Marine Ostracoda (Crustacea) from the late Oligocene Gellibrand Marl, Otway Basin, Victoria, Australia

A subsurface sample from Heywood-10 borehole, Otway Basin, Victoria, has provided the first ostracod assemblage of Oligocene age from the Gellibrand Marl (Heytesbury Group). Previous Gellibrand Marl ostracod assemblages were Miocene. This Late Oligocene assemblage of 384 specimens includes 50 species and subspecies from 34 genera across 18 families; 24 taxa are placed in open nomenclature. Of the taxa discussed, several appear to be new species but with too few specimens for them to be described as such. The reciprocal of Simpson’s Diversity Index was applied to assist assemblage comparisons. The Gellibrand Marl assemblage is larger, contains more families, genera and taxa but is less diverse than a smaller assemblage from the Early Oligocene Narrawaturk Formation (Nirranda Group) at the same location, and more diverse than an assemblage from the Early Oligocene/Ruwarung Member, South Australia. There are notable differences in the dominant taxa present in each assemblage. In the Gellibrand Marl, Pontocyprididae predominate; in Narrawaturk Formation, Cytheruridae and Xestoliberididae are most abundant; and in the South Australian assemblage, Bairdiidae by far the most numerous. This Gellibrand Marl collection has the characteristics of an at least partly allocthanous assemblage, the habitat a well-oxygenated mid-shelf environment. No cold or deep-water taxa are present.