The Great Ordovician Biodiversification Event (GOBE) represents a diversification at lower taxonomic levels of most body plans that had appeared during the Cambrian explosion. Among trilobites, several novelties related to enrolment evolved during the GOBE. A kinematic analysis by means of 3D modelling of some new enrolment types shows no relationship with those of the Cambrian trilobites. While some structures emerged for the first time in Ordovician trilobites (e.g. articulations and panderian organs), other structures (e.g. anterior and posterior arch in head and tail) appear earlier in Cambrian trilobites. Our results suggest that the evolution of some groups was clearly rooted in the Cambrian explosion while others clearly appeared during the GOBE.

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 trilobites towards reduction in the number of thoracic segments in phylogenetically derived taxa in order to enhance the efficiency of enrolment.