Trilobite Cephalon Morphospace Through Time

Abstract

Variation in the morphology of trilobites has long been used in classification, to infer mode of life, and as an indicator of morphological diversity. While taxonomic diversity is, by definition, related to extinction and radiation events, its relation to morphological diversity is not so clear. Here, we use cephalon shape to reconstruct morphospace occupation for the proetids, phacopids and harpids over the post-Cambrian Palaeozoic. Through Elliptic Fourier analysis of 210 images representing 112 phacopid, 87 proetid, and 11 harpid genera from the published literature, we analyse cephalon morphospace occupation and disparity through time. Each order shows distinct patterns of morphospace occupation and disparity dynamics through time (sum of ranges, sum of variances, average nearest neighbour distances). Dramatic reductions in all measures took place following the Ordovician and Devonian extinction events, and increases took place during Devonian radiation (maxima both in terms of both volume and density). Morphospace occupation through the Devonian shows a restricted morphospace leading up to the Frasnian-Famennian extinction and asymmetric trimming of morphospace is evident in both the Ordovician and Devonian events. Overall, phacopids have the highest morphological diversity, whilst harpids exhibit a more constrained cephalon shape. These results thus provide insights beyond that of previous studies of taxonomic diversity and cranidia disparity and reveal the morphological evolutionary dynamics of trilobites in the Phanerozoic.

Date of Award	31 Dec 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	David Legg (Supervisor), Russell Garwood (Supervisor) & Rob Sansom (Supervisor)