Globorotalia inflatalineage andG. crassaformisfrom Blind River, New Zealand: recognition, relationship, and use in uppermost Miocene-lower Pliocene biostratigraphy

Abstract

The biostratigraphic use of variations in shell form in the Globorotalia inflata lineage and in G. crassaformis from an upper Miocene-Pliocene sequence in central New Zealand is studied. In the G. inflata lineage compressed, angular shells in early populations of G. puncticulata sphericomiozea are succeeded by progressively more globose forms (G. puncticulata puncticulata, G. inflata). The gradual changes in morphology are a problem in biostratigraphic applications. A keel, which is developed at the periphery of angular shells, is absent in more inflated shells and its existence is suggested as a biostratigraphic criterion that is simpler to apply than estimates (qualitative or quantitative) of chamber shape. For biostratigraphy, it is suggested that the appearance of G. puncticulata sphericomiozea occurs when shells without a keel comprise at least 5% of a sample and that the appearance of G. puncticulata puncticulata occurs when a keel is absent from all shells. No simple rule, potentially suitable for biostratigraphy, has been found that will discriminate G. inflata from G. puncticulata puncticulata. A few angular shells with the slit-like aperture characteristic of G. crassaformis occur with advanced samples of G. puncticulata sphericomiozea. Specimens of G. crassaformis with rounded axial profiles appear higher in the sequence, close to the entry of G. puncticulata puncticulata. The taxa intergrade. Two biostratigraphic data are suggested: the appearance of G. puncticulata sphericomiozea and the lowest joint occurrence of G. puncticulata puncticulata and G. crassaformis.