Taphonomy and palaeoecology of deep-water chemosymbiotic bivalves from the Eocene of Outer Eastern Carpathians, Ukraine

Abstract

The paper reports rare occurrence of deep-water solemyid and lucinid bivalves from Cenozoic sequences of Outer Eastern Carpathians, Ukraine. The studied bivalves occur in the Eocene Pasichna Formation, represented by deep-water marls and pelagic limestones intercalated with thin-bedded calcareous sandstones yielding material re-deposited from shallow water. This find offered a chance to study the unaltered shell breakage pattern of these deep-burrowing solemyid bivalves buried in situ by turbidites. The breakage includes arcuate breaks representing likely syn-vivo damage related to active burrowing of the animal while telescopic breaks are most likely post-mortem distortions produced by the load of the heavy sediment covering sea bottom after a turbiditic event. Taxonomic study indicates that both solemyid and lucinid bivalves from the Eocene of the Pasichna Formation are most similar to deep-water members of their respective clades, known from roughly coeval deposits from the Tethys. The available evidence indicates that bivalves in question lived on the deep-sea muddy bottom, influenced by turbidite deposits, and were buried by them syn-vivo or early post-mortem. This is in contrast to the great majority of molluskan faunas from the Carpathian Paleogene, reworked during transport downbasin from shallow-marine settings by mass gravity flows. The detailed study of the occurrence mode of deep-water bivalves in the Carpathian part of the Tethys during the Paleogene, influenced by frequent build-up of oxygen-depleted waters, offers implications for the evolutionary histories of both groups in question. For both solemyids and lucinids, adaptations to oxygen-depleted environments are primordial, and are carried from the Silurian until today, as supported by the presented fossil and actualistic evidence. This is in contrast to their histories in high-redox potential environments, with solemyids known from cold seeps from the Devonian onwards and lucinid seep record only from the Jurassic onwards.