291-6 Comparative Visualization of Ammonite Ornament in 3D Morphospace

Session: Cephalopods Through Time: Insights into Evolution, Ecology, and Environmental Reconstruction

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Abstract:

Over the course of their ~344 million years on earth, ammonoids exhibited a vast array of shell forms reflecting environmental, functional, and evolutionary controls on morphological diversity. Previous morphological research has an extensive history which has primarily focused on the quantification of whorl shape and whorl expansion rate. External ornament has been categorized quantitatively, but not much has been done in terms of quantitative assessment. Shell ornament such as ribbing, bullae, tubercles, and clavi, are qualitative categorizations that have been used to study ammonoid taxonomy and paleobiology. Semi-quantitative scoring methods have been used to interpret such forms of ornament as the result of sexual selection, a response to predation, and an improvement of hydrodynamic efficiency. Miao et. al (2024) developed a quantitative metric for 2D and 3D data using indices which permit the investigation of the relationship between ornamental complexity and longevity in planispiral ammonoids. The indices capture general relief, but do not consider trends in the dimensions of specific forms of ornament.

We use 3D geometric morphometrics to quantify all aspects of ornament in both planispiral and heteromorph ammonites. Over 200 ammonite specimens with well preserved shell ornament were 3D scanned and assigned uniform landmark placement using a 3 x 3 grid of landmarks with semilandmarks in between from the umbilical shoulder to the middle venter in Stratovan Checkpoint©. Two preliminary morphospaces were created to visualize different forms of ornament. The first uses principal component analysis to plot distance from “normal” (in mm) on the X-axis, and types of ornament (bullae, tubercles, clavi, etc.) on the Y-axis. The second focuses on ribbing and plots parameters like rib form (plicate, falcate, bifurcated, etc.), rib density (spacing), rib dimensions (height and width) (both in mm) on the XYZ axes of a ternary diagram. Some potential applications of these methods may eventually be used to examine sexual dimorphism, ontogeny, intraspecific variability, and the role of environmental stress in the construction of ammonite shells. 

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9659

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