Morphometric analices of Mactra Linné (Bivalvia) from the marine Quaternary of Argentina (Southwestern Atlantic)

AGUIRRE M.L.; PEREZ S.I.; FARINATI E.A.

Londres

Congreso; III International Paleontological Congress; 2010

Paleontological Association

Mactra is a common and distinctive bivalve in Pleistocene and Holocene marine sediments of Argentina. Along the Bonaerensian area, M. isabelleana is constant and abundant, exhibiting wide morphological variation. Scarcer modern records in the Argentine and Magellanean Malacological Provinces (SW Atlantic margin) include similar species originally described on shell shape, i.e., petiti, isabelleana, patagonica, marplatensis, jane\iroensis. Relative Warps analysis (RW) on coordinates of 16 landmarks and 25 semi-landmarks along the inner outline (digital images of the right valve) for 1,200 shells (fossil and modern) of different morphs (species) from 24 Bonaerensian sites (beach ridges, tidal flat, coastal lagoonal facies, beach) improve objective evaluations of intraspecific and interspecific variation of Mactra in this area. Shape similarity of morphs suggests that interspecific differences are not important and shell shape variation represents a sequence of decreasing umbonal inflation and increasing elongation (conditioning length of pallial sinus and elongation of hinge features) in a chain of transitional morphs. The modern variation of Mactra correlates with environmental parameters (mainly substrate nature and salinity, secondarily water energy and depth). Subtrigonal inflated shells (isabelleana, petiti) predominate in muddy substrates of quieter, shallow mixo-polyhaline waters; ovate-elongate, less convex shells (marplatensis, janeiroensis, patagonica) are typical of fine sands in poly-euhaline deeper waters, close to higher energetic open marine conditions. Therefore, we suggest that M. isabelleana is a polymorphic species, with ecomorphs potentially useful as palaeoenvironmental indicators during the last high sea-level stands up to present.