Abstract
Although clypeasteroid echinoids are very common in the shallow water sediments of the Miocene of Sardinia, they have received little attention as far as their origin is concerned. In this paper, three sand dollar dominated mass accumulations from the Lower Miocene of Sardinia are studied and compared in order to understand their genesis and palaeoecology. Two clypeasteroid assemblages were found in the Sarcidano region (Central Sardinia) within the Nurallao Formation (Late Oligocene–Burdigalian). The third sand dollar mass occurrence crops out in the Logudoro region (northern Sardinia) within the Calcari di Mores Formation (Late Burdigalian). These fossil echinoid deposits show a low taxonomic diversity; two assemblages are dominated by the genus Amphiope while the other is dominated by Parascutella with subordinate Clypeaster. An integrated stratigraphic, taphonomic and sedimentological approach allows different genetic mechanisms for these shallow water, sandy, shoreface deposits to be inferred. The origin of these deposits is related to ecological, morphological and sedimentological factors. The sand dollars of Cuccuru Tuvullao (Nuragus) show high degrees of abrasion and fragmentation and represent a multiple in situ reworked accumulation while the assemblage of Duidduru (Genoni), with low abrasion and variously oriented complete and fragmented specimens constitutes an example of a proximal storm deposits. The sand dollar bed of Monte Sa Loca (Chiaramonti) consists of a dense accumulation of well-preserved, mainly concordant-oriented, juvenile and adult specimens and is considered to represent an autochthonous assemblage. This paper explores the role of detailed sedimentological and taphonomic analysis in the palaeoecological and genetic interpretations of the sand dollar beds and points out the widespread distribution and diversification of these echinoids in the Miocene deposits of the Mediterranean area.
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