The “Mesozoic marine revolution” was introduced by Vermeij in 1977, and refers to the hypothesis that a considerable reorganization in the marine communities during the Mesozoic led to a substantial increase in predation pressure. As a consequence, prey species developed various adaptations such as thicker shells, more effective ornamentation or spines, or the ability to hide faster and deeper. We will look further at this in the coming lecture. The main goal with this simple sketch, is to show that tiering has changed markedly from the Cambrian Fauna to the Palaeozoic and Modern Evolutionary faunas. But what is tiering? Ausich and Bottjer in 1982 defined tiering as the distribution of benthic organisms within the space above and below the seafloor. As the sketch shows, the Cambrian was characterised by a very low level of tiering among infaunal as well as epifaunal organisms. During the interval from the Ordovician period to the Permian, the tiering levels grew markedly. It reached the maximum during the lower and middle part of the Mesozoic, more precisely in the Triassic and Jurassic, as illustrated by giant over 3 m long Triassic crinoids from China. The period from the Cretaceous until present was dominated by deep infaunal tiering, while the epifaunal tiering decreased slightly. We have seen that the marine Modern Evolutionary Fauna was dominated by bivalves, echinoids, gastropods, cephalopods and vertebrates. Most of these were mobile. Infaunal burrowers such as echinoids and bivalves became especially common. The teleosts, which is today's dominant group of bony fishes, evolved and diversified. During the Jurassic and Cretaceous, modern groups of gastropods, bivalves and bryozoans became dominant. With regard to cephalopods, the evolution of ammonoids, particularly the so-called true ammonites allowed expansion of cephalopods into the pelagic realm as predators. The coleoid cephalopods, which also include the extant 8 and 10 armed cephalopods, did also diversify rapidly, and especially the belemnites are commonly found as fossils because of their calcified rostrum also called the guard. During the Mesozoic, many groups of “reptiles” became adapted to life in the seas, including the ichthyosaurs, mosasaurs and plesiosaurs, some of which may have reached a length of 15 m or more. It is probable that also the evolution of the large marine reptiles was related to the increasingly abundant pelagic prey in the Mesozoic marine environments. In the Cretaceous, often large, sessile bivalves named rudists, formed large reefs. The rudists is a group of peculiar bivalves that evolved during the Late Jurassic to Late Cretaceous and lived in warm, shallow oceans in equatorial areas. They became extinct at the Cretaceous/Tertiary boundary. They had two asymmetric valves with one valve attached to the sea floor. Let us take a little look at the microscopic organisms that were part of the marine microplankton. Unicellular plankton including planktic foraminifers, coccolithophorids and dinoflagellates evolved and became abundant in the oceans. Despite that the single coccolithophorids were only about 5–100 micrometers in diameter, they reached enormous quantities in the oceans during the late Cretaceous and Danian in northwest Europe and also other places. Their calcium carbonate exoskeletal scales are named coccoliths, and these are the main constituent of the white chalk that is exposed in the cliffs of for instance Denmark, Germany, England and France. The end of the Cretaceous is marked by the K/T mass extinction, using the abbreviations for Cretaceous and Tertiary. Ammonites, belemnites, rudists, many groups of marine reptiles, and many families of invertebrates and planktic protists, especially coccolithophorids and foraminifers, became extinct. The probably best known group that became extinct at the end of the Mesozoic was the non-avian dinosaurs. We can summarize some of the important differences between the marine Palaeozoic Evolutionary Fauna and the Modern Fauna. The Modern Evolutionary Fauna is characterised by: 1) A clearly higher diversity of marine organisms, including both protists, invertebrates and vertebrates. 2) More abundant and diverse predators in all environments; and drilling and crushing are important. 3) More abundant, diverse and effective deposit feeders and other burrowing organisms, which means a deeper level of infaunal tiering. 4) Higher diversity of suspension feeders. 5) More abundant and more diverse primary producers (phytoplankton and benthos). Especially diatoms, dinoflagellates and coccolithophorids are important.
