Interpreting drag consequences of ammonoid shells by comparing studies in Westermann Morphospace

Differently-shaped ammonoid shells present varied hydrodynamic properties, but the relevance of shell shape to ammonoid paleoecology is unclear. To examine trends in ammonoid shell shape that relate to hydrodynamic consequences, we project ammonoid shell data into Westermann Morphospace. Operationally similar to other multivariate methods, Westermann Morphospace features a fixed frame and scaling calibrated around the most common planispiral morphotypes. First, results of hydrodynamic experiments are projected into the space, to test for associations between recognized morphotypes and measures of drag force. Discocone shell shapes produce minimal drag at small sizes and/ or low speeds, while oxycone shells produce minimal drag at higher sizes and/or faster speeds. If hydrodynamic efficiency was a first-order selective pressure on shell shapes produced by ammonoids, an association is expected between larger adult shells and oxyconic geometry. To assess this, published shape data are shown in Westermann Morphospace to examine intervals of evolutionary interest. A rough analysis of Late Triassic ammonoid shell shapes shows the expected association between larger shells and oxyconic geometry, but the association is completely reversed immediately after the end-Triassic mass extinction, and the association does not return among the recovered'' middle Jurassic ammonoids. This suggests that hydrodynamics suited for high-metabolism rapid locomotion were not a first-order influence on shell shape immediately after the extinction, or that other hydrodynamic influences require assessment by different methods. Lastly, shells of a series of endemic chronospecies of Cretaceous Neogastroplites are compared, showing a shift to inclusion of more discoconic shells at smaller sizes, resulting in minimal drag for more of the ontogenetic series as a whole. Together, these results indicate that size must be considered when interpreting the hydrodynamic efficiency of ammonoid shells. Flank shape, ornament and soft tissue behaviors may be as or more important than first-order shell geometry, and quantification of their importance and influence on varied shell geometries will benefit from further experimental results.