Unusual Miocene hydrocarbon-seep faunas from the Brisighella area in northern Italy: embedded in clastics and first records of the lucinid bivalves Megaxinus and Miltha

Abstract

Ancient hydrocarbon-seep sites known as "Calcari a Lucina" are common in Miocene strata of northern Italy and typically consist of carbonate deposits dominated by large lucinid, bathymodiolin, and vesicomyid bivalves. Here we report two new sites found in Upper Miocene strata at Monte Mauro near Brisighella in the Emilia-Romagna province. One is unusual by being embedded in unconsolidated siltstone without any carbonate, but yet, consisting of the typical, seep-restricted bivalves Bathymodiolus moroniae and Archivesica aharoni vesicomyid clams and bathymodiolin mussels. The second deposit is dominated by the lucinid Megaxinus bellardianus, which has never been reported from a Miocene seep deposit in this region, despite being common in coeval siliciclastic sediments nearby. This species emphasizes biogeographic relationships between Upper Miocene seep faunas in the Mediterranean region and the tropic Indo-West Pacific Ocean.

Mollusks (Gastropoda, Bivalvia) from Miocene cold-seep deposits in northern Italy: revisions and additions

“…Thalassonerita megastoma has been reported and at times figured from various Calcari a Lucina sites of the Romagna Apennines (Moroni 1966;Taviani 1994Taviani , 1996Taviani , 2001Taviani , 2011Sami & Taviani 2015Kiel et al 2023). Specimen MSF 1223 from Le Colline shows scars of the left and right columellar muscles, consistent with the anatomical sketch of the closely related extant species Thalassonerita naticoides (Clarke, 1989) provided by Warén & Bouchet (1993: fig.…”

“…Bivalves are more commonly endemic at seeps than gastropods, especially taxa with an intimate symbiosis with, and dependence on, chemotrophic bacteria. At the Calcari a Lucina deposits, these include the mytilids Bathymodiolus Kenk & Wilson, 1985and Samiolus Kiel & Taviani, 2017(Taviani 1994Kiel & Taviani 2017;Kiel et al 2023), large vesicomyids belonging to Archivesica and Wareniconcha, and the enigmatic Sisonia ultimoi sp. nov.…”

“…This species was originally reported from the Calcari a Lucina deposits by Moroni (1966) as Phasianema taurocrassum, and was subsequently cited as such from other Calcari a Lucina sites in the Romagna Apennines (Sami & Taviani 2019;Kiel et al 2023). Sacco (1895) introduced Phasianema costatum var.…”

“…We believe Moroni (1966) correctly assigned this species to Homalopoma. Initially reported from outcrops at Santa Sofia (Moroni 1966;Taviani 1994Taviani , 1996, it has since been reported from other Calcari a Lucina sites only rarely (Sami & Taviani 2019;Kiel et al 2023). Homalopoma sp.…”

“…Over the last one-and-a-half decades, we (in different constellations) have published several faunistic and taxonomic updates of the mollusks of the Calcari a Lucina deposits. These included most bivalve groups (Taviani et al 2011;Kiel & Taviani 2017 and various reports on individual new sites or noteworthy taxa (Berti et al 1994;Lucente & Taviani 2005;Sami & Tabanelli 2013;Sami & Taviani 2015;Kiel et al , 2023. The purpose of this study is to revise all species that we have not covered previously (mostly the gastropods), to describe new species, to provide new records of known species from various sites, and to supply new taxonomic data for known species.…”