ONTOGENY AND CONSTRAINTS IN THE EVOLUTION OF AMMONOID SHELL COILING

The coiled shell of ammonoid cephalopods has a first-order influence on hydrodynamic interactions with the environment, with implications for the biology and ecology of the organism. Over their 360 million-year history, ammonoids evolved a wide range of coiling morphologies; however, the variety of planispiral forms was greatest in the Paleozoic and greatly reduced in the Jurassic and Cretaceous. This reduction in disparity occurred because depressed forms with large whorl width rarely evolved in the Jurassic or Cretaceous. However, why those forms rarely evolved is more puzzling, and the roles of functional, constructional, and developmental constraints are unclear. To investigate the contribution of developmental constraints, I examined ontogenetic shape trends from digitized ammonoid whorl sections. Disparity is lower among Jurassic-Cretaceous ammonites at all size classes, from post-hatchlings at diameters of only a few millimeters to adult sizes, despite pronounced shifts in ecology and function during growth. Planispiral ammonoids exhibited strong preferential shape change during ontogeny, almost always towards more compressed shells with slightly greater whorl overlap. As a result, adult shell shape is strongly constrained by the range of available juvenile forms. Jurassic-Cretaceous ammonites lacked juvenile shells with large whorl widths, and the rare depressed adult forms evolved through an atypical ontogenetic pathway. This suggests that developmental constraints at the earliest ontogenetic stages played an important role in disparity of shell coiling in planispiral ammonoids.