Does intraspecific variation in juvenile Late Cretaceous ammonoids correlate with their systematic position,...

Phenotypic intraspecific variation of organisms is essential for evolution and, thus, has the potential to provide crucial insights into evolutionary dynamics. Additionally, ontogeny is often intricately linked with the evolutionary trajectories of organisms. In this study, we explore the relationship between the magnitude and ontogenetic pattern of intraspecific variation, and the interrelationships of organisms, their geographic distribution, and species duration. We analyzed the intraspecific variation in the whorl expansion rate (WER) of several Late Cretaceous ammonoid species, including Hypophylloceras ramosum, Phyllopachyceras ezoense, Gaudryceras tenuiliratum, Tetragonites glabrus, T. popetensis, Damesites damesi, Tragodesmoceroides subcostatus, Subprionocyclus minimus, Yezoites puerculus (all from Hokkaido, Japan), as well as Scaphites whitfieldi and Hoploscaphites comprimus (both from North America). Our results reveal a weak to moderate, negative correlation between the magnitude of intraspecific variation and geographic distribution. The correlation between intraspecific variation and species duration is weak. Notably, scaphitid and phylloceratid ammonoids exhibit a higher degree of intraspecific variation compared to other species, although no significant differences are apparent within each family. Additionally, scaphitid species from both Japan and North America display similar ontogenetic patterns of intraspecific variation. Hypophylloceras ramosum exhibits a pattern of intraspecific variation, differing from other normally coiled ammonoids. In other taxa, the pattern among species cannot be distinguished. These discoveries suggest that intraspecific variation, geographic distribution, and species duration are, at least, not positively correlated. However, contrary to previous studies, our data suggest a potential link between intraspecific variation and the interrelationships of species (relative phylogenetic position). Further research involving the analysis of more taxa, multiple morphological parameters examined over longer ontogenetic stages, and the development of a robust phylogenetic hypothesis are necessary to better understand these associations.

The online version contains supplementary material available at 10.1186/s13358-025-00397-y.