Taphonomy of a Lithiotis problematica mound-core.

Lithiotis problematica is a Lower Jurassic aberrant bivalve characterized by a strongly inequivalve shell consisting of a long stick-like umbonal part and a short spoon-shaped body cavity. It was a gregarious semi-infaunal bivalve, which inhabited muddy substrates characterized by a relatively high sedimentation rate. The species thrived in tropical shallow marine environments, throughout the Tethys and Panthalassa carbonate platforms, where it had a relevant role as mound builder. The term Lithiotis has been often used in literature also to indicate other more frequent Lower Jurassic gregarious bivalves, such as Lithioperna and Cochlearites, but the identification in the field of these three genera is not easy and they are generally indicated with the generic term “lithiotids”. For these reasons the paleoecology of the Lithiotis problematica build-ups is scarcely known and detailed taphonomic analysis are not available in literature. The L. problematica mound studied here is located in the Rotzo Formation (Calcari Grigi Group) in the Tonezza del Cimone – Folgaria area (NE Italy). The bivalves make up a low lens-shaped accumulation, about 4 m wide and 20 cm high. The shells occur in a hard cemented limestone and are infilled by sparry calcite or greenish micrite. Lithology and preservation preclude the study of isolated specimens from the matrix. The reconstruction of the shells was, therefore, obtained through serial sectioning of some limestone blocks, originating from bed TO 41. The limestone blocks were sectioned in parallel slabs about 10 mm thick, which were then polished. The slab surfaces were scanned by an optical scanner at 1200 dpi. The most part of the Lithiotis shells are still articulated and characterized by an unusual short shells with a broad body chamber and a long umbonal notch. The biofabric of the biogenic concentration consists of almost parallel and inclined (60° in mean) shells with respect to the upper bedding surface. The orientation of commissure plane is related to the shell density. In densely packed aggregates the commissure are parallel, while they are randomly oriented in loosely packed aggregates. At the base of the mound, dispersed horizontal disarticulated valves of Lithiotis, preserved as casts, probably represented the hard surfaces for larval cementation and the colonization stage of the mound. The lower part of the mound is composed by marly limestone. Here, the shells were affected by a rapid dissolution, which destroyed both the apical regions of individuals in life position and the valves laying horizontally. In the middle-upper part of the mound, the shells are mostly infilled by calcitic cement or greenish mudstone coming from the overlain bed. The matrix of the mound ranges from mud-supported in the lower part, with sparsely distributed bioclasts, to grain- and bioclastic-supported in the upper part, where the bioclasts consist of abundant larger foraminifera (e.g., Orbitopsella), green algae, brachiopods (Lychnothyris) and gastropods. Rare sponges and solitary corals are also present. The shell fabric and fossil assemblages suggest the transition from relatively quite and muddy bottom conditions, favourable to the settlement and growth of Lithiotis, to a very shallow marine (euhaline) environment near the normal wave base. The occurrence, at the top of the mound, of abundant filter-feeding and sessile benthic forms suggests a drop in sedimentation and, probably, the emersion of the mound. The changes in the sedimentary rates controlled the shell shape (short and large) and the mound demise. On the basis of the growth increments of Lithiotis, the duration of the mound is discussed.