Modelling enrolment in Cambrian trilobites

Trilobites were capable of enroling in different ways based on the flexible articulation of thoracic segments and associated interlocking devices; the type of enrolment (spiral or sphaeroidal) is thought to have largely depended on the coaptative devices that each trilobite used to enclose the body. Based on X-ray microtomography scans of complete enrolled specimens from the Cambrian, we created three-dimensional (3D) computer models to assess the kinematics needed to achieve both enrolment types. We demonstrate that closely related trilobites with little morphological variation (Bailiaspis?, Conocoryphe and Parabailiella) developed different enrolment types as a result of small variations in the number of thoracic segments and the angle between adjacent segments. Moreover, our models indicate that sphaeroidal enrolment, which is associated with a smaller number of thoracic segments, enabled faster encapsulation. This supports the hypothesis that there was a trend in the evolution of tr...

三叶虫可通过胸节的柔性关节结构与配套联锁装置实现多种卷背姿态；其卷背类型（螺旋形或球形）被认为主要取决于各三叶虫用于闭合躯体的适配咬合装置。基于寒武纪完整卷背标本的X射线显微断层扫描（X-ray microtomography）数据，我们构建了三维（3D）计算机模型，以评估实现这两类卷背姿态所需的运动学机制。研究表明，形态差异极小的近缘三叶虫类群（疑似Bailiaspis、Conocoryphe及Parabailiella），其卷背类型的差异源于胸节数量及相邻胸节夹角的细微变化。此外，模型结果显示，与较少胸节数量相关的球形卷背姿态，可实现更快的躯体包裹过程。这一发现支持了三叶虫演化存在某一趋势的假说。