Reading shell shape: implications for palaeoenvironmental reconstructions. A case study for bivalves from the marine Quaternary of Argentina (south-western Atlantic)

Abstract

Most research on bivalves from the south-western Atlantic used morphological (shell) characters for taxonomic discriminations. Dominant Veneroids from Argentinian Quaternary coastal deposits exhibit wide morphological variation – often making objective discriminations difficult/impossible, which could be objectively described and compared through geometric morphometrics techniques. This work focuses on comparison of geometric morphometrics methods applied to fossil and modern shells, to assess inter- and intra-generic variations. Three approaches were considered: landmarks (L), semi-landmarks (SL) and outlines. Shell shape analyses for different time spans (Pleistocene, fossil Holocene and modern) and areas (Patagonia and Bonaerensian) showed that Elliptic Fourier analysis (EFA), Landmarks and Landmarks plus Semilandmarks (L+SL) can discriminate at generic levels: Mactra, Mulinia (Mactridae) vs. Pitar, Protothaca, Eurhomalea, Clausinella (Veneridae). L and L+SL are powerful for inter/intraspecific distinctions of Mactra. Variability of Mactra isabelleana includes the remaining nominal ‘species’ (transitional morphs). Causal environmental factors of (phenotype) variation could be addressed for modern environments (substrate, salinity and energy). Subtrigonal-inflated shells predominate in muddy, quieter, shallow mixo-polyhaline waters; ovate-elongate-compressed in sandy, poly-euhaline, deeper habitats. Differential spatial distribution (and abundance) across time responds to Late Quaternary high sea-level stands: transgressive maxima allowed higher salinity in marginal-marine areas and optimal conditions for Mactra isabelleana contrasting with scarcer records in the Mar Argentino today.

Keywords::

• Molluscs
• Holocene
• Pleistocene
• variability
• geometric morphometrics
• palaeoenvironments

Acknowledgements

We thank Ivan Perez (División Antropología, MLP) for his help from the beginning of this study. We also thank Diego Giberto and Claudia Bremec (INIDEP, Mar del Plata, Argentina), Alejandro Tablado (MACN, Buenos Aires, Argentina) and Cristina Damborenea and Mónica Tassara (MLP, La Plata, Argentina) for allowing us to examine molluscan collections at national institutions; Kathie Way (Natural History Museum, London) for help and advice in examining types and historical molluscan collections from South America and for providing information regarding type materials of Veneridae and Mactridae from the SWA; Jorge O. Codignotto, Roberto Kokot, Karen Davies, Ignacio Castellanos and Rubén Medina for their help during field work and Liliana Mormeneo and Perla Imbellone regarding taphonomic aspects of Mactra.

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplementary material

Supplemental data for this article are available online.

Notes

^1. Email: [email protected]

^2. Email: [email protected]

^3. Email: [email protected]

Additional information

Funding

This work was supported by Agencia Nacional de Promoción Científica (ANPCYT) (Project PICT 468), Consejo Nacional de Investigaciones Científicas y Técnicas (Conicet) (Projects PIP 0080 and PIP 11220120100372CO), Universidad Nacional de La Plata (Project PI 11/N587 and 11/N726 of Facultad de Ciencias Naturales y Museo) and Universidad Nacional del Sur Bahia Blanca) (Project PI 24/H099), DAAD and DFG-Conicet bilateral cooperation projects also facilitated collections of modern Mactridae and other Veneroid bivalves from southern Patagonia.