Systematics and evolution of the brachiopods Pachymagas and Waiparia in New Zealand
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
New Zealand brachiopod species previously referred to Pachymagas and the associated genus Waiparia are systematically revised, the profusion of questionably substantiated species names and the inappropriate and untenable generic equivalence given (hence "Pachymagas" of the title) with younger, morphologically dissimilar South American species providing the rationale and impetus for the revision. Through excavation to expose cardinal characters it was found that a relatively consistent morphology was retained throughout the "Pachymagas"-Waiparia lineage, the progressive thickening of the posterior of the shell interpreted to reflect the freelying habit adopted, the thickening helping to weight the shell and hold it in a stable position on the substrate. Further, it was determined through examination of excavated interiors that at least two distinct genera have been placed In "Pachymagas"; a third distinct genus may also have been incorporated, but this possibility requires further study before any confident conclusions can be made. Of the twenty-four species assigned to "Pachymagas" by previous authors, eight are recognised, one tentatively, with one new species erected. The extensive synonymisation required results from the typological species model employed in the creation of species by the two primary authors for the genus, J. A. Thomson and R. S. Allan. Both authors recognised many distinct species based on minor differences in exterior shape, but here it is considered that these species are rather phenotypic variates of a phenotypically plastic genus. Phenotypic variation is a far simpler explanation for the minor differences in shell shape that are observed within and between localities, and it affords well with the observation that interior characters are highly variable, the phenotypic variation being global in its expression. The establishment of evolutionary relationships are compounded by the phenotypically plastic nature of "Pachymagas", the occurrence of ecophenotypes further blurring genotypic identity. However, it was determined that the evolutionary sequence can be considered a peramorphocline - a "sequence of increasingly more peramorphic species".