WESTERMANN MORPHOSPACE DISPLAYS AMMONOID SHELL SHAPE AND HYPOTHETICAL PALEOECOLOGY

Westermann Morphospace is a ternary diagram of shape and hypothetical life mode of planispiral ammonoid shells. Each apex of the morphospace maximizes one of three components of variation in planispiral ammonoid shells: exposure of the umbilicus, degree of inflation, and expansion of the whorl. Insofar as different shell shapes facilitated different life modes, the morphospace is a tool for estimating paleoecological occupation represented by a collection of shells. Data presented explore mass extinction recovery, intraspecies variation, and ontogenetic change.

Shell measurements and calculations required for Westermann Morphospace are similar to those of Raup (1967), with the exception that the logarithmic model is removed. Westermann Morphospace communicates shape of the final shell without assertion of a specific growth model. In light of research of the complexity of shell development, and broad variation of shell shape within a species, Westermann Morphospace is useful for investigation of intraspecies shape variation and ontogenetic shape change.

Ammonoid paleobiology includes experimental hydrodynamics, analytical biomechanics, and paleontological studies of facies association and apparent habitat range. Westermann (1996) summarized support for life habit interpretations of commonly-recognized morphotypes, and depicted ammonoid shell shapes as gradations between three end-members. Westermann Morphospace quantifies this interpretive scheme.

For each measured specimen, calculated shape component values are scaled by comparison to Treatise monographs of Dactyloceras, Eurycephalites, and Oxynoticeras. Scaled shape components are then normalized to account for one hundred percent of a specimen’s shape characterization. These percents fix the position of a data point in Westermann Morphospace.

Ecospace occupation is highly relevant to studies of ecology and mass extinctions. Life mode designations taken directly from a diagram by Westermann are associated with areas of Westermann Morphospace. As further research produces quantitative thresholds and limits of morphospace-life mode associations, the quantitative Westermann Morphospace will improve as a paleoecological tool.