamentation as diagnostic and phylogenetic characters for decades. Such tooth crown features include facets, flutes, striations, serrated carinae, and coarse anastomosing texture. This study investigates the relative contribution of dentine and enamel to the development of these dental characters and assesses possible homologies between these structures. Histological analysis of isolated mosasaur teeth revealed that flutes and facets develop initially from the dentine, and the external enamel morphology we observe macroscopically mirrors the shape the underlying dentine. Striations combine underlying contributions from the dentine with additional and irregular enamel deposition that results strictly from amelogenesis. In both serrated carinae and anastomosing texture the border between the dentine and the enamel is smooth, and these external ornamentations form through variations in enamel development. Based on these observations, we infer that flutes and facets are part of a morphological spectrum and should not be treated as separate phylogenetic characters. Conversely, striations develop differently than flutes and facets, and should therefore be treated as a distinct character. We recommend referring to the “serrations” on mosasaur carinae as crenulations to differentiate these enamel-only structures from true denticles possessing a dentine core. Anastomosing texture can also coincide with significant apical thickening, both of which could be adaptations for processing hard-shelled prey. Care must be taken when using tooth crown features as diagnostic or phylogenetic characters because seemingly different morphologies can have similar developmental origins, and tooth morphology can be more closely tied to diet than to common ancestry.
Highlights
The modes of tooth attachment and replacement were among the features that first allowed nineteenth century naturalists to recognize that a giant fossil recovered from the phosphate mines in Maastricht, the Netherlands, represented an aquatic lizard and not a crocodile or a whale (Camper 1800; Cuvier 1808)
Facets were examined in a Mosasaurus sp. tooth (UALVP 57353), flutes were examined in Gavialimimus almaghribensis (MHNM.KHG.1231), striations were examined in two examples of Platecarpus tympaniticus (UALVP 55497, UALVP 57046), serrations were examined in Mosasaurus sp. (UALVP 57352) and in Prognathodon sp. (TMP 86.036.0463), and coarse texture was examined in Globidens simplex (MHNM.KHG.221) and Prognathodon sp. (TMP 86.036.0463)
All of the mosasaur tooth crowns we examined were predominantly composed of dentine with a thin cap of clear enamel
Summary
The modes of tooth attachment and replacement were among the features that first allowed nineteenth century naturalists to recognize that a giant fossil recovered from the phosphate mines in Maastricht, the Netherlands, represented an aquatic lizard and not a crocodile or a whale (Camper 1800; Cuvier 1808). The teeth of these extinct aquatic lizards, known as mosasaurs, have been studied extensively, and multiple histological analyses of their teeth have demonstrated how the dental tissues contribute to the modes of implantation and replacement (Caldwell 2007; Caldwell et al 2003; LeBlanc et al 2017; Luan et al 2009; Rieppel and Kearney 2005; Zaher and Rieppel 1999). Despite the thorough histological examination of mosasaur tooth attachment, examination of tooth crowns at the tissue level has focused primarily on microstructures of the dentine and enamel (Owocki and Madzia 2020; Sander 1999, 2000) This lack of investigation is all the more surprising considering that tooth crown ornamentation figures prominently in the character lists used in phylogenetic analyses of Mosasauridae (Bell 1997; Bell and Polcyn 2005; Dortangs et al 2002; LeBlanc et al 2012; Palci et al 2013; Simões et al 2017b).
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