Bathymetric distributions of modern populations of some common Cenozoic Bryozoa from New Zealand, and paleodepth estimation

Abstract

Paleodepth inference from fossils often involves application of the principle of taxonomic uniformitarianism using known depth ranges of modern populations belonging to the same or closely related species. Bryozoans are abundant on the continental shelf around New Zealand at the present day but their potential as paleodepth indicators in the Cenozoic, where they dominate many fossil assemblages, has not been explored. Here, we survey the bathymetric distributions of four readily identifiable and common species (Cinctipora elegans, Attinopora zealandica, Diaperoecia purpurascens, and Celleporaria emancipata) plus one distinctive ecological group of bryozoans (tube‐building symbionts of hermit crabs), all of which are well represented in the Neogene fossil record of New Zealand. Data on the benthic stations containing these bryozoans in the extensive oceanographic collections at the National Institute of Water & Atmospheric Research reveals wide bathymetric distributions which constrain their use in estimating paleodepth from fossil occurrences. Total depth ranges were found to be: Cinctipora elegans, 17–914 m; Attinopora zealandica, 35–1156 m; Diaperoecia purpurascens, 0–1156 m; Celleporaria emancipata, 68–690 m; hermit crab tube builders, 0–252 m. A localised analysis of occurrences and abundances of Cinctipora elegans and Diaperoecia purpurascens off Otago Peninsula was also conducted. This analysis confirmed that these species occur over a large depth range on the Otago shelf. In C. elegans, colony abundance peaked strongly between 75 and 100 m off Otago Peninsula, a relatively narrow depth range. Despite their wide bathymetric distributions, several of the species surveyed here may eventually prove useful as paleodepth indicators because of their morphological plasticity which, by analogy with other bryozoans, is probably a response to environmental variables, some correlated with depth. Data are also presented on the geographic distributions of the target bryozoans.