Morphological diversity and disparity in trilobite cephala and the evolution of trilobite enrolment throughout the Palaeozoic

Morphology permits the extracting of information to study patterns of disparity and diversity of a particular group of animals through time. Enrolment is a characteristic behaviour of trilobites, having been first recorded in the Cambrian and continued until their demise at the end of Permian mass extinction. Morphological trends related to enrolment strategies have been documented previously but are yet to have been analysed in a morphospace framework. Here we analyse the cephalic shape of a wide range of trilobite taxa using geometric morphometrics. These methodologies were implemented to explore whether the relationship between the shape of the head and the evolution of different enrolment styles correspond with an increase in morphospace or disparity during the Palaeozoic. Our results show distinct patterns of diversity and disparity in the functional morphology of trilobites during the Palaeozoic. Furthermore, these analyses show that enrolment types in trilobites without interlocking devices (e.g. cylindrical) permit more flexibility in the head shape, promoting higher rates of disparity, possibly leading to an enhanced ability to occupy a broad range of ecological niches. On the contrary, lower disparity cephalic shape constrains the way to enclose the body and consequently places limitations on the ability to adapt to new niches. Our results indicate that the higher rates of morpho-functional diversity and disparity coincide with the increase of morphospace in radiations in the early Palaeozoic (Cambrian and Ordovician) but also immediately after the Carboniferous-Permian biodiversification event. Low disparity rates constrain the success of trilobites with a specific enrolment type. Finally, the results show that sphaeroidal enrolment style was the most successful type that evolved independently in all trilobite orders.