Abstract Scaphitid ammonoids were ubiquitous and significant components of the Western Interior Seaway during the Late Cretaceous. This group is characterized by a recurved hook at maturity that deviates from the juvenile whorls. Such a modification seems counterproductive to active locomotion and to manage a biologically effective orientation that facilitates efficient feeding and swimming. Virtually reconstructed 3D hydrostatic models reveal that the examined mature scaphitids had the capacity for neutral buoyancy while assuming a stable, upward-facing orientation in the water column during life. Models of juvenile Hoploscaphites nicolletii suggest that scaphitid apertures were oriented only slightly more horizontal than adults. The hydrostatic influence of sexual dimorphism was explored with the species Hoploscaphites crassus. The inflated macroconch has a lower stability and higher hydrodynamic drag compared to its microconch counterpart. The effect of shell compression was investigated by comparing H. crassus and the more compressed H. nicolletii. The latter species has a relatively high stability and much less hydrodynamic drag during movement. The mature U-shaped body chamber distributes organismal mass in a way that increases stability, and simultaneously orients the soft body so that propulsive energy is efficiently transmitted into horizontal backwards movement with minimal rocking. Swimming velocities computed from hydrodynamic drag experiments suggest that scaphitids were relatively slow swimmers with compressed forms attaining slightly higher velocities (when scaled by mass). Hydrodynamic lift was investigated with computational fluid dynamics simulations. These experiments revealed that the overall shape of the shell is responsible for significant lift in the upwards direction, which is not heavily influenced by ornamentation. This explains how a reduced soft body can overcome and manage a slightly negatively buoyant condition during life. Therefore, the seemingly cumbersome shape and orientation of the scaphitid morphotype may not have been a hindrance during locomotion.

中文翻译：

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来自美国西部内陆的 scaphitid ammonoids 的体内流体静力学和流体动力学特性

摘要 Scaphitid ammonoids 是晚白垩世西部内陆海道中普遍存在的重要组成部分。该组的特征是成熟时背弯钩偏离幼鱼。这种修改似乎对主动运动和管理促进有效进食和游泳的生物有效方向适得其反。虚拟重建的 3D 流体静力模型显示，所检查的成熟舟状体具有中性浮力的能力，同时假设生命期间在水体中保持稳定、朝上的方向。幼年 Hoploscaphites nicolletii 的模型表明，scaphitid 孔的方向仅比成年人略多水平。用 Hoploscaphites crassus 物种探索了性二态性的流体静力学影响。与微型海螺相比，膨胀的大海螺具有较低的稳定性和较高的水动力阻力。通过比较 H. crassus 和更压缩的 H. nicolletii 来研究壳压缩的影响。后者在运动过程中具有相对较高的稳定性和更小的流体动力阻力。成熟的 U 形体腔以增加稳定性的方式分配机体质量，同时定向软体，以便将推进能量有效地传递到水平向后运动中，同时将摇摆降至最低。从流体动力学阻力实验计算出的游泳速度表明，舟形类动物是相对缓慢的游泳者，具有压缩形式，获得稍高的速度（按质量缩放时）。通过计算流体动力学模拟研究了流体动力升力。这些实验表明，贝壳的整体形状是向上方向显着提升的原因，而这不受装饰的严重影响。这解释了减少的软体如何在生活中克服和管理轻微的负浮力状况。因此，舟骨形态型看似笨重的形状和方向可能不是运动过程中的障碍。