Evaluating morphological species recognition in fossil and modern gastropods (Littorinidae, periwinkles)

Supplementary materials for the article, 'Evaluating morphological species recognition in fossil and modern gastropods (Littorinidae, periwinkles)' in Paleobiology (2025). Article abstract: Species recognition is an essential part of biological and paleontological study. In gastropods, although species are genetic entities, shell morphology continues to be used as the primary source of information to recognize most species. While there are few directly tested cases, variations in conchological characters for modern species are expected to reflect underlying genetic differences that define a biological species, an assumption that is also applied to identify species in the fossil record. Additionally, how consistently shell shape differentiates gastropod species remains poorly understood. In this study, shell shape of Recent and Pliocene–Pleistocene fossil specimens of well-known intertidal gastropods (Littorinidae, periwinkles: †Littorina petricola, Littorina keenae, and the sister-species pair Littorina plena and Littorina scutulata) from the east Pacific was analyzed using landmark-based morphometrics and compared with published molecular data. For the extant species, there is a general positive relationship between shell shape and genetic differences. Discriminant function analyses indicate distantly related species can be more reliably recognized byfrom their shells, while closely related species have a higher error. Fossils and recent specimens were classified with similar consistency. More work is needed to illuminate whether this case applies more widely.