Shell morphometric variations in a Patagonian Argentina clam (Ameghinomya antiqua) from the Mid-Pleistocene (MIS 7) to the present

This study analyses the shell morphological variations in the venerid clam Ameghinomya antiqua recovered from Bustamante Bay, in Patagonia Argentina, from the Pleistocene (MIS 7–MIS 5) to the present. Fossil and modern A. antiqua shells were analyzed using the Elliptic Fourier Analysis (EFA) and landmark methods. Both methods were successful in separating the studied bivalves into four chronologic groups -MIS 7, MIS 5, Holocene and Modern-, showing significant shell shape and size variations throughout the considered period. The MIS 7 shells have an ovoid rounded shape, the MIS 5 shells exhibit an ovoid and marked prosogyrous condition, while Holocene and Modern shells are more rounded to subquadrangular. MIS 5 shells are also larger than the others groups. These morphological variations could be related to the phenotypic plasticity of A. antiqua as a response to different environmental conditions, representing the ability of this species to adapt to different sub-environments as an answer to substrate variations and the hydrological changes which apparently took place during the period.