The bivalve Retrotapes antarcticus, from the Eocene of Antarctica, includes two different morphotypes of which one presents flat valves while the other one exhibits globoid valves. Previous authors have identified the same two morphotypes in the extant species Retrotapes exalbidus. This paper assesses the flat versus globoid morphotypes via comparing the two species by analyzing Elliptic Fourier Contours of 17 valves of R. antarcticus and 40 valves of R. exalbidus. A principal component analysis was performed and the age of both morphotypes was estimated by way of counting growth lines. Several authors have argued that genetic differences between the two forms of R. exalbidus are not sufficient to separate them into two distinct species and suspected that the globoid morphotype may correspond to gerontic specimens. Our results demonstrate that the morphological transformation detected in R. antarcticus is similar to that observed in R. exalbidus. The analysis of growth lines confirms that the globoid morphotype corresponds to gerontic specimens in both species. This constitutes an example of gerontic intraspecific variation. Additionally, this contribution allows us to further understand the life-history of the fossil bivalve R. antarcticus for it presents a growth pattern akin to that of the living species.