Pulsed cephalopod diversification during the Ordovician

Abstract

During the Ordovician Radiation cephalopods expanded their habitat from the shallow marine environments of the low latitudes to global occurrences in almost all marine environments. The expansion of habitat was accompanied by a drastic morphological and taxonomical diversification. The diversification was not a constant linear process, but involved intervals of diversity crises and fundamental faunal change at the global scale. A new database, comprising more than 5100 cephalopod occurrences with high stratigraphical resolution allowed for a subsampled and normalised calculation of mean standard diversities at the scale of the IGCP 503 time slices. The resulting curves reveal a pulsed diversification of Ordovician cephalopods with three main diversification phases which were in each case preceded by a significant faunal change and a diversity crisis. Each diversification pulse led to higher diversity levels reaching a Katian diversity climax. The cephalopod turnover rates decreased during the Ordovician. But only a weak diversity dependence of turnover rates and diversity can be detected. The diversification events are characterised by (1) a poor correlation with major physical events in the Ordovician world, but good agreement with important events of the diversity evolution of trilobites, brachiopods and reefbuilders, and by (2) a statistically significant general trend of decreasing origination rates. A major cephalopod faunal change at the Lower/Middle Ordovician boundary can be directly compared with the contemporaneous faunal changes in brachiopods and trilobites. We interpret the canonical, pulsed diversification of Ordovician cephalopods as a result of ecosystem-dependent processes. Potentially an increasing ecosystem complexity, an increasing stability of the trophic structure and an increasing ecosystem resilience led to the expansion of cephalopod habitats and the multiplication of niches.