Morphological variability of Brachidontes Swainson (Bivalvia, Mytilidae) in the marine Quaternary of Argentina (SW Atlantic)
Abstract Morphometric analyses of abundant shells of Brachidontes from 27 localities (19 littoral deposits of Late Quaternary age and eight modern sites) in Argentina, where Brachidontes rodriguezi d'Orb., 1846, Brachidontes darwinianus d'Orb., 1846 and Brachidontes purpuratus Lamk., 1819 have been reported, show implications for palaeoenvironmental reconstructions. Only shell characters have been generally used for taxonomic discrimination of these taxa (i.e., general shape, size, external ornamentation, colour). Morphometric variation ranges from elongate, comparatively small shells (R), typical of coarse sandy substrates and warm or warm-temperate water masses in typically marine and polyhaline environments (dominant in the mid-Holocene), to bigger, more strongly ribbed and more globose shells (P), characteristic of pebbly or rocky substrates, and shallow, cold water masses in typically open marine habitats (dominant in the Magellanean province at present). Traditional and geometric morphometric techniques performed on large amounts of fossil and modern shells help to objectively define the morphometric variation of Brachidontes in the area. The coordinates for the shell outline (165 specimens) and five linear dimensions (485 specimens) were used for morphometric analyses of shell specimens of the three main morphs (D = darwinianus; R = rodriguezi; P = purpuratus) identified within the samples collected, for two main reasons: 1) They represent the most common features used for taxonomic discrimination at the species level; 2) comparative analysis of outer ornamentation and colour could produce deceiving results due to the varying preservation grades between fossil and modern shells. Results both by Relative Warps and Principal Component techniques show no difference among shell morphs, implying that B. darwinianus and B. rodriguezi (= B. solisianus d'Orb. = B. exustus Linn.) might have to be synonymized with B. purpuratus (P), which dominates in colder waters at higher latitudes. The morphotypes are extremes within a wide morphological range and can be linked together in a chain of transitional forms. They respond to different habitats (ecomorphs) controlled by temperature (geographical variation), substrate, energy and water-depth, and by salinity, and by competitive strategies for space (availability of food resources), all influencing growth. The morphometric variation of organisms in fossil strata could represent a valid tool to reconstruct palaeoenvironmental changes that occurred during high sea-level stands since ca. 300 ka (Pleistocene, MOIS 9) to ca. 7.6–4.5 ka (mid-Holocene Climatic Optimum, transgressive maximum of MOIS1) and up to present.