WESTERMANN MORPHOSPACE: AMMONOID SHELLS, LOCOMOTION, AND METABOLISM

Westermann Morphospace (Paleobiology 38:3) is a simple plotting tool to compare ammonoid shells to hydrodynamic and life mode interpretations. It suggests paleoecological interpretations, even with collections of limited quantity or taphonomic quality, such as are common in field samples of mixed faunas. Here we present evidence to support physiological interpretations based on comparison to modern cephalopods, as well as updates explaining the plotting mechanism.

The method compares three parameters of shell shape: umbilical exposure, whorl expansion, and overall inflation. Comparing scaled values in a ternary diagram effectively projects shell shape data from three-dimensional space onto a fixed frame. The newly published scaling values are based on analysis of data from Raup (1967) and Treatise volume L (1957, 2009). The diagram clearly distinguishes serpenticones, oxycones, and sphaerocones. Platycones and planorbicones can overlap in the ternary diagram, but are easily distinguished in 3D space.

Life mode interpretation fields are initially laid over the morphospace based directly on conclusions by Westermann (1996), and the dashed lines separating them should be refined. There is strong support, however, for Westermann’s interpretations: specimens’ position in the ternary space is significantly correlated with hydrodynamic properties (drag, efficiency). Rather than distinct categories, the morphospace could be contoured with gradients of hydrodynamic efficiency.

Comparison to emerging biological data for ammonoids’ closest living relatives, the coleoid cephalopods, further improves the life mode interpretations. To calculate potential swimming speeds, Jacobs (1992) used a single value to characterize the power of all ammonoids. In coleoids, however, metabolism varies more widely than other measured pelagic faunas combined. Locomotion style, rather than size or habitat depth, is the chief constraint of coleoid cephalopod metabolism. We suggest ammonoids also employed varied metabolic rates matched to the different locomotory limitations of their shells. Varied metabolic rates may explain differential success during environmental and biological crises in Earth history, such as mass extinctions and climate change.