Shell sculpture variation as a response to energy change in littoral settings: Chlamys actinodes, a Miocene case of study

DEL RÍO CLAUDIA JULIA; SANTELLI, MARÍA BELÉN; MARQUEZ, FEDERICO

Burgos

Congreso; XXI Reunión Bienal de la Real Sociedad Española de Historia Natural; 2015

Real Sociedad Española de Historia Natural

C. actinodes Sowerby is a highly frequent and widely distributed byssated scallop in the Late Miocene Puerto Madryn Formation exposed in northern Patagonia, a region situated in the southernmost tip of the South America.The Puerto Madryn Formation is represented by a 150 m thick upward shallowing sedimentary sequence composed by ochreous sandstones, sandymudstones, whitish tuffs, grayish muddy sandstones, and thick yellowish complex shell-molluscan beds, where a Transgressive and Maximum Highstand Phases are recognized. Each one of these phases are characterized by particular fossiliferous assemblages with unique taphonomic histories. C.actinodes is present in two of these associations, being abundant in the Ostrea patagonica-Chlamys actinodes-Aequipecten paranensis-"Turritella"ambulacrum Assemblage (OCAT) and a common species in the Amusium paris-Ostrea patagonica-Aequipecten paranensis Assemblage (AOA). Application of 2D geometric morphometric method to more than 80 left valves recorded from the OCAT Assemblage recorded from the Transgressive Phase and from the AOA Assemblage identified in the Maximum Highstand Phase, allowed the identification of two morhotypes, herein named as high-plicae and low-plicae morphotypes. Coarse-ribbed specimens (high-plicae morphotype) are characteristic of the OCAT Assemblage and inhabited firm sandy or shell-gravel substrates in high energy environments of the lower shoreface, where shells needed to be reinforce to face to stronger tidal currents than the fine-ribbed specimens (low-plicae morphotype) from sandy substrate swept by soft currents in the upper and middle -offshore marine settings. A solid sequence stratigraphic frame of this unit, the detailed knowledge of taphonomic stratigraphy of fossiliferous accumulations, and an accurate composition of assemblages involve that yield C. actinodes, fully support conclusions reached in the present research.This is a contribution supported by the Research Projects ANCyP PICT57 and CONICET PIP 320