The evolution and initial rise of pelagic caryocaridids in the Ordovician

The ‘Great Ordovician Biodiversification Event’ represents the rapid diversification of marine organisms during the Middle Ordovician. As the initial stage of the ‘Great Ordovician Biodiversification Event’, the ‘Ordovician Plankton Revolution’ played a crucial role in the increase of biodiversity in the Ordovician. The occurrence, phylogeny, diversity and abundance of caryocaridids, an important and representative group of planktonic arthropods in the Ordovician, may provide important information for studying the ‘Ordovician Plankton Revolution’ and the evolution of pelagic food webs. Based on systematic collection of paleontological data of caryocaridids, combined with phylogenetic methods, we determined the taxonomic position of caryocaridids, discussed the evolutionary path of the group, reconstructed the biological migration process of caryocaridids and its paleoecological and paleogeographical setting. Our results show that caryocaridids were a clade of planktonic arthropods, which are commonly preserved in black shales along with graptolites and pelagic trilobites. The caryocaridids evolved from the late Cambrian phyllocarids. They first appeared in the Tremadocian, rapidly diversified in the Floian to Darriwilian, then declined in the Late Ordovician, and went extinct at the end of the Ordovician. From the late Cambrian to the Early Ordovician, a large number of marine organisms switched to the planktonic mode. The emergence, rise and diversification of planktonic caryocaridids provide new evidence for the diversity of marine plankton, the complexity of the Ordovician pelagic food webs, and the occurrence of the ‘Ordovician Plankton Revolution’.