DECOUPLED RECOVERY OF ECHINOID DIVERSITY AND DISPARITY AFTER THE END-CRETACEOUS EXTINCTION

Biodiversity recoveries after extinction can vary in timing, in ecological dynamics, in geographic signal, and in the immediate and long-term responses of surviving clades to extinction and its biotic and abiotic consequences. Much of this variation has been documented in terms of taxonomic diversity, and modeled in terms of equilibrium ecology, fixed ecospaces, and the differential filling or refilling of canonical “empty” niches. However, rediversification also unfolds in morphospace, and quantification of morphological disparity may reveal different signatures of recovery as well as suggest new models that incorporate contrasts between diversity and disparity. In this regard, the recovery from the Cretaceous-Tertiary (K-T) mass extinction has been understudied, inviting assessments of how disparity relates to the pace, taxonomic composition, and evolutionary context of rediversification. Sea urchins (class Echinoidea), an important component of marine invertebrate assemblages, suffered 36% genus-level extinction at or near the K-T transition. Here we document and compare the recovery of global disparity and diversity after the K-T in the main clade of deposit-feeding echinoids, the heart urchins.

Morphological disparity after the end-Cretaceous extinction was decoupled from changes in taxonomic diversity differently within the two subclades of the main clade, the Superorder Atelostomata. In one group, disparity remained stable despite long-term diversity increase, yet a peak in the disparity of originations 5-10 million years after extinction reflects unprecedented morphological experimentation and a nonrandom shift in morphospace occupation. In the sister-group, preferential diversity loss left isolated survivors in morphospace, increasing disparity as a by-product. These results show that disparity expresses distinct postextinction signatures of clade success, failure, and evolvability, and can enrich recovery models exclusively focused on taxonomic diversity.