Diversification and disparity in a major Palaeozoic clade of Brachiopoda: the rise and fall of the Plectambonitoidea

The Plectambonitoidea Jones, 1928 is an abundant and diverse Palaeozoic clade of the phylum Brachiopoda. The shells were generally small and pursued a recumbent mode of life resting on a convex ventral valve or possibly on a concave dorsal valve. A phylogeny is presented for the c. 125 genera comprising the superfamily, based on 50 characters. The changing disparity through time has been evaluated from the morphological matrix based on the sum of variances and the diversity trajectory of the clade mapped using corrected range data from the Paleobiology Database. The group expanded its reach during the Tremadocian, accelerating in numbers during the Floian and Dapingian, and peaking in the Darriwilian. Originations occurred during a window within the Tremadocian-Floian ages. The group suffered a major extinction at the end of the Ordovician and in the late Silurian, continuing as a dead clade walking until the Early Carboniferous; extinctions were most marked during the Katian-Aeronian ages. On the other hand the expansion of disparity predated the hikes in diversity, accelerating during the early part of the Tremadocian. In common with other major groups, experimentation and innovation amongst new body plans exploded prior to bursts of diversity at lower taxonomic levels.