Taphonomy and Palaeoecology of multi-element skeleton invertebrates: a genetic model for exceptional preservation

The aim of the research is to improve the understanding on the origin of fossil accumulations by investigating their taphonomic and sedimentological features and by using stratigraphic data and the autecology of the organisms forming the shell beds. A number of highly different oligospecific high dense fossil accumulations of wellpreserved, multi-element skeleton invertebrates, such as arthropods and echinoids, were studied. The first case studied concerns the Ordovician trilobite-like arthropod Tariccoia arrusensis, which is found locally within the grey siltstones of Riu Is Arrus Member (Monte Argentu Formation, Upper Ordovician), cropping out along the valley of the creek Riu Is Arrus, SE Fluminimaggiore (SW Sardinia). The locality Roia Srappas is the most distinctive whit regards to the fossil content. The facies analysis allows documenting a clastic sequence whose origin is related to turbidity currents(hyperpycnal flows) generated by a flood-dominated fluvio-deltaic system entering in lagoonal or protected shallow marine settings. Two taphofacies are recorded: tf1 consists of a high density of articulated remains, which constantly show convex down orientation, while tf2 consists exclusively of disarticulated remains randomly oriented. The preservation state and preferred orientation of arthropods in the tf1 leads to infer gentle transport by mud plumes, related to hyperpycnal flows, rapid burial and low scavenging and bioturbation. The second case focused on four clypeasteroid mass occurrences from the Lower Miocene of Sardinia, which are analyzed and compared. Two clypeasteroid assemblages were found within the Nurallao Formation (Late Oligocene-Burdigalian). The other two investigated clypeasteroid assemblages that were investigated, are found within the Calcari di Mores Formation (Late Burdigalian). These fossil echinoid deposits show a 2 low taxonomic diversity: two assemblages are dominated by the genus Amphiope while the other is dominated by Parascutella with subordinate Clypeaster. Although the four sand dollar deposits, which are the object of this study, can be assigned to a shoreface environment, there are differences in their features and origin. Three different genetic mechanisms, such as multiple in situ reworking, storm deposition and autochthonous accumulation can be inferred. The third case focused on regular echinoid and spatangoid mass accumulations, which are extremely rare in the fossil record of the Miocene of Sardinia. In the Lower Burdigalian sediments of the Marmilla Formation two monospecific regular echinoid and spatangoid mass occurrences were found. The regular echinoid assemblage consists of a dense accumulation of phymosomatid echinoid Anisophyma n.g.n.sp., which is decribed and illustrated. The other echinoid assemblage is a monospecific Brissopsis beds. The third regular echinoid mass accumulation, which is found within the Gesturi Marls (Upper Burdigalian to Langhian), is a sea urchin spine bed with rare and poorlypreserved complete tests. Although the three studied echinoid mass accumulations can be trace to a storm-dominated offshore environment, observed variations in the taphonomic and sedimentologic features are due to three different genetic mechanisms such as, rapid buried living community (obrution), time-averaged fossil bed by sediment starvation, and distal tempestite event. The studied mass accumulations allowed explaining the nature and the distribution of the fossil concentrations across a wide spectrum of environments. Nearshore depositional environments are dominated by sedimentological concentrations but can also include sedimentologically overprinted biological concentrations. In deeper water environments shell-bed development can mainly be the results of ecologic (biologic) mechanisms, shell accumulation during episodes of sediment starvation and rapid burial by storm events. The analysis of multi3 element skeleton mass accumulations showed that these fossil deposits are a useful tool for palaeoenvironment and palaeoecological reconstructions.