Comparing morphology and cranial osteology in two divergent clades of dice snakes from continental Europe (Squamata: Natricidae: Natrix tessellata)

Abstract The Western Palearctic harbours a diverse snake fauna, including numerous endemic species and yet unnamed clades, identified through molecular analyses. However, morphological characteristics of these clades, even of common species, often remain relatively unexplored. In this study, we provide an examination of the morphology and cranial anatomy of the semi-aquatic snake species Natrix tessellata (Laurenti, 1768), with a focus on populations of the so-called ‘Europe’ and ‘Greece’ clades. Utilising both museum collections and field data, we first morphologically examined 541 individuals of N. tessellata , categorising them according to previously established clades and lineages that resulted in relatively low morphometric and meristic variation across the species’ range. When assessing the 448 specimens from the ‘Europe’ and the ‘Greece’ clades separately, we similarly observed little variation in meristic characteristics. On the other hand, individuals of the ‘Greece’ clade displayed smaller and more slender body and head proportions compared to those of the ‘Europe’ clade and the pigmentation of the labial scales is distinctively paler in the ‘Greece’ clade, whereas the overall body colouration remains largely similar between the two. Our osteological analysis of 47 N. tessellata skulls also indicated slight differences in the frontoparietal portion of the braincase between the ‘Europe’ and the ‘Greece’ clades, warranting further examination with a larger dataset and extending to other skull components. These findings hold significance for ongoing enquiries into the species’ biogeography, morphology and ecological adaptations. In summary, the integration of morphological and osteological data with genetic information offers a promising avenue for potential taxonomic revisions of N. tessellata in the future.