Morphological and taxonomic diversity each provide insight into the expansion and contraction of major biological groups, while the nature of the relationship between these two aspects of diversity also has important implications for evolutionary mechanisms. In this paper, I compare morphological and taxonomic diversity within the classes Blastoidea and Trilobita, and within the trilobite clades Libristoma, Asaphina, Proetida, Phacopida, and Scutelluina. Blastoid morphology is quantified with homologous landmarks on the theca, and trilobite form is measured with a Fourier description of the cranidium. Morphological diversity is measured as the total variance among forms in morphological space (proportional to the mean squared distance among forms). Blastoid taxonomic diversity is based on published compilation of stratigraphic ranges of genera. The Zoological Record was used to determine the number of new species of trilobites described since the publication of the Treatise; temporal patterns in species richness are similar to those for generic richness based on the Treatise, suggesting a common underlying signal.
Morphological variety and taxonomic richness often increase together during the initial diversification of a clade. This pattern is consistent with diffusion through morphospace, although some form of adaptive radiation cannot be ruled out. Morphological diversity varies little throughout much of the history of Proetida, a pattern that may suggest major constraints on the magnitude and direction of evolution, and that agrees with the perception of Proetida as a morphologically conservative group. Two major patterns are seen during the decline of clades. In Blastoidea, Trilobita, Libristoma, and Asaphina, morphological diversity is maintained at substantial levels, and in fact continues to increase, even in the face of striking reductions in taxonomic richness. This pattern suggests continued diffusion through morphospace and taxonomic attrition that is effectively non-selective with respect to morphology. In Phacopida, Scutelluina, and to some extent in Proetida, morphological diversity decreases along with taxonomic diversity. This pattern suggests heterogeneities such as elevated extinction and/or reduced origination in certain regions of morphospace. As found previously for the echinoderm subphylum Blastozoa, all studied clades of trilobites except Proetida show maximal morphological diversity in the Mid–Late Ordovician and maximal taxonomic diversity sometime during the Ordovician, suggesting some degree of common control on diversification patterns in these groups.