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

The Plectambonitoidea Jones form 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 morphological matrix was constructed for the 123 genera comprising the superfamily, based on 43 characters. The changing disparity through time was evaluated from the morphological matrix based on four different metrics and the diversification and diversity dynamics of the clade revealed 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. The group suffered a major extinction at the end of the Ordovician, continuing as a dead clade walking until the Middle Devonian. On the other hand, the expansion of disparity predated the hikes in diversity, accelerating during the early part of the Ordovician. In common with other major groups of brachiopods, experimentation and innovation amongst new body plans expanded prior to bursts of diversity at lower taxonomic levels.