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Extinction Events Ver2.Pub GEOS272 Extinction Events Macquarie Universty Department of Earth and Planetary Sciences GEOS 272 Earth’s Evolving Environments Extinction Events (John A. Talent: A personal view) “[We] never step foot in the same river twice!” —Heraclitus (6th century BC): We have been brought up on a gradualist view Back in the early 1800s there was a of the world. We are well aware of individual fairly widespread view that Earth history had extinctions―exemplified by the Dodo (Fig. 1), been punctuated by a series of major catastro- Aepyornis, Diprotodon, dinosaurs, woolly rhi- phes. This scenario, owing much to the obser- noceros, and various species of mammoths vations of pioneer French vertebrate palaeon- and mastodons. It is the mass extinctions, re- tologist Georges Cuvier (1769-1832), was ea- gionally and even globally, rather than the ex- gerly received in France but found resistance in tinctions of individual lineages, that are of par- the United Kingdom where several earth scien- ticular interest. Some of these extinction events tists, principally Sir Charles Lyell, had argued resulted in swift reductions in biodiversity persuasively for a gradualist (uniformitarian) around the globe, decimating terrestrial as well view of Earth history. The gradualist view be- as marine life, and thus greatly influencing the came overwhelming following publication of subsequent course of evolution. In the case of Charles Darwin’s Origin of Species (1859) and the largest of these events, the end-Permian remained so until a series of papers by the late extinction, as much as 98% of all life forms Norman D. Newell between 1956 and 1967 laid ceased to exist in a very short time. the foundations for a new catastrophism. He pointed out that there seemed to have been five spectacular cutbacks in diversity globally during the history of life. Textbook authors often refer to these Figure 2 at the “Big Five” and, curiously, continue to do so, apparently unaware of research in the past 40 or so years that has demonstrated punctua- tion of the history of life by numerous global and regional events at various scales. Our aim in this segment of the course is to examine these events, focusing on their scale, fre- quency, the organisms involved in some of the Figure 1 major events, and to consider why some 1 GEOS272 Extinction Events groups of organisms seem to have been more sensitive to such events than other groups – and to probe what might have been the causes, and whether the pattern of such events displays any periodicity Fig. 2. The first three of these five major events (Fig. 3) was close to but not quite at the end of the Ordovician (the Ashgill Extinction); the sec- ond halfway through the Late Devonian (the Up- per Kellwasser Event: at the Frasnian- Famennian boundary); and the third, the greatest of all global extinction events (the “Mother of all Extinction Events”), is now known to have ex- tended as a series of sub-events through the lat- est Permian, culminating at the Permian-Triassic boundary, and continuing intermittently to about the end of the Early Triassic. In aggregate, more than 90% of then-known life forms disappeared with this event. The other two “grand events” Figure 4 (stages) at the end of the Triassic, Norian and Rhaetian, with the event occurring at the end of Figure 3 the Rhaetian, but presently available data indi- cate they are virtually time-equivalent—the No- were at the end of the Triassic and at the end of rian, better-known, based on normal marine se- the Cretaceous (K/T boundary event). The last, quences, the Rhaetian on essentially shallow associated with the extinction of the dinosaurs, is marine and hypersaline sequences]. very familiar to the general public but, as we will There are, however, other significant see later, many other groups of organisms went global extinction events in addition to the “big abruptly into extinction at the same time. five”. These include the Ireviken and Lau events Thirty five years ago I presented the (in the Silurian), and the Taghanic (late in the story then current, based on Newell’s data and Middle Devonian) and Hangenberg (very late in the then reigning explanation: that these extinc- the Late Devonian) and possibly a major event at tions coincided with major refrigeration events, the base of the Cambrian when a great diversity but the this explanation was not compelling (Fig. of soft-bodied animals—jellyfish, seapens and 4). The database has become much larger since many other bizarre creatures (Fig. 5 and recon- Newell’s time, but more recent plots of the data struction, Fig. 6)—of the latest Proterozoic (the have produced exactly the same result, implying Ediacaran stage named for Ediacara in South these five events were very real and of major im- Australia) disappeared. portance in the history of life. Let us look briefly at the “big five” before The late J. J. Sepkoski about 20 years considering a few of the lesser events. ago plotted the numbers of families through time. Decreases in diversity associated with “big five” 1. Ashgill or Hirnantian Event (Very late Ordo- show up well as V-shaped incisions in Sep- vician) koski’s graph. Slight errors in positioning these The first of the “big five”, late in the Late events resulted from the gross way the database Ordovician, coincided with a dramatic reduction of time-ranges had been compiled. [Textbooks, in reef-building organisms, especially the stro- incidentally, generally show two subdivisions matoporoids (a group of sponge-like organisms) 2 GEOS272 Extinction Events Figure 5a Figure 6 Figure 5b and an extinct group of corals, the Rugosa. This lia), especially about a century ago. Quarrying event, in the Hirnantian stage of the Ashgill, was ceased when the commercially unattractive dark associated with a major glaciation with a spec- horizon was encountered. tacular ice-cap extending over much of Africa. The upper surface of this “killing field” consists of millions of ammonoids the size of 2. Upper Kellwasser Event in the Late Devo- saucers. Not only ammonoids suffered severely nian (at the Frasnian-Famennian boundary) during this event, but numerous other groups, As with the latest Ordovician event, the among them the brachiopods (“lamp shells”) that Upper Kellwasser Event saw a dramatic de- were the main structuring organisms in many crease in diversity of brachiopods (“lamp shells) communities at that time, and the stromatopor- that had been the most diverse component of oids, the principal frame-building organisms in communities during Ordovician, Silurian and De- reef structures at that time. The same Upper vonian times, as well as ammonoids (resembling Kellwasser dark interval occurs over much of the the present-day Nautilius – lower left of Fig. 7) world where this time-interval is represented in reduction in reef-building organisms, especially rock sequences. I have seen it in numerous lo- the sponge-like stromatoporoids (lower right of Fig. 7 shows upper surface) and rugose corals (Fig. 7,upper). The Lower Kellwasser Event, about 1.5 million years earlier, had less impact, but has a similar dark-band “signature” to the Upper Kellwasser Event. It has yet to be inten- sively investigated. What do extinction events look like in the field? The illustration (Fig. 8) is a quarry at Cou- miac in southern France where a dark layer about 0.5 m in thickness in mottled pinkish lime- stones (“building marbles”) represents the “killing field” of the Upper Kellwasser Event (one of the “big five”). The mottled pink limestones were ex- tensively quarried for tabletops and mantlepieces and shipped all over the world (including Austra- Figure 7 3 GEOS272 Extinction Events Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 4 GEOS272 Extinction Events Figure 8 Figure 9 rock) where “sedimentary signatures” like those cited above are lacking. For instance, excursions in isotopes of oxygen, carbon and strontium (representing significant changes in the composi- tion of the world ocean) in the superb Late Devo- nian rock sequences of the Canning Basin in northwestern Australia are virtually identical to excursions in isotopic data for the Upper Kell- wasser Event in the superb rock-sequences of the Montagne Noire in southern France. Total extinction of a species implies something like a 100% effective field of “bullets”, Figure 10 so to extinquish all species of a genus and to do so for 75% of, say, all brachiopod genera in exis- calities in Morocco, southern France, Germany, tence at the onset of the Upper Kellwasser Event central Asia (Kyrgyzstan – Fig. 9) and south should convey an impression of how devastating China—about 15,000 km—but it did not extend this was to the marine communities in existence through to Australia, though it can be readily rec- at that time. It was a very, very real event with ognized from isotope data in wonderful rock se- important consequences in the history of the quences winding their way across the landscape globe. for hundreds of km E and W of Fitzroy Crossing in NW Australia (Fig. 10). In this picture the 3. Latest Permian-Early Triassic Event rough country is the tract of Devonian limestones Commencing towards the end of the Permian with reefs. The river, which has sliced a gorge and apparently extending through the Early Tri- through this tract, is the Lennard River. assic, though perhaps with its apogee at the Per- Global extinction events are readily rec- mian-Triassic boundary was the “Mother of all ognizable by the pattern of extinctions in the vari- Extinctions” (Fig. 11) when 92-95% of all genera ous fossil groups and by changes in isotopic of organisms, including 98% of marine planktonic composition of fossil shells (and even of whole- life (the basis of food chains in the oceans), and sizeable proportions of the then extant inverte- brates, vertebrates and plants perished during one phase or another of this event/events–but, seemingly, mostly at or extremely close to the Permian-Triassic boundary.
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