A taxonomic and palaeobiogeographic study of the fossil fauna of Miocene hydrocarbon seep deposits, North Island, New Zealand

Abstract

Hydrocarbon seeps are geological features characterized by the release of fluids rich in methane and hydrogen sulfide from predominantly continental-margin sediments, usually diffusely over extensive areas. These settings typically support lush chemosynthetic faunal communities distinct ecologically from the surrounding sea floor. Discovery of these modern 'extreme environments' has allowed recognition of vent-seep deposits in the geological record. Palaeontological study of fossil seeps contributes to assessment of evolutionary and palaeobiogeographic trends in these environments. Proposals for introduced and standardized taxonomic terminology are made herein, including a new fundamental adjective relating to the core Linnaean level of class. A diverse fossil fauna from 13 hydrocarbon seep deposits in the Hawke's Bay region, North Island, New Zealand is described. Three formally described and named species are introduced, a provannid gastropod (Provanna marshalli) and two mytilid mussels [Bathymodiolus (s. l.) heretaunga and Gigantidas coseli]. The fossil record of provannids and vent and seep mussels is reviewed in detail, and the current state of knowledge on modern vent and seep mussels is synthesized. Provanna marshalli is the most variable species of the genus described to date, and its shell microstructure is reported and found to be comparable to the fossil species Provanna antiqua and some modern specimens of the genus. Both new mussel species are small compared to their modern congeners, and it is suggested that an evolutionary trend toward gigantism occurred in vent and seep mussels since the first known fossil species of the group appeared in the Middle Eocene. Furthermore, four new records of named taxa with two emended descriptions, eight other new (sub)species and 35 taxa left in open nomenclature are described herein and compared against occurrences of similar taxa in the hydrocarbon seep fossil record and at modern New Zealand hydrocarbon seeps. Miocene New Zealand hydrocarbon seep diversity, endemicity and palaeoecology is thereafter evaluated. Description of the ichnofossil content of the New Zealand hydrocarbon seep deposits adds to a relative paucity of such work in the fossil hydrocarbon seep literature. Taxonomic similarities between New Zealand hydrocarbon seep localities and analogous (circum- Pacific and/or Miocene-age) seep deposits were evaluated and found to have greater correlation with age than geographical location. Faunal migratory pathways between New Zealand and the most similar (contemporaneous) localities, that are not available today, were available in the Miocene. An overview of the entire global hydrocarbon seep fossil record is presented, with patterns of metazoan occurrences since the earliest (Silurian) reports of hydrocarbon seep fossils elucidated. It was found that there was a lull in seepage throughout the Permian and Triassic, with clear faunal differences either side of this hiatus, brachiopods dominating before, molluscs dominating from the Jurassic onwards.