GEOMETRIC MORPHOMETRIC EXAMINATION OF SHAPE CHANGE THROUGH ONTOGENY AND PHYLOGENY OF CENOZOIC NEW ZEALAND TURRITELLID GASTROPODS FAMILY TURRITELLIDAE

The turritellid gastropods (family Turritellidae) of New Zealand are an ideal group for the study of shape change through both ontogeny and phylogeny.

Due to the non-overlapping spiral growth of an individual shell, the entire life span of each individual, from the larval shell through metamorphosis to the time of death, is visible. This allows extremely detailed description of shape change through an individual's life time and therefore also allows the accurate description of the ontogenetic shape change of each species.

Since Marwick first introduced them in 1971, “Marwick diagrams” (ontogenetic diagrams showing estimations of major ornament development points and whorl sizes through ontogeny) have been used as a visual descriptive tool in discussing the development of shell sculpture through the ontogeny of turritellid species. Previous versions, however, have been drawn by hand and therefore subjective and potentially omit information. Image analysis software now makes possible automated and therefore more accurate versions of these diagrams. Geometric Morphometric methods, which have been usefully employed in the study of organismal shape change in a range of groups, are applied to these new diagrams to analyze the shape change of several features through individual turritellid species' shell ontogeny. Using outline analysis allows comparison between points where homologous shell ornamentation isn't necessarily present, making it especially useful for examining developmental changes. The ontogenies of different species can then be compared.

In addition, the New Zealand turritellids have both modern representatives and an extremely detailed, high resolution fossil record, which appears to show both anagenetic and punctuated phases of evolution. The ability to incorporate information from the entire ontogenetic sequences into comparisons between species across the phylogeny, especially where linked to age and growth rate information, will allow extremely accurate description and quantification of modes of evolution occurring through the group's history.