Welcome everybody! My name is Jan Audun Rasmussen and I'm a paleontologist at the Natural History Museum of Denmark. I work mainly on microfossils and cephalopods. Today we will take one step further on the road following the evolution of life on Earth. More precisely, we will dive into the massive evolutionary changes and biotic replacements in the marine life that succeeded the end-Permian mass-extinction 252 million years ago. This is the largest mass extinction known to have occurred on Earth ever. Many new creatures and ecological strategies showed up, and innovative predator-prey interactions accelerated the evolution. The Modern Evolutionary Fauna did evolve! You have already heard that Jack Sepkoski interpreted the marine life in the sea during the Phanerozoic as consisting of three marine, so-called evolutionary faunas: the Cambrian, the Palaeozoic and the Modern Evolutionary Fauna. This subdivision of the Phanerozoic, marine, metazoan faunas, is widely used today, although Alroy in a series of papers published since 2004, has argued that the composition of the three faunas should be partly revised and reinterpreted. The marine Modern Evolutionary Fauna evolved rapidly after the significant mass-extinction at the Permian-Triassic boundary 252 million years ago, and it was by far the dominant evolutionary fauna through the Mesozoic and Cenozoic, where the number of marine invertebrate families almost constantly increased. The Permian-Triassic boundary marks both the boundary between dominance of the Palaeozoic and Modern faunas, and the decline of the Cambrian fauna. Because the most significant changes in the marine Modern Fauna took place in the Mesozoic, the lecture will have special focus on this era. As you may remember, the Mesozoic era is divided into the Triassic, Jurassic and Cretaceous periods, and span the time interval from 252 to 66 million years. The supercontinent Pangaea was fully assembled during the earliest Permian almost 300 million years ago and the first significant phase of break-up and separation took place in the Early Jurassic, but north-eastern North America, Greenland, and western Europe remained connected until well into the Cretaceous. It has been suggested that the break-up of Pangaea in the Mesozoic had a significant influence on the evolution. Basically, it promoted diversification because of the related changes in climate, oceanic conditions, sea-level and paleogeography. The end-Permian extinction, which is the most severe extinction event recorded on Earth, caused that about 95% of all marine species became extinct. The reason for the mass-extinction is still debated, but it is probable that factors like the substantial volcanism in Siberia during this time, together with oceanic anoxia and release of methane hydrate gasses, played a major role. For instance, trilobites, cephalopods, corals, bryozoans, brachiopods and crinoids were badly affected by the mass-extinction. Many of the shelly fossils that became extinct at the Permian-Triassic boundary represented filter-feeding communities, for example the brachiopods, sponges, tabulate and rugose corals and the bryozoans. Because these animals consumed microscopic organisms by filter-feeding, it has been speculated that their extinction may have been related to the collapse of the planktic food source. The Modern Evolutionary Fauna was less dependent on filter-feeding plankton from surface waters than the Palaeozoic Fauna, and animals with other feeding strategies such as deposit feeders, herbivores, grazers and carnivores became increasingly more common after the end-Permian extinction. Let us take a closer look at a couple of the groups that was heavily affected by the mass-extinction. The Brachiopoda, usually called brachiopods, was an important part of the marine Palaeozoic Evolutionary Fauna, especially the pedicle-bearing, articulate (or rhynchonelliform) brachiopods. This group was heavily affected by the Permian-Triassic mass-extinction, and the superficially similar bivalves became much more common than brachiopods during the following Mesozoic-Cenozoic period. Although articulate brachiopods and bivalves both have two valves, there are distinct biological and morphological differences. The plane of symmetry of a brachiopod cuts the valve in half along its length. In bivalves the mirror image in many (but not all) species runs along the edge of the valves where they close together. Another difference is that the brachiopod valves close when the muscles relax, whereas the bivalve valves open when they are relaxed. The inner organs are very different in the two groups. Brachiopods have a lophophore, which is a coiled, rigid internal apparatus adapted for filter feeding and respiration. Instead, bivalves are characterised by an open circulatory system with tentacles, gills and a mantle. Bivalves show a much wider range of use of the substrate than brachiopods. In addition, research has shown that most bivalves are more efficient at filtering water than brachiopods, and it is possible that this influenced on the change in dominance from the Palaeozoic to the Mesozoic and Cenozoic. Another important group that was affected by the end-Permian mass-extinction was the echinoderms. Surviving crinoids, which also are echinoderms migrated to deeper water and mobile echinoderms like the echinoids and asteroids flourished in the shallower water. A group of echinoid taxa, the irregular echinoids such as heart urchins and sand dollars, evolved from regular echinoids during the Jurassic, and were adapted to living conditions below the sea-bottom. The shell flattened relative to the regular echinoids, and the anus was moved from the top towards the posterior - and importantly - the siphons lengthened so they gradually could reach oxygenated water from a deeper burrow. The evolution of numerous new taxa during the Mesozoic took place at the same time as the developing break-up of Pangaea and a general rise in the sea level through the Mesozoic, which caused that many continental regions were covered by shallow epicontinental seas. As previously mentioned, the extinction at the end of the Permian period wiped out great numbers of marine organisms including trilobites, fusulinid foraminifers, rugose and tabulate corals, and bryozoans. Reef-forming, scleractinian corals and sea urchins expanded relatively quickly during the Triassic and Jurassic periods. At the end of the Triassic Period, another mass extinction occurred in marine ecosystems. For instance, it killed the conodonts and resulted in a serious extinction among ammonoids, and also among cephalopods and bivalves. It has been shown that the timing of the Triassic-Jurassic extinction coincides with floral evidence for a climatic shift to arid conditions in terrestrial environments of Gondwana, and with a significant drop in sea-level.
