Thyasirid bivalves from Cretaceous and Paleogene cold seeps

We present a systematic study of thyasirid bivalves from Cretaceous to Oligocene seep carbonates worldwide. Eleven species of thyasirid bivalves are identified belonging to three genera: Conchocele, Maorithyas, and Thyasira. Two species are new: Maorithyas humptulipsensis sp. nov. from middle Eocene seep carbonates in the Humptulips Formation, Washington State, USA, and Conchocele kiritachiensis sp. nov. from the late Eocene seep deposit at Kiritachi, Hokkaido, Japan. Two new combinations are provided: Conchocele townsendi (White, 1890) from Maastrichtian strata of the James Ross Basin, Antarctica, and Maorithyas folgeri (Wagner and Schilling, 1923) from Oligocene rocks from California, USA. Three species are left in open nomenclature. We show that thyasirids have Mesozoic origins and appear at seeps before appearing in “normal” marine environments. These data are interpreted as a record of seep origination of thyasirids, and their subsequent dispersal to non-seep environments. We discuss the age of origination of thyasirids in the context of the origin of the modern deep sea fauna and conclude that thyasirids could have deep sea origins. This hypothesis is supported by the observed lack of influence of the Cretaceous and Paleogene Oceanic Anoxic Events on the main evolutionary lineages of the thyasirids, as seen in several other members of the deep sea fauna.Financial support to KH was provided by the Polish National Science Centre (NCN) research grant 2014/15/B/ST10/04886 “The influence of Paleocene/ Eocene Thermal Maximum on oceanic chemosynthesis-based ecosystems” (for field work on Svalbard), by the Institute of Paleobiology, Polish Academy of Sciences (for the visit to the collections in Vienna) and by SYNTHESYS grant SE-TAF-5990 (for a visit to collections in Stockholm). Financial support to SK for various field work was provided by the Deutsche Forschungsgemeinschaft (grant Ki802/6-1), by the European Commission through a Marie-Curie fellowship (MEIF-CT-2005-515420), and by the Joetsu University of Education. Field work in Hokkaido was financially supported by a Grant-in-aid for Scientific Research from the Japan Society for Promotion of Science (C, 26400500, 2014–2016; C, 17K05691, 2017–2019) to KA and RGJ. Material from Montrose, Nebraska, was collected thanks to the support of the Polish National Science Centre (NCN) research grant 2012/07/B/ ST10/04189 to Andrzej Kaim (ZPAL).