Intraspecific variation throughout ontogeny in the Cenomanian (Late Cretaceous) ammonoid taxon Neogastroplites muelleri Reeside and Cobban

Abstract

Intraspecific variation of Neogastroplites muelleri Reeside and Cobban throughout ontogeny was studied based on 100 well-preserved specimens ranging in diameter from 10 to 75 mm that were extracted from a single calcareous concretion in the Mowry Shale Formation of the Colorado Group (lower Cenomanian) in central Montana, USA. The embryonic shell is exceptionally large among Ammonitina, and the width of the initial chamber exhibits a wide range of intraspecific variation in relation to its diameter, and thus a large variation exists in the thickness of the initial chamber. There is a positive correlation during the post-hatching stage between shell growth ratios (whorl width to whorl height and umbilical diameter to shell diameter) and relative initial chamber thickness, such that those specimens with more depressed initial chambers have greater growth ratios than those that are more compressed. This may imply that the initial chamber morphology could constrain subsequent shell growth. Due to the differences in these growth ratios, the degree of intraspecific variation of relative whorl thickness and relative umbilical size increases as the shell grows. Our analysis suggests that this high level of intraspecific variation in N. muelleri is directly related to the large morphological differences observed in the initial chambers, and these corresponding differences are maintained in the shell growth ratios of the subsequent post-hatching stages. The large embryonic shell suggests that the inhospitable environment of the Mowry Sea may have reduced the survival of newly hatched ammonoids, thus allowing Neogastroplites to survive by evolving a larger hatching size. Although the cause of the large variation observed in the relative initial chamber thickness is unclear, the wide intraspecific variation observed in post-hatching shell morphology implies that the physical and biological environment did not reduce the survival of individuals with different shell morphologies. This observation suggests that the Mowry Sea may have been an environment lacking external constraints for post-hatching Neogastroplites, when compared to other post-hatching environments.