Revealing the use of dental indices to infer taxonomic variation in sauropod dinosaurs

Abstract

<p>Sauropod teeth are commonly categorized taxo-nomically by two well-established measurement indices: slenderness index (SI" apicobasal height/mesiodistal width),which quantifies breadth, and compression index (CI" labio-lingual width/mesiodistal width), which quantifies cross-sectional circularity. Although both indices are used to infer high-level taxonomic affinities, little is known about the linear relationships between the constituent measurements or how the indices vary intra-cranially and at lower taxonomic levels. Here, we evaluate these relationships using a novel dataset of sauropod teeth (N = 898) spanning all major sauropod groups. Results for both indices indicate significant differential scaling within Sauropodomorpha, both in slope and elevation. Broad-crowned sauropods mostly dis-play positive allometry in SI compared to isometry in narrow-crowned sauropods. However, this distinction is less clear with CI as most sauropods display isometry, whilst non-sauropod sauropodomorphs (e.g. Plateosauridae) display positive allometry. An ANOVA reveals SI varies significantly with genus and tooth position. Specifically, jaw type(maxilla and dentary) is significant within Plateosauridae, whilst tooth position is significant within titanosaurs. Over-all, variation within CI is restricted to genus. Our findings reveal that whilst indices have taxonomic utility, there are caveats. The measurements used to calculate the indices exhibit significant allometry, indicating that index values are size-dependent. Furthermore, the indices may not accurately reflect size heterodont conditions present among early-branching sauropodomorphs. Our study highlights the importance of quantifying taxonomic relationships of measurement data, which can be used to inform hypotheses regarding the physiological and palaeoecological driversinfluencing tooth shape evolution.</p>