Abstract
Morphological changes along ontogeny, based on size, are analysed in five species of Gryphaea from the Middle Jurassic of southern South America. Three of these taxa are new: G. apiculata n. sp., G. varillasensis n. sp. and G. euteicha n. sp. All five species have thin radial striae on the left valve. They are regarded as closely related on the basis of the similar configuration of the main part of the left valve (especially during the first ontogenetic stages), the associated development of the posterior flange (and sulcus), the comparable variability and/or recurrent morphology of these and other structures, and their relative stratigraphic position or age. They show differences in the relative appearance time and development of several characters, and similar changes probably have also occurred stratigraphically, at least, in G. oxytropis philippi and G. euteicha n. sp. All these features allow the recognition of size (morphology)-based heterochronic processes (peramorphic and paedomorphic), either in the origin of most of these taxa or in morphologic changes which developed along time in some of them. This work provides preliminary evidence of the most remarkable iterative occurrence of heterochrony known to date in oysters.