Allometric growth pattern of the genus Ptychomya Agassiz in the Neuquen Basin, west-central Argentina

Abstract

Ptychomya koeneni Behrendsen is a frequently recorded heterodont bivalve in Lower Cretaceous marine rocks of the Neuquén Basin, especially in the Agrio Formation. It shows a characteristic divaricate ornamentation, which makes it easy to be identified. The sole published study to date on its taxonomy has established the existence of six varieties of subspecific rank. However, a recent unpublished study by the authors based on newly collected specimens from the Agrio Formation and type specimens of the mentioned varieties from the Burke Museum of Natural History & Culture (Seattle, USA) found out that there is a wide morphological gap between one of the varieties, namely P. koeneni coihuicoensis, and the rest of them, suggesting that they could be two distinct groups. The aim of this work was to compare the allometric growth patterns of P. k. coihuicoensis and the P. koeneni cluster in order to determine if they share a common ontogeny. Two aspects of external morphology of the shell were antified (general outline and ornamentation) for specimens of each group at different ell sizes (i.e. growth stages) using geometric morphometrics. Allometric curves were statistically compared through regressions with categorical predictors for each morphological variable. Results showed that while some morphological variables seemed to share a common allometric curve, there are other variables that differ in their growth attributes (i.e. regression curves have a statistically different origin and/or slope). Shape trajectories of P. k. coihuicoensis and the P. koeneni cluster differed along the ontogeny in the multivariate morphospace, meaning that the overall growth pattern was different between these two groups. This evidence supports the hypothesis that P. k. coihuicoensis is a distinct group separated from the rest of the P. koeneni varieties, and points out the need for a taxonomical reevaluation of the genus Ptychomya in the Lower Cretaceous of the Neuquén Basin.