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.
Highlights
What constitutes heterodonty in vertebrates is often difficult to delineate, with different qualitative definitions in place depending on the clade being studied (Shimada, 2001). Kieser et al (1993, p.195) referred to the definition of heterodonty as ‘‘a bone of contention,’’ and since the issue has not been fully resolved
When regression scores were plotted against centroid size (CS), the regression had a goodness of fit accounting for less than 10% of the variance (y = 0.313x-0.981; r2 = 0.090; p < 0.0001; 95% = 0.296,0.328)
Most of the shape variance in Crocodylia was along a single PC axis
Summary
What constitutes heterodonty in vertebrates is often difficult to delineate, with different qualitative definitions in place depending on the clade being studied (Shimada, 2001). Kieser et al (1993, p.195) referred to the definition of heterodonty as ‘‘a bone of contention,’’ and since the issue has not been fully resolved. Kieser et al (1993, p.195) referred to the definition of heterodonty as ‘‘a bone of contention,’’ and since the issue has not been fully resolved. This lack of clarity is most pronounced within members of Crocodylia. Peyer (1968, p.17) defined the term as lacking the discrete dental categories seen in mammals (incisors, canines, premolars, molars), even though he admitted ‘‘a sharp distinction between homodont and heterodont is not possible.’’ Ferguson (1981) referred to Alligator mississippiensis as ‘‘pseudoheterodont,’’ because it showed a gradual, as opposed to punctuated, change in tooth shape along the tooth row (see Grigg & Gans, 1993; Hendrickx, Mateus & Araújo, 2015a). Some researchers have argued certain modern crocodylians are heterodont, and claim dental categories do exist (Aoki, 1989; Kieser et al, 1993)
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