Quantitative heterodonty in Crocodylia: assessing size and shape across modern and extinct taxa

Abstract

Heterodonty in Crocodylia and closely related taxa has not been defined quantitatively, as the teeth rarely have been measured. This has resulted in a range of qualitative descriptors, with little consensus on the condition of dental morphology in the clade. The purpose of this study is to present a method for the quantification of both size- and shape-heterodonty in members of Crocodylia. Data were collected from dry skeletal and fossil specimens of 34 crown crocodylians and one crocodyliform, resulting in 21 species total. Digital photographs were taken of each tooth and the skull, and the margins of both were converted into landmarks and semilandmarks. We expressed heterodonty through Foote’s morphological disparity, and a principal components analysis quantified shape variance. All specimens sampled were heterodont to varying degrees, with the majority of the shape variance represented by a ‘caniniform’ to ‘molariform’ transition. Heterodonty varied significantly between positions; size undulated whereas shape was significantly linear from mesial to distal. Size and shape appeared to be primarily decoupled. Skull shape correlated significantly with tooth shape. High size-heterodonty often correlated with relatively large caniniform teeth, reflecting a prioritization of securing prey. Large, highly molariform, distal teeth may be a consequence of high-frequency durophagy combined with prey size. The slender-snouted skull shape correlated with a caniniform arcade with low heterodonty. This was reminiscent of other underwater-feeding tetrapods, as they often focus on small prey that requires minimal processing. Several extinct taxa were very molariform, which was associated with low heterodonty. The terrestrial peirosaurid shared similarities with large modern crocodylian taxa, but may have processed prey differently. Disparity measures can be inflated or deflated if certain teeth are absent from the tooth row, and regression analysis may not best apply to strongly slender-snouted taxa. Nevertheless, when these methods are used in tandem they can give a complete picture of crocodylian heterodonty. Future researchers may apply our proposed method to most crocodylian specimens with an intact enough tooth row regardless of age, species, or rearing conditions, as this will add rigor to many life history studies of the clade.

Australia’s prehistoric ‘swamp king’: revision of the Plio-Pleistocene crocodylian genusPallimnarchusde Vis, 1886

“…29 ) provides useful information of the dental morphology. Paludirex displays evident size differentiation between the alveoli (size-related pseudoheterodonty/size-related heterodonty; D’Amore et al, 2019 ), especially obvious among the anterior maxillary dentition (i.e., at least the first five maxillary teeth) of CMC2019-010-2 (character 222, state 1). Size-related heterodonty is present to various degrees in all Cenozoic Australian crocodylians for which their dentition is at least partially known, both extant (albeit to a much lesser degree in C. johnstoni as opposed to C. porosus ) and extinct (although in Quinkana the heterodonty tends to be very subtle; Molnar, 1977 , 1981 ).…”

Functional implications of dentition-based morphotypes in piscivorous fishes

“…This issue was discussed by D'Amore et al . [24], who noted the need for a broader evaluation of tooth form and function.…”

Reconstructing the archosaur radiation using a Middle Triassic archosauriform tooth assemblage from Tanzania

“…In this study, we use teeth as an indicator of ecological disparity because they have relatively high preservation potential (Turner-Walker, 2008) and offer a direct link to ecology through diet (Lucas, 1979; Dessem, 1985; Scanlon & Shine, 1988; Sander, 1997; Linde, Palmer & Gómez-Zurita, 2004; Santana, Strait & Dumont, 2011; Zahradnicek et al, 2014; Melstrom & Irmis, 2019). Quantitative analyses of tooth morphology have previously been used to document fossil assemblages with an emphasis on diet (Larson, 2008; Frey & Monninger, 2010; Larson & Currie, 2013; Hendrickx, Mateus & Araújo, 2015; Larson, Brown & Evans, 2016; D’Amore et al, 2019; Melstrom & Irmis, 2019). We consider diet as the aspect of ecology of interest in this study because the relative ease of its inference from morphology alone and the use of diet in previous studies of evolutionary radiations (Slater & Friscia, 2019).…”