The rise of a novel, plankton-based marine ecosystem during the Mesozoic: a bottom-up model to explain new higher-tier invertebrate morphotypes

Major radiation events amongst a range of phytoplanktonic and zooplanktonic microbiota such as calcareous nannofossils, calpionellids, diatoms, dinoflagellates, planktonic foraminifera and radiolarians are characteristic of the time interval between the Late Jurassic (c. 160 Ma) and Late Cretaceous (c. 100 -66 Ma). Both directly and indirectly, these radiations in the marine water column led to a proliferation of various benthic groups such as burying and swimming crabs and irregular echinoids as well as nektonic groups such as ancyloceratine heteromorph ammonites. For each of the invertebrate groups studied we have plotted all available data on their diversity through time. The resultant histograms form the basis of our new model of the 'infill' of Mesozoic ecosystems. The impact on detritus and plankton feeders was direct, from the Late Jurassic onwards, in that an increased supply of planktonic food particles became available. However, the radiation of burying raninoid crabs, which comprised more complex scavengers/detritivores, was intimately linked to the bloom of (part of) their food source (i.e., marine meiofauna). During the Cretaceous Period new innovations and adaptations brought about additional faunal turnovers. For instance, amongst frog crabs (Raninoidia), there was a transition from the palaeocorystid type, via the lyreidid type to the raninid morphology. Similarly, coeval turnovers are documented for irregular echinoids (i.e., from Toxasteridae via Micrasteridae to Schizasteridae) and for heteromorph ammonites: from Ancyloceratoidea via Turrilitoidea to Scaphitoidea. Here we present, for the first time, an elaborated bottom-up model to explain the emergence and dispersion in time of different invertebrate groups at a range of higher-tier trophic levels.