The origin and paleoecology of clypeasteroid assemblages from different shelf settings of the Miocene of Sardinia, Italy

Clypeasteroid echinoids can be common in both siliciclastic and carbonate sedimentary sequences of the Miocene of the Mediterranean and Paratethys areas where they frequently form highly fossiliferous deposits. Two echinoid assemblages dominated by clypeasteroids from the early-middle Miocene of Logudoro region (Northern Sardinia) are compared using a detailed sedimentological and taphonomic analysis along with functional morphological considerations. The assemblages differ widely with respect to taxonomic composition, sedimentological features, and taphonomic signatures, such as the orientation and degree of abrasion, encrustation, and bioerosion. The first assemblage (Ardara) is characterized by a low-diversity echinoid fauna, consisting exclusively of the two clypeasteroid genera, Amphiope and different morphotypes of Clypeaster. This deposit represents a proximal tempestite originating in a high-energy, shallow-water, shoreface setting. Higher diversity characterizes the second assemblage (Ittiri) with clypeasteroid, spatangoid, and cassiduloid echinoids. This deposit originated by multiple in situ reworking events within a deeper, low-energy, sublittoral environment. The difference in faunal diversity between these assemblages reflects ecological factors such as substrate characteristics and food supply. Differences in preservation potential of the different echinoid tests also influence their representation within the deposits. These described echinoid deposits represent two examples of a wide spectrum of clypeasteroid-dominated assemblages from the Miocene of Sardinia, which are compared with respect to taxonomic makeup, sedimentological and taphonomic features, and depositional environments.