390 Invertebrates, Marine, Overview noise associated with an incomplete fossil record This does tend to make the mass extinction events appear less dramatic For example, only 54% of marine families were lost in the Permian extinction event, but it is estimated that 77 to 96% of all species became extinct The early Cambrian shows a rapid increase in diversity that tends to flatten out in the middle and late Cambrian (Figure 9) Most animal phyla appear in the record in this phase The Cambrian diversity includes a number of ‘‘archaic’’ forms such as trilobites, hyoliths, and inarticulate brachiopods that decline after the Cambrian period Diversity is not high in the Cambrian and rather unspecialized detritus and low suspension feeding organisms functionally dominate communities, suggesting a simple ecology The Ordovician sees a steep rise in diversity but the curve then flattens for some 200 million years Periods of mass extinction are detectable, notably at the end of the Ordovician and in the late Devonian, but in general diversity stabilized This new Palaeozoic diversity was associated with an evolutionary radiation of sessile benthic organisms such as crinoids, articulate brachiopods, stenolomate bryozoans, and tabulate and rugose corals The Palaeozoic period ended with the catastrophic Permian-Triassic mass extinction Diversity then apparently rose steadily, with a small extinction at the end of the Cretaceous, past the levels achieved in the Paleozoic until the unique peak of the present day Current family diversity is apparently twice as high as the Palaeozoic stable level The Palaeozoic community never seems to have recovered and the modern high diversity is associated with a new fauna of shell-breaking predators and sediment movers This fauna includes the familiar sea urchins, bivalve and gastropod mollusks, and crustaceans such as shrimps and crabs A variety of explanations have been given for this pattern A number of authors have pointed to the association between the rise in taxonomic diversity and a rise in functional diversity This almost certainly explains the Ordovician increase in diversity, which is essentially because of increased exploitation of marine sediments Explanations for the Mesozoic-Cenozoic explosion in diversity are more controversial One argument is that the breakup of the Pangaea supercontinent caused increased diversity through increased climatic variation leading to more endemicity It is not clear how this argument would Figure The family diversity of marine animals as measured by their fossil record from 600 M years ago to the present day Graph C shows the rise and fall of the diversity of the Cambrian fauna of unspecialized detritus and low suspension feeding organisms, P show the rise and fall of the diversity of the Palaeozoic fauna of sessile benthic organisms, and M shows the rise of the diversity of the modern fauna of sediment movers and shell breaking predators Note that sampling error may have exaggerated modern diversity The arrows indicate periods of mass extinction, is at the end of the Ordovician, in the late Devonian, at the end of the Triassic, and at the end of the Cretaceous (the dinosaur extinction) P indicates the massive Permian extinction (Graph by Sepkoski, 1992, reproduced with permission from Heywood VH (ed.) (1995) Global Biodiversity Assessment UNEP, Cambridge: Cambridge University Press.)