Morphological innovation did not drive diversification in Mesozoic–Cenozoic brachiopods

Abstract

Over long spans of geological time, various groups of organisms may wax and wane, experiencing times of apparent success and contraction. These rises and falls are often said to reflect either opportunities created by climate change or the relative success of innovative characteristics. Phylum Brachiopoda was one of the most successful marine clades before the Permian/Triassic mass extinction (PTME), but after this event, they became marginal components of marine communities through to the present day. How brachiopod morphological innovations reacted to swiftly declining diversity has long remained poorly understood. Here we analyse morphological evolution over the 300 Myr (Permian–Quaternary) history of the four major Mesozoic–Cenozoic brachiopod orders (Terebratulida, Rhynchonellida, Spiriferinida, Athyridida). Unexpectedly, their disparities reached or exceeded pre-PTME levels, but were decoupled from generic richness, which was generally low. Distribution of taxa in morphospace and shifts in centroid indicate that all four orders exploited new morphospaces when adapting to post-Permian environments. A comparison of morphospace occupation and diversity evolution suggests that the high extinction rate of brachiopods and the limited diversification of new forms may have accounted for the depauperate nature of modern-day brachiopods. Analysing more than 1,000 genera of four major orders of brachiopods spanning the Permian to the Quaternary, the authors find that despite low levels of taxonomic richness after the Permian/Triassic mass extinction, brachiopods continued to innovate morphologically, indicative of high evolutionary adaptability.