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Trilobites were a successful group of marine arthropods that thrived for much of the Palaeozoic Era. Akey innovation of trilobites involved the capacity to enrol their biomineralized exoskeletons for protection; a trait that appeared early in their evolutionary history during the Cambrian Period. However, even though there has been much interest in the description of enrolment-related anatomical adaptations, and different styles of enrolment, the kinematic aspect has just begun to be explored—not just as means of understanding trilobite movements, but also as a potential driver of evolution. In this study, with the use of the open-source software Blender, four models of three different species of Ordovician trilobites(two phacopines and two illaenines) were constructed and later submitted to enrolling simulations. Different angle metrics were measured and then compared between simulations to analyse the performance of the developed methodology, and between the different models to reveal potential evolutionary or developmental trends. Results suggest a clear relationship between the kind of higher-level taxonomic groups analysed and the angle arrangement of the trunk elements in an enrolled position. In contrast, the same was not as evident when comparing the measured angle metrics against the total number of segments. Illaenina models consistently resulted in lower angles between segments required to enrol, suggesting a more efficient enclosure of the body favoured by bigger and more spherical proportions of cephalon and pygidium. This study contributes to a growing field in palaeontology and provides new, detailed enrolment data for several trilobite species. In future, such data should be synthesized with those from previous and future studies to analyse the evolutionary and developmental history of trilobites from this novel perspective.

Trilobites were a successful group of marine arthropods that thrived for much of the Palaeozoic Era. Akey innovation of trilobites involved the capacity to enrol their biomineralized exoskeletons for protection; a trait that appeared early in their evolutionary history during the Cambrian Period. However, even though there has been much interest in the description of enrolment-related anatomical adaptations, and different styles of enrolment, the kinematic aspect has just begun to be explored—not just as means of understanding trilobite movements, but also as a potential driver of evolution. In this study, with the use of the open-source software Blender, four models of three different species of Ordovician trilobites(two phacopines and two illaenines) were constructed and later submitted to enrolling simulations. Different angle metrics were measured and then compared between simulations to analyse the performance of the developed methodology, and between the different models to reveal potential evolutionary or developmental trends. Results suggest a clear relationship between the kind of higher-level taxonomic groups analysed and the angle arrangement of the trunk elements in an enrolled position. In contrast, the same was not as evident when comparing the measured angle metrics against the total number of segments. Illaenina models consistently resulted in lower angles between segments required to enrol, suggesting a more efficient enclosure of the body favoured by bigger and more spherical proportions of cephalon and pygidium. This study contributes to a growing field in palaeontology and provides new, detailed enrolment data for several trilobite species. In future, such data should be synthesized with those from previous and future studies to analyse the evolutionary and developmental history of trilobites from this novel perspective.