The Roles of Cameral Liquid Transport and Shell Strength in Ammonoid Suture Complexity

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Ward 9531892 Ammonoid and nautiloid cephalopods are externally shelled molluscs with an extensive and diverse fossil record. The shell is subdivided into a number of chambers; the partitions dividing these chambers (the septa) have shown a remarkable evolutionary pattern of increasing complexity. The intersection of these septa with the lateral walls of the shell form a complex and foliate suture. These sutures, in turn, have long been recognized as one of the most complex and rapidly evolving structures in the fossil record. But their function ("the ammonoid suture problem") remains one of the oldest unresolved questions in paleontology. The prevailing interpretation, the ammonoid septa and their sutures were buttresses to prevent implosion, is now largely unchallenged. In this proposal, however, we provide evidence that strongly challenges this view. This evidence follows from a combination of approaches including finite element analysis of stress distributions in models of septa exposed to pressure loads, data from the fossil record that are inconsistent with the idea of buttressing, and a new modeling effort that suggest septal surface elaboration may be related to the transport of liquid from the chambers. Since this cameral liquid transport is related to both the growth rate of the animals and to short term buoyancy regulation, we suggest that one selective process in the evolution of these complex characters is related to the physics and physiology of cameral liquid transport. The proposed research revolves around two central hypotheses: (1) that septal surface elaboration reduces the stress in the chambered portion of the shell (the phragmocone) and (2) septal surface elaboration augments cameral liquid transport rate. These hypotheses with a combination of computational and mathematical approaches as well as physical modeling and morphometric approaches. Limited set of clades for which there is an ample fossil record that contains well preserved specimens. The fossils will provide morphometric data with which we will examine the above hypotheses. Then use a new mathematical method to quantify the shapes of septa and sutures from digitized data. This method will also used to prescribe shapes for both stress and cameral liquid transport analyses. Currently developed finite element analyses for stress distributions and will apply this method to both real and hypothetical ammonoids to examine the relationship between stress and surface elaboration. Theoretical models that will describe relationships between cameral liquid transport and surface elaboration, we will develop models to test our transport ideas. In sharp contrast to conventional views, our preliminary work suggests that surface elaboration leads to augmented stresses on portions of the phragmocone. Moreover, our initial modeling efforts on cameral liquid transport indicate that septal surface elaboration is manifested as greater rates of chamber liquid flux. Thus, an exciting consequence of this work is that we suggest that this surface elaboration involves two conflicting selective processes: increased cameral liquid transport at a considerable mechanical risk. The consequences of this trade-off, let alone its possible existence, have not been previously proposed.