Systematics and assessment of the chemosynthetic bivalve family Vesicomyidae

Abstract: The deep ocean has fascinated researchers for centuries with discoveries of new ecosystems and species, yet there are still many biogeographic and evolutionary questions unanswered. Along active and passive tectonic margins, specialized ecologies thrive in a symbiosis between chemosynthesizing bacteria and their host animals. This specialized fauna exists in particular in areas where hydrocarbon seepage occurs. Hydrocarbon seep environments have been of research interest since their discovery in the Gulf of Mexico in 1984. They are home to a diverse chemosynthetic fauna whose fossil record extends back to the Paleozoic. This project strives to increase the understanding of the biodiversity, biogeography and evolution of seeps and their diverse fauna. It is focused around the Cenozoic seep fauna, and especially the bivalve family Vesicomyidae that have dominated chemosynthetic systems since the late Eocene. The taxonomy of this family is under revision; discrepancy between molecular and morphological characteristics has resulted in cryptic species. To help improve the taxonomic and evolutionary framework, the main focus of the PhD project is to create a phylogeny based on the combination of morphological data and molecular data, with including most of the fossil vesicomyid species. Vesicomyid literature and specimens from museum collections have been thoroughly examined and analyzed using cladistics to assess their taxonomy and evolution. Stratigraphic dispersal of vesicomyids is recorded in a stratophenic comparison to visualize first occurrences and early species radiation, providing an overview of their fossil record and compiling the appearance of evolutionary traits. Reassessment of taxonomic groupings lead to the description of the new genus Squiresica, which includes the small-sized Arhicvesica knapptonensis and Archivesica marincovichi. Furthermore, from the Humptulips Formation in northwestern USA and the Talara Basin in northern Peru, we describe four new seep sites. Hydrocarbon seep sites in Italy, Cuba and Taiwan are examined for a biostratigraphic purpose. From USA and Peru, 11 new seep-mollusk species are identified and the fossil content from all locations are indicative of a modern type seep fauna. The phylogenetic analysis reveals that several morphology-based groupings correspond well to molecular groups, and that climate may have played a role in the shell size difference we see in fossils around the Eocene-Oligocene transition.