Contrasting patterns of disparity suggest differing constraints on the evolution of trilobite cephalic structures during the Cambrian ‘explosion’

Trilobites are an abundant group of Palaeozoic euarthropods that appear abruptly in the fossil record c. 521 million years ago. Quantifying the development of morphological variation (or ‘disparity’) through time in fossil groups like trilobites is critical in understanding evolutionary radiations such as the Cambrian ‘explosion’. Here, I use geometric morphometrics to quantify ‘cumulative disparity’ in functionally-important structures within the trilobite cephalon across their initial radiation during Cambrian Series 2. Overall cephalic disparity increased rapidly and attained a maximum within several million years. This pattern is dominated by the cephalic outline (in particular the genal spines), reflecting rapid, convergent expansion to the extremes of morphospace in a few early families. In contrast, removing the outline and focusing on structures such as the glabella and eye ridges (associated with feeding and vision, respectively) showed a more gradual increase in disparity, closer in line with taxonomic diversity and supporting the hypothesis of a relatively accurate trilobite fossil record. These contrasting patterns suggest that disparity in different structures was constrained in different ways, with extrinsic (ecological) factors likely having the biggest impact on overall disparity. It also implies that patterns of disparity in isolated substructures cannot necessarily be taken individually as representative of overall morphologies.

Methods
Methods are described in the main text of the article and in the 'Detailed Methods' section of Appendix S1.