Plio-Pleistocene bryozoan faunas of the Wanganui Basin, New Zealand : diversity, distribution and paleoecology

Abstract

New Zealand's temperate seas are especially rich in bryozoans and they are important in the Cenozoic fossil record. This study outlines bryozoan occurrences during the Pliocene-Pleistocene of the Wanganui Basin, North Island, New Zealand. A large collection of fossil material was sampled from 48 marine-deposited lithostratigraphic units in the cyclothemic sequence. Bryozoan colonies encrust hard substrata, namely mollusc valves, in deposits of near-shore and shelf origin. From a total of some 1200 colonies, the occurrence of 103 bryozoan taxa, including three new species, are recorded here, and a biostratigraphic range chart is presented. Paleoecological characteristics of the fauna, including diversity, vary through each depositional sequence. The availability of hard substrates, sedimentation rates and water currents is likely to have influenced bryozoan colony growth. Distributional patterns of 692 encrusting bryozoans found on oysters andpectinid valves in particular were analysed. The locations of bryozoan colonies on valve surfaces provide some information about past habitat, including substratum orientation and mortality at the time of encrustation. On some surfaces colonies encountered each other. The results of 465 interactions (mostly interspecific) were recorded to provide insight into bryozoan competition. Fifteen common bryozoan taxa are ranked in terms of their ability to overgrow one another. Using multivariate statistics three recurring bryozoan assemblages were identified, (Paleocommunities 1, 2 and 3) that correlate with the local sequence- stratigraphic systems tracts. The relative abundances of colony growth form also vary among samples from different depositional settings. Erect bryozoans are important in many Wanganui horizons; however, encrusting species dominate the assemblages overall. Some bryozoan growth forms characterise certain environments (e. g. free-living forms in shallow-shelf sandy settings), but most were not exclusive in their distribution. Taphonomic effects on each fossil assemblage are evident. Erect bryozoan remains found in lower transgressive systems tracts are typically disarticulated, transported and worn. In mid-cycle shellbeds however, the in situ encrusting bryozoan colonies often show little taphonomic alteration and erect branching colonies are often well-preserved. The recurring growth-form associations and diversity-based bryozoan paleocommunities allow for general environmental interpretations, but are not necessarily strict indicators of habitat limitations for these taxa. The fluctuating nature of the faunal changes through the Wanganui sequence suggests that bryozoans were responding to an alternating, repetitive process, probably cyclic sea-level change, as has been already noted for the local molluscan fauna. Lateral variations in bryozoan distribution probably occurred via migration or habitat tracking of the fauna through the Plio-Pleistocene.