The byssate pectinid “Chlamys” actinodes (Sowerby, 1846) is an abundant and widely distributed species in the sedimentites of the Puerto Madryn Formation (late Miocene), exposed in the northeastern sector of the Patagonian region (Valdés Peninsule, Chubut Province). The exterior sculpture of this species is highly variable and the 2D geometric morphometric method herein performed enabled the recognition of two morphotypes that reflect differential development of shell-sculpture in response to variation of the hydraulic energy conditions in littoral settings. On one hand, a high-plicae morphotype is defined, represented by specimens with prominent coarse-sculptured shells inhabiting firm-sandy or shell-gravel substrates which developed in high energy environments of the lower shoreface recorded in the Transgressive Phase of the sequence. On the other hand, a low-plicae morphotype is recognised as consisting of fine-sculptured specimens associated to sandy substrates of the upper and middle offshore marine settings recorded in the Maximum Highstand Phase and placed immediately above the maximum flooding surface. In these facies, hydraulic energy conditions are restricted to mild tidal currents of much lower energy than those developed in the environments inhabited by specimens presenting coarse-sculptured shells.
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