Vermetid reefs and their use as palaeobathymetric markers: new insights from the Late Miocene of the Mediterranean (Southern Italy, Crete)

Vermetids are a poorly known family of reef-building gastropods characterized by an extremely irregular growthof the shell. Vermetid reefs are known from the Badenian (Middle Miocene) to present-day and their mainimportance is related to their use as sea-level indicators, strictly associated to the intertidal or immediate subtidalzone. During their evolution, two different vermetid genera have been recognized as reef-builders: Petaloconchuswas the major component of vermetid reefs until the Holocene when, for still unknown reasons, it was almostcompletely replaced by Dendropoma. In the Mediterranean Basin, Petaloconchus reefs have been frequently foundin close association to Late Miocene coral bioconstructions. New finding of Petaloconchus reefs within the slopefacies of the Salento Messinian reef complex (southern Italy), together with new data from late Tortonian–earlyMessinian reefs of Crete (Greece) provide further information on the palaeoecology of these peculiarbioconstructions. Stratigraphical and sedimentological contexts of Salento and Crete vermetid reefs have beendescribed, together with an account of their size, shape and inner structure. Biotic (vermetids, serpulids,bryozoans and coralline algae) and abiotic components (earlymarine cements and intra-reef sediment) have beenidentified by microfacies analysis, and their percentage-abundance has been reported. These data allow thedescription of vermetid reefs from two different depositional settings: along the shelf edge and, for the first time,within the coral reef slope. Shelf-edge and slope Petaloconchus reefs show the same inner structure andcomposition, but they differ by their shape, by the density of the framework, and by the relative proportion of theassociated reef-building organisms (serpulids, bryozoans and coralline algae). Additional information on theirpalaeobathymetric setting have been acquired from the presence of other reef-building biota (Porites corals andHalimeda green algae)which occur in close proximity to slope vermetid reefs, together with observations on someanatomical feature of Petaloconchus shells. Results clearly indicate for Petaloconchus reefs a palaeobathymetricrange spanning from the upper subtidal zone down to 30–50 m of depth. The present study confirms vermetidreefs as important tools for the reconstructions of ancient sea-level, but great attention has to be paid to theidentification of the main reef-building vermetid. Actually, while a great number of present-day Dendropomareefs can be associated to the mean sea-level, Late Miocene vermetid reefs from Salento and Crete indicate forPetaloconchus bioconstructions a wider palaeobathymetric range.