Epidermal scale growth, allometry and function in non-avian dinosaurs and extant reptiles.

Epidermal scales in sauropsids perform a wide array of biological functions, which can relate to their shape and size. Accordingly, growth-related changes in scale morphology may reflect distinct functions between juvenile and adult individuals, such as use in mating interactions. Such patterns are poorly explored in both extant reptiles and non-avian dinosaurs, limiting functional interpretations. Here, we investigate scale growth in the ornithischian ceratopsid Chasmosaurus belli and hadrosaurid Prosaurolophus maximus by comparing scale morphologies between juveniles and adults of each taxon. Scale shape is generally consistent across growth stages in both taxa, and changes in C. belli feature scale length cannot reject isometry. However, there is a greater increase in C. belli feature scale width. In practical terms, the magnitude of these size differences rejects the hypothesis that feature scale morphology played a role in mating interactions, suggesting instead that their size was largely non-adaptive. To contextualise the patterns in the sampled dinosaurs, we assessed scale growth and allometry using an ecologically diverse sample of eight extant reptile species belonging to Crocodylidae, Scincidae, Elapidae and Pythonidae. While isometry is the overall most frequent pattern of scale growth in our sample of extant reptiles, most species demonstrate positive scale allometry in at least one area of their bodies, which is likely a response to changing body proportions. Scale shapes in the studied extant species, as in both dinosaurs, are largely retained through growth. This study provides the first detailed assessment of skin growth in non-avian dinosaurs, supporting morphological stasis in the growth of most of their scales.