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

鹦鹉螺类和鹦鹉螺类头足类是具有广泛而多样的化石记录的外壳软体动物。外壳被细分成若干腔室；分隔这些腔室的隔板（隔层）显示出一种显著的进化模式，其复杂性日益增加。这些隔与壳的侧壁相交形成一个复杂的叶状缝合线。反过来，这些缝合线长期以来一直被认为是化石记录中最复杂、进化最快的结构之一。但它们的功能（“氨类缝合问题”）仍然是古生物学中最古老的未解决问题之一。普遍的解释是，氨气隔膜及其缝合线是防止内爆的支撑，这一解释现在基本上没有受到质疑。然而，在这一建议中，我们提供了有力挑战这一观点的证据。这一证据来自多种方法的结合，包括对暴露在压力载荷下的鼻中隔模型的应力分布进行有限元分析，来自化石记录的数据与支撑的观点不一致，以及一项新的建模工作，表明鼻中隔表面的细化可能与腔室的液体运输有关。由于这种相机液体运输既与动物的生长速度有关，也与短期浮力调节有关，因此我们认为这些复杂性状的进化中有一个选择过程与相机液体运输的物理和生理有关。所提出的研究围绕两个中心假设：(1)隔膜表面的加工减少了壳体腔室部分（隔膜）的应力；(2)隔膜表面的加工增加了壳体液体运输速率。这些假设结合了计算和数学方法以及物理建模和形态测量方法。有限的一组分支，有大量保存完好的化石记录。这些化石将为我们检验上述假设提供形态计量学数据。然后用一种新的数学方法从数字化数据中量化间隔和缝合线的形状。这种方法也将用于规定应力和照相液体输运分析的形状。目前开发了应力分布的有限元分析，并将这种方法应用于真实和假设的菊石，以检查应力和表面精加工之间的关系。理论模型将描述相机液体传输和表面细化之间的关系，我们将开发模型来测试我们的传输想法。与传统观点形成鲜明对比的是，我们的初步工作表明，表面加工导致对phragmocone部分的应力增加。此外，我们对相机液体输送的初步建模工作表明，间隔表面的细化表现为更大的腔室液体通量率。因此，这项工作的一个令人兴奋的结果是，我们认为这种表面加工涉及两个相互冲突的选择过程：在相当大的机械风险下增加相机液体运输。这种权衡的后果，更不用说其存在的可能性，以前从未被提出过。

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.