Hydromechanical Constraints on the Ontogeny and Evolution ofScaphitid Ammonites

我们计划在其系统发育框架及其时间、岩相和地理分布的背景下，研究密切相关的舟形菊石物种群的流体力学特性。我们选择了一个特有的单系群，大约有25个物种，范围从北美西部内陆的Turonian到Santonian（上白垩纪）。这些物种在它们的壳中保存了它们生长的记录，并在个体发育过程中表现出深刻的形态变化，最终在成熟时形成一个开卷的体腔。此外，这些物种在成壳形状上表现出广泛的多样性，从具有发达的“舟状”体室的相对压缩的物种到相对凹陷的体室“通常”卷曲的物种。我们将使用一个流量箱和阻力测量装置，研究每个物种在大小和形状上的个体发生变化的流体力学含义。这将使我们能够构建涵盖所有胚胎后生长的物种特定的流体力学个体发育轨迹（HOT's）。我们会透过对虹吸管和隔膜的研究来补充这些资料，以提供有关相对生境深度的资料。我们将把这些HOT与西部内陆航道中有充分记录的各物种的古环境和进化分布进行比较。因此，这一比较将首次阐明，在何种程度和方式上，功能性的流体力学限制对一组密切相关的菊石的演化和地理分布作出了贡献。

We plan to investigate the hydromechanical properties of closely related group of scaphitid ammonite species in the context of their phylogenetic framework and their distribution with respect to time, lithofacies, and geography. We have chosen an endemic, monophyletic group of approximately 25 species ranging from the Turonian to Santonian (Upper Cretaceous) of the Western Interior of North America. These species preserve in their shells a record of their growth and manifest profound morphological changes during ontogeny, eventually developing an uncoiled body chamber at maturity. In addition, these species display a wide diversity in adult shell shape ranging from relatively compressed species with a well-developed "scaphitid" body chamber to relatively depressed species in which the body chamber is "normally" coiled. Using a flow tank and drag measuring apparatus, we will study the hydromechanical implications of ontogenetic changes in size and shape for each species. This will allow us to construct species specific hydromechanical ontogenetic trajectories (HOT's) covering all of postembryonic growth. We will supplement this information with studies of the siphuncle and septa, which may provide information on relative habitat depth. We will compare these HOT's with the paleoenvironmental and evolutionary distribution of the respective species within the well documented context of the Western Interior Seaway. Thus, this comparison will elucidate for the first time the extent and way in which functional hydromechanical constraints have contributed to the evolution and geographic distribution of a closely related group of ammonites.