Mass Extinctions, Notable Examples of 168 5 3000 Number of genera 800 Number of families 600 400 2000 1000 200 0 E Cm O S D C Ne Paleozoic 500 400 P 300 Tr J K Mesozoic 200 100 Pg N Cz Geologic timescale (Ma) Figure Past diversity of marine families The uppermost line shows the total number of marine families recorded in the fossil record through the geologic periods of the Phanerozoic and uppermost Neoproterozoic The circles correspond to the five major mass extinctions: (1) Late Ordovician; (2) Late Devonian; (3) Late Permian; (4) Late Triassic; and (5) end-Cretaceous The lower, yellow shading shows the families assignable to the Cambrian evolutionary fauna; the green shading shows the families assignable to the Paleozoic evolutionary fauna, and the upper, purple shading shows the families assignable to the modern evolutionary fauna E, Ediacaran; Ne, Neoproterozoic; Cm, Cambrian; O, Ordovician; S, Silurian; D, Devonian; C, Carboniferous; P, Permian; Tr, Triassic; J, Jurassic; K, Cretaceous; Pg Paleogene; N, Neogene; Cz, Cenozoic; Ma, million years ago Modified from Sepkoski Jr JJ (1984) A kinetic model of Phanerozoic taxonomic diversity III Post-Paleozoic families and mass extinction Paleobiology 10: 246–267, with permission from Palcantological Society Early Ordovician as the Earth was experiencing a greenhouse climate, with carbon dioxide levels and temperatures much higher than those of the present day Through the Early Ordovician there is evidence that climate gradually cooled, and from the Middle Ordovician tropical sea surface temperatures were much more similar to those of the present day (Trotter et al., 2008) Global climate remained fairly equable through the Middle and into the Late Ordovician, but sometime in the Katian (the penultimate stage of the Ordovician, about 450 Ma) temperatures begin to fluctuate It appears that a tipping point was reached toward the end of the Katian and global temperatures plummeted by some 1C through the early part of the Hirnantian Stage, plunging the world into the grips of a global icehouse (Finnegan et al., 2011) As climate cooled ice sheets would have expanded, causing sea level to drop, eventually exposing the continental shelves The first extinction event, which eliminated nearly half of all genera, is intimately linked to cooling and sea level fall at this time, as organisms succumbed to pressures such as lowered temperatures, changes in ocean circulation and reduced space on shallow marine shelves The reason for the climatic instability and onset of cooling are still unresolved Plate tectonics might have had an influence as a large continental landmass moved close to the South Cm O S D C Paleozoic 500 P Tr J K Mesozoic 400 300 200 Geologic timescale (Ma) 100 Pg N Cz Figure The past diversity of marine genera The line shows the total number of marine genera recorded in the fossil record through the geologic periods of the Phanerozoic The circles correspond to the five major mass extinctions: (1) Late Ordovician; (2) Late Devonian; (3) Late Permian; (4) Late Triassic; and (5) endCretaceous Cm, Cambrian; O, Ordovician; S, Silurian; D, Devonian; C, Carboniferous; P, Permian; Tr, Triassic; J, Jurassic; K, Cretaceous; Pg Paleogene; N, Neogene; Cz, Cenozoic; Ma, million years ago Modified from Alroy J, Aberhan M, Bottjer DJ, et al (2008) Phanerozoic trends in the global diversity of marine invertebrates Science 321: 97–100, with permission from AAAS Pole during the Ordovician Elsewhere, the chemical weathering of mountain belts formed by plate collisions may have helped reduce the level of carbon dioxide in the atmosphere Some evidence suggests that ice sheets may have been present, at least transiently, as early as the Middle Ordovician, implying a lengthy and complex interval of climate change preceding the onset of catastrophic cooling (Finnegan et al., 2011) In the wake of the first extinction event, as global temperatures and sea level fell, the low-diversity, cool-water Hirnantian fauna became widespread Around the middle of the Hirnantian stage, global cooling ceased and temperatures began to rise once more through the latter part of the Hirnantian As the Earth emerged from global icehouse conditions and ice-sheets retreated once more, sea level and ocean temperatures would have increased The warm, rising seas, associated with the spread of low oxygen, or anoxic, conditions, led to the second episode of extinction at the end of the Hirnantian The sea-level fall of the Late Ordovician glaciation prevented the deposition of shallow marine rocks in many areas, as the shelves became exposed This complicates the scientific analysis of biodiversity changes during this interval Do certain species disappear because of real biological extinction, or simply because the types of rocks that preserve their fossils are no longer present? This is a problem that affects the analysis of all past extinction events to a greater or lesser extent, especially further back in time when the global rock record is generally less complete (Smith, 2007) Although rocks recording the Late Ordovician crisis are common in Europe and China, for example, deposits of that age in North America are limited to a few places in central Nevada, USA, and Anticosti Island,