Cambrian explosion and Ordovician biodiversification or Cambrian biodiversification and Ordovician explosion?

In this study, we examine how metazoan biodiversity has accumulated from the late Precambrian until the Silurian at various scales of taxonomic organization using compilations of the First Appearance Data (FAD) of global marine Metazoa from the datasets available in the Paleobiology Database (PBDB) and primary literature. The results indicate that all animal phyla appear during the late Precambrian and the earlier parts of the Cambrian, which corresponds to the usual concept of the Cambrian Explosion. However, at lower taxonomic ranks, a significant increase of first appearances is observed during the Ordovician, corresponding to an Ordovician Explosion of animal orders, families and genera. The cumulative counts of metazoan FADs at these lower taxonomic ranks reveal a gradual and long-term increase of diversity, reflecting a single large-scale radiation that started in the late Precambrian and lasted at least until the Silurian. This scenario corroborates recent studies that point towards a single long-term radiation during the early Palaeozoic, without clearly distinguishable global diversity explosions during discrete intervals.