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; MÁRQUEZ, 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 byssatedscallop in the Late Miocene Puerto Madryn Formation exposed in northernPatagonia, a region situated in the southernmost tip of the South America.The Puerto Madryn Formation is represented by a 150 m thick upwardshallowingsedimentary sequence composed by ochreous sandstones, sandymudstones , whitish tuffs, grayish muddy sandstones, and thick yellowishcomplex shell-molluscan beds, where a Transgressive and MaximumHighstand Phases are recognized. Each one of these phases are characterizedby particular fossiliferous assemblages with unique taphonomic histories. C.actinodes is present in two of these associations, being abundant in the Ostreapatagonica-Chlamys actinodes-Aequipecten paranensis-?Turritella?ambulacrumAssemblage (OCAT) and a common species in the Amusium paris-Ostrea patagonica-Aequipecten paranensis Assemblage (AOA). Application of 2Dgeometric morphometric method to more than 80 left valves recorded fromthe OCAT Assemblage recorded from the Transgressive Phase and fromthe AOA Assemblage identified in the Maximum Highstand Phase, allowedthe identification of two morhotypes, herein named as high-plicae and lowplicaemorphotypes. Coarse-ribbed specimens (high-plicae morphotype) arecharacteristic of the OCAT Assemblage and inhabited firm sandy or shell-gravelsubstrates in high energy environments of the lower shoreface, whereshells needed to be reinforce to face to stronger tidal currents than thefine-ribbed specimens (low-plicae morphotype) from sandy substrate sweptby soft currents in the upper and middle -offshore marine settings. A solidsequence stratigraphic frame of this unit, the detailed knowledge of taphonomicstratigraphy of fossiliferous accumulations, and an accurate composition ofassemblages involve that yield C. actinodes, fully support conclusions reachedin the present research.This is a contribution supported by the Research Projects ANCyP PICT57 and CONICET PIP 320