Intraspecific morphological variation in late Miocene Bolboforma, and implications for their classification, ecology, and biostratigraphic utility

Abstract

A preliminary morphological study of large, well preserved bolboform populations from the late Miocene section of DSDP Site 593, in the southern Tasman Sea, reveals a relationship between test size and intraspecific morphological variation interpreted as a gradual transformation in test shape and surface ornamentation. The succession begins with small, smooth, thin-walled subspheroidal tests. As test size increases, rudimentary ornaments become more common and the shape more globose. With further size increase, the walls thicken and ornaments become more structured, but specimens still retain a rounded globose shape. In still larger tests, the ornament becomes more prominent and tests develop a squatter shape with flatter aboral surfaces. The transformation generally ends with large, thick-walled, highly ornate forms with squat globose tests, robust ornaments, and relatively flat aboral surfaces. Although the size of the test increases notably through the transformation, there appears to be no corresponding change in the internal chamber size from its initial value, and so the transformation is largely due to the addition of calcite to the outer surface of the test wall. This appears to be an inherent biological feature and does not involve calcite dissolution/reprecipitation or other retrograde processes. The detail of the transformation varies between species and is presented here for Bolboforma subfragoris s.l., B. gruetzmacheri n. sp., and B. metzmacheri s.s., which exhibit different types of ornament — spinose, irregularly ridged, and cancellate, respectively. When different morphological forms of a single species occur in the same sample, they have the potential to be interpreted as different species. Taxonomic confusion is further compounded by the morphological similarity of many small, smooth-walled, and weakly ornamented forms of different species. These features are common in bolboform populations and have contributed to the misinterpretation of some species. Differences in test shape and ornament are most notable in large ornate forms and taxonomic interpretations should be based wherever possible on such forms. Once the taxonomy is clear, the biostratigraphic utility of the late Miocene bolboforms from Site 593 has been in the recognition of single species (or at most two) in a given sample. This provides an excellent biostratigraphic framework for a period of time that lacks significant alternative microfossils for age control in New Zealand sediments. The distribution of bolboforms in (mainly) southern hemisphere marine sediments appears to track the palaeocirculation of Southern Component Intermediate Water (SCIW). Hence addition of calcite to the outer surface of the test wall probably occurred while the bolboforms were entrained in subsurface flows of SCIW. In this respect, the increased complexity of ornamentation may have been a morphological adaptation to counter the loss of buoyancy as calcite was added to the outer surface of the test wall. This would have presumably contributed to the entrainment and dispersal of bolboforms in the flow of subsurface water masses, and it suggests that the tests of some bolboforms at least may represent a dispersal phase in the life cycle of this enigmatic group of microfossils.