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

Abstract

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. □ Arthropods, ecology, ecomorphology, evolution, functional morphology.

Morphological disparity trends in Devonian trilobites from North Africa

“…In addition to diet and habitat, trilobite morphologies are known to be influenced by their lifestyle, such as living in low‐oxygen habitats (Fortey 2004) or the enrolment type (Suárez & Esteve 2021). In the same way, the competition between species is also a driver of diversification recognized in arthropods (Eberle et al .…”

“…Similarly, the predator/scavenger trilobites showed an increasing morphospace occupation possibly linked to a food partitioning and to an interspecific competition (Schluter 2000; Grant & Grant 2006). The increase of enrolment type in the Devonian was also a source of morphological disparity (Suárez & Esteve 2021). More generally, the diversification phases could be partly attributed to interspecies competition in arthropods (Eberle et al .…”

Long-term clade-wide shifts in trilobite segment number and allocation during the Palaeozoic

“…Several enrolment types have been observed across trilobites. The greatest variety of types occurred in the Cambrian, and there was a decrease in the number of enrolment strategies as well as an increase in the dominance of some strategies over the Palaeozoic [ 38 , 39 ]. In addition to local coadaptive features on sclerite margins, shifts in enrolment type may have been facilitated by changes in segment number and allocation in the body, which would have impacted the flexibility of the trunk region as well as contributed to ‘caudalization’ [ 40 , 41 ].…”

“…In addition to local coadaptive features on sclerite margins, shifts in enrolment type may have been facilitated by changes in segment number and allocation in the body, which would have impacted the flexibility of the trunk region as well as contributed to ‘caudalization’ [ 40 , 41 ]. We used the dataset of Suárez & Esteve [ 39 ] to investigate changes in enrolment strategies as a possible driver for the contraction in thoracic segment range and increased proportion of the trunk segments being allocated to the pygidium. Across the Palaeozoic, there is no significant difference in the median number of segments in the thorax with the exception of the discoidal strategy, which had a significantly lower median number of tergites compared to species using other enrolment strategies ( figure 4 ; electronic supplementary material, table S14).…”

“…These results suggest that change in the prevalence of certain enrolment strategies does not provide strong explanatory power for associated shifts in segment number and allocation.

Figure 4 Number of segments in the thorax for trilobites with different enrolment strategies [ 39 ]. Short orange line = median; long blue line = mean; numbers adjacent to distributions represent sample sizes.

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