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)

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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

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Figure 8 Figure 9

Figure 10 Figure 11

Figure 12

Figure 13

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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. Rugose corals that had been important in reef building, massive bryozoans (order Trepostomata—prominent like small cannon balls among other fossils in Per- mian rocks south of Wollongong, as at Ulladulla), several major groups of crinoids (“sea lilies”), the last of the trilobites, several gastropod (“snail”) lineages and numerous groups of brachiopods— some brachiopod lineages survived into the very early Triassic only to disappear seemingly in the Figure 11

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FigureFigure 1214 Figure 15 first of the minor events that occurred intermit- oceans the rudistids (a group of large bivalves— tently until the end of the Early Triassic. Of about 75 families of ammonoids, perhaps only two families survived into the Triassic to then diver- sify into the galaxy of ammonoids that were so characteristic of Triassic, Jurassic and Creta- ceous times.

4. End-Triassic Event Again that spectacular free-swimming group, the ammonoids (Fig. 12) suffered badly. Only three ammonoid lineages survived into the Jurassic; these swiftly re-diversified. Reef building had be- come spectacular during the Triassic but crashed again when the important sponge group, the sphinctozoans, a frame-building group with large vesicles up the centre of the organism, al- the most important frame-builders in reefs at that most went into extinction. The great white moun- time—experienced 100% extinction. Perhaps tains extending from Austria and the Dolomites even more devastatingly, microscopic plants and of northernmost Italy through the Balkans to animals, the basis of food chains in the oceans, Greece consist of an enormous reef complex went into extinction and near extinction (c. 98%) that suddenly ceased at the end of the Triassic. with obvious consequences for life forms ulti- Brachiopods again suffered badly. mately dependent on these food chains. got off better. The rubbish eaters, the "pigs of the 5. End-Cretaceous Event globe", you might say, got on better than other The best know of “big five” global extinction organisms. events occurred at the Cretaceous-Cainozoic boundary when on land the dinosaurs and in the Lesser events

Figure 16 Figure 17

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Figure 18

Figure 19

Figure 20

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shown in Fig. 13, proved to be saw-toothed, showing that, initially, the increase in diversity of brachiopods was greater, overall, than the rate of extinction, but, after reaching a maximum of about 200 genera (aggregating more than 1,000 species) in the Emsian, the clustering of extinc- tion events increased, producing a situation in which loss of diversity with each successive event was greater than the rate of diversification between events. Putting this and all other avail- able data together we came up with 10 events (some previously suggested) during Silurian and Devonian time which needed to be characterized and tested for their global impact. (Figs 14 and 15). In order to get better time-control on these events, we then directed our attention to the conodonts (Figs 16 and 17) —a group of small pelagic organisms with which we had been work- ing for many years. They are tiny teeth (some grouped in an external “tooth-basket” in some genera; reconstructed in the mouth of Fig. 17) of an extinct group of jawless fish-like creatures. The way these teeth changed during the deep past has enabled us to discriminate intervals of time of about a million years, sometimes much less. In the hands of someone like Lennart Jeppsson (figure on right in Fig. 18), a friend of ours at the University of Lund who has a pen- Figure 21 chant for huge amounts of data. In a centimeter- by-centimetre study he undertook of the Ireviken But there were other extinction events, some on Event (see below) he employed people on the nearly the same scale as the “big five”. Let us dole to pick conodonts for him. His database for look at a few of these events, large and small that event on the island of Gotland consists of through the Silurian, Devonian and Early Car- about 500,000 conodonts. With that amount of boniferous ones on which we have undertaken data (?overkill) and Lennart’s eyes, time- research on brachiopods, conodonts and, more intervals as small as 100,000 years or perhaps recently, marine gastropods (“snails”). even as little as 10,000 years can be discrimi- The Late Ordovician to Early Carboniferous nated from the changing morphology of cono- (end-Tournaisian) witnessed a sequence of at least a dozen extinction events (including two of the “big five”), global in extent, and having vari- ous levels of impact on saw-toothed marine (especially shallow marine) life. The intervals be- tween these events were of such short duration during the late Middle to end-Late Devonian: the “rebound” of diversity from each event was less than the reduction in diversity that occurred dur- ing each event. The result? A graph when diver- sity is plotted against time through mid- Devonian-Early Carboniferous (Tournaisian). This is exemplified by the diversity of brachio- pods (“lamp shells”) that were the dominant Figure 22 group in many, even most, communities during that time. donts. Conodonts became abundant in the Ordo- A colleague and I working on brachio- vician, diversified rapidly and dispersed quickly pods plotted their diversity through time from the through the oceans of the world until the end- latest Ordovician (Ashgill) through to the end of Triassic event when they were extinguished. the Devonian period (a dozen time-subdivisions In order to get more precise information of these periods, called stages). The result, about ocean chemistry, we then selected the

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Figure 23: Lau Event, Gotland best possible rock sequences in Australia and Europe (Figs 19 and 20) for study of isotopes of oxygen (an indicator of temperature), carbon (closely tied to life) and strontium on opposite sides of the globe to determine if ocean chemis- try had changed through these events and if the changes had been propagated through the ocean system worldwide. Some results:

Three of the Many Minor Global Ex- tinction Events 1. Ireviken Event Fig 18 shows the cliffs at Ireviken on the island of Gotland, Sweden, and a trench cut through the soft calcareous clays for sampling for isotope data. The event occurred without detectable change in rock type well above the dog-leg in the trench. Fig. 21 shows data for isotopes of carbon and oxygen through this same event at Boree Creek, near Orange, NSW. Note the displace- ment (excursion) to the right for both oxygen 18 (indicating cooling) and carbon 13 in the upper part of the graphs. The data from Ireviken is vir- tually the same, showing that the changes in ocean chemistry associated with these events was propogated worldwide. Of special interest regarding the Ireviken Event is that 6 sub-events can be deciphered, showing that it was not in- Figure 24: Lau Event, Broken River, Qld stantaneous but displayed a discerninle “package of history”.

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Figure 25

Figure 27 Figure 26

Figure 28

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from rock sequences in Germany, It is especially 2.Lau Event characterised by a dramatic reduction in ammon- The Broken River (Fig. 22) , about 200 km W of oid faunas worldwide, almost to the point of com- Townsville, has elegant rock sequences in which plete extinction, and by a short, sharp refrigera- several of the global extinction events can be tion event well documented from bores in the discriminated. One of these was named the Lau Amazon Basin of Brazil. Event from a locality on the island of Gotland, Some global extinction events are readily Sweden. Like the Ireviken Event is was rather identified in stratigraphic sequences by unusual major, decimating the fabric of marine faunas packages of sediments, notably, in the case of that had been in existence prior to the onset of the Hangenberg Event by a brown shale interval the event at the beginning of an interval know as and a thin limestone (“Hangenberg Shale and the crispa Conodont Zone. This event is ele- “Hangenberg Limestone” respectively) found in gantly displayed by rock-sequences in southern rock sequences of seemingly identical age Sardinia, Scania (southern Sweden), in subur- across 4,000 km of North Africa and Europe, just ban Prague, and above all in the Broken River as (see earlier), the Upper Kellwasser Event is region and on the island of Gotland. characterised by a 0.5 to 2 m black shale interval The Lau Event affected corals, brachio- found in rock sequences in the 15,000 km be- pods (“lamp shells), trilobites, marine worms, mi- tween Morocco and southern China. cro- and macro-plankton and fish. Changes in For this event we sampled the classic sediments at that time indicate a change in Drewer Quarry locality in Germany, a rock se- depth. Changes in isotopes reflect changes in quence in the Montagne Noire of southern ocean chemistry. The relative timing of isotopic, France where it is beautifully represented, and sediment and faunal changes is essentially the undertook trenching (450 m of trench) to get an same at the Broken River as on the island of adequate rock sequence in the Fitzroy Crossing Gotland. area of Western Australia (Figs 25–27). This Prior to the onset of the Lau Event, the event proved to have a short, sharp strontium marine communities in the Broken River area excursion for which we do not have an adequate explanation (Fig. 28).

?Periodicity from Computer-modelling Computer wizard David Raup, from computer modeling of J.J. Sepkoski’s compilation of family ranges coupled with Schumachers’s asteroid data, concluded that the size of bolide that could strike the Earth every 100,000 years may be dis- astrous locally but would have negligible effect globally. The size of bolide that might impact with the Earth every million years could result in a 5% kill, but a largish one every 100,000,000 years, according to Dave, could result in a 65% kill. To get a 95 % kill, as some people argue for the lat- est Permian-Early Triassic event(s), would need Figure 29 a “real beauty”—and the evidence (see earlier) suggests it was not a simple event. It is impor- were dominated by tabulate corals and stro- matoporoids. A few trilobites and brachiopods (“lamp shells”) survived through the event. After the event, water depths became shallower and more strongly tide and storm-influenced, but, when examined closely, there is remarkable similarity between the rock sequences at the Broken River and on Gotland. Note the sharp excursion in carbon 13 in a section we sampled at Botvide, Gotland (Fig. 23). From our counter- part section at the Broken River (Fig. 24) we took 238 samples for isotope analyses.

3. Hangenberg Event This event, just a gasp before the Devonian- Carboniferous boundary, was first discriminated Figure 30

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tant to note that such modeling tells us about To improve the database so that it might pro- likelihoods, not when. There was plausibility duce more compelling results from computer- this far. However, Dave’s modeling suggested modeling would be a Herculean task. a significant extinction event every 24+ million years, but this was shown to be an artifact of Explanations the way Sepkoski’s data had been presented. Explanations of global extinctions are various,

Figure 31

Figure 32

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some to be taken seriously, others, such as al- the carrying capacity for plants is about 35–40 kaloids causing constipation in dinosaurs, not! species. Extinction of a few species, such as A moment’s thought for such a suggestion by severe drought, leaves vacant niches for raises the questions of how to explain the si- successful colonization by more or less the multaneity (or near simultaneous) decease of same number of species (but not necessarily all dinosaur lineages at or very, very close to exactly the same species). For bigger islands, the Cretaceous-Cainozoic boundary, and the there would be a greater carrying capacity, simultaneous extinction of vast numbers of tested by data from the islands in Gatun Lake families and genera, marine and non-marine, at in the Panama Canal, and also by fumigation of the same time. The explanations proffered fall tiny islets in Florida and monitoring re- into several categories, terrestrial ones, such colonization by plants and especially insects. as volcanic outbursts; grand scale marine Schopf’s extension of the Macarthur- transgression/regression events (a plate tec- Wilson theory to semi-continent or continent tonic scenario), and extraterrestrial ones such scale, specifically for Permian time, did not as supernova explosions and comet or asteroid work because of the enormous differences in impacts (Fig. 29). diversity of terranes and climate from continent to continent, and failure to appreciate that a 1. Grand-scale trangressions and regressions global marine regression (with decrease in di- That marine transgression/regression events versity of marine biota) would, necessarily, be were the trigger was much favoured by the late complemented by an increase in area available Tom Schopf about 20 years ago. He sug- for colonization, leading to, eventually, an in- gested, plausibly, that theMacarthur-Wilson crease in diversity of terrestrial floras and fau- theory of Island biogeography could be applied nas. at continent/global scale, and that the carrying capacity in genera or species would be con- 2. Extraterrestrial theories (principally asteroid/ trolled by area available—the greater the area comet impacts) (with marine regression), the greater the carry- These have been much favoured since ing capacity for species, genera or families of the influential paper by Louis Alvarez et al., organisms. published in 1980, suggesting that impact of a For a tiny island, such as Heron Island, c. 10 km-diameter nickel-iron asteroid would neatly account for the end-Cretaceous global extinction event. An asteroid impact, with Chicxulub crater in the Yucatan Peninsula of Mexico as the favoured site, has remained the ruling hypothesis since 1980 and has, too often as we shall see, been accepted as fact for the end-Cretaceous as well as all other global ex- tinction events, especially by people not ac- quainted with the data. Though everyone loves the drama of an asteroid impact, some caution is needed. If you are going to have an exotic cause such as something extraterrestrial slam- ming into the Earth, you must have good evi- dence pointing, as they say, to the “smoking gun”. Let us look at some of the data and something of the debate (Figs 30, and for iden- tified craters Fig. 31). If the scenario requires asteroids or comets to have done the job, there must be craters around. How many craters are there? How big are they? Did they occur at the same time as the extinctions? And is there some sort of cor- relation between size of crater and scale of ex- tinction? The most striking feature of the surface of Mars, Mercury or the various moons con- nected with the planets of our solar system, in- cluding our moon, is the pattern of impact cra- Figure 33: Shocked quartz ters. The history of our solar system involvesan

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abundance of impacting, though with diminish- 3. Volcanic scenarios ing frequency through time. Despite impact Some people prefer volcanic to bolide scenar- sites on Earth tending to be obliterated rela- ios—well summarized for the end-Cretaceous tively quickly (in terms of geologic time) by ero- Extinction by A. Rice (Geol. Soc. America Sp. sion and sedimentation, many craters (Fig. 31) Pap. 244: 39–56. Such workers point to vast have been identified that, clearly, have been outpourings of Late Cretaceous–earliest Caino- produced by bolides (asteroids, comets) of zoic volcanics, the Deccan Traps, making up various sizes. But, for many of these, there is the Western Ghats, the mountain tract extend- ing down the west side of peninsular India. The cause would seem unlikely to be volcanic out- pourings extending over a few million years if the event were swift, possibly instantaneous, as appears to have been the case with the end-Cretaceous event. A similar scenario has been advanced for the latest Permian-Early Triassic event/ events: vast volcanic outpourings known as the Siberian Traps, best known from the Noril’sk nickel-mining area of NW Siberia. Some have hypothesized that the Noril’sk occurrence, by far the largest nickel-bearing orebody in the world, resulted from a huge bolide: a nickel-iron Figure 34 asteroid. This is an interesting suggestion, but I am not sure how the volcanic outpourings can little precision regarding the exact time of im- be linked to the sequence of anoxic events in pact. Witness the superb Acraman crater in the oceans that Swiss geologist Aymon Baud South Australia and its halo of debris found in and his group have documented (with isotopic rock sequences in the Flinders Ranges (Fig. data) and insist was the prime cause of the lat- 32) 250–300 km to the east, but this impact oc- est Permian-Early Triassic extinction event/ curred well back during the Proterozoic before events. the appearance of multicellular life. Some au- thors have suggested a connection with a sig- 4. Gross changes in ocean-circulation nificant extinction event in the unicellular life patterns (acritarchs) during the Proterozoic, but others The opening or closing of major gate- argue that the two events are very much out of ways between landmasses of continental scale kilter as regards age. Other craters in Australia is an obvious prime control on gross oceanic seem not to have been of sufficient size to ac- circulation patterns. For example, opening of count for any of the extinction events that oc- the gateway between South America and Ant- curred during Phanerozoic time (Cambrian to arctica, of fairly recent origin, enabled develop- Recent). Incidentally, the shock-pattern in- ment of the Circum-Antarctic Current, isolating duced in quartz (Fig. 33) by an extreme pres- Antarctica from warm waters from equatorial sure impact is distinctive and often used, with regions and resulting in refrigeration of the con- other data, as evidence for bolide impacts. tinent and profound cooling of the Southern Ocean, with drastic effect on the distribution and composition of marine communities. A friend and co-researcher of ours, Lennart Jeppsson (University of Lund, Sweden), has pointed out repeatedly that Silurian events with which he is most concerned (the Ireviken, Mulde and Lau events) were not instantane- ous—and therefore facilely attributale to bolide impacts—but were a sequence of sub-events, six in the case of the Ireviken Event—and he has developed a scenario, influenced by con- sideration of the dynamics of the of the Pacific and Indian Oceans and of the North Atlantic Ocean, particularly the Gulf Stream. [At the present day, heavy, cold water Figure 35 sinks in the polar region of the far north Atlantic

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Figure 36 and flows southwards in the reverse direction to the Gulf Stream. The increasing rate of melt- ing of the Greenland icecap, melting and disap- pearance of the Arctic ice and, above all, the rapidly increasing flow of freshwater from the great Siberian rivers is causing rapid freshen- Figure 37 ing (and reduction in specific gravity) of the cold waters that sink to form what, in the deep quartz) with abnormally high levels of platinoid ocean, we might call the “Reverse Gulf elements, especially the readily-analysed irid- Stream”. Bottom measurements have shown ium, that had blanketed much and possibly all the rate of flow of the latter has decreased by of the globe very close to if not at the Creta- 20% in the past century. It and, by implication, ceous-Cainozoic boundary had been produced the Gulf Stream may therefore cease flowing in by a 10 km diameter nickel-iron asteroid, the the next 50-100 years with catastrophic conse- explosion equating with blast by 100 million quences for the climate of western Europe…]. megatonnes of TNT. Allen Hildebrand seems to have been 5. A look at the Cretaceous-Cainozoic first to identify a huge crater, 300 km across, in scenario (Fig. 34) Cretaceous limestones, buried beneath a few Let us start with Max de Laubenfels, a km of Cainozoic sediments on the Yucatan prominent worker on fossil and living sponges. peninsula of Mexico. The scale of the buried Because Max was a bit of an oddball, nobody crater was appropriate for the scenario pro- took much notice of a paper he had published posed by the Alvarez group, as was the signifi- in the Journal of Palaentology in 1946 in which cant “spike” in the greenhouse gas carbon di- he presented some back-of-an-envelope calcu- oxide (and brief temperature increase of about lations suggesting the end-Cretaceous event 10°C) that was observed. Evidence was pro- could have resulted from impact of an asteroid duced of mercury anomalies (implying acid of about 10 km diameter. That Max was a rain) and soot from global wildfires. Florentin prophet before his time was forgotten, even af- ter the Alvarez group (see below) suggested the same scenario, admittedly with much more data, over 40 years later. Subsequently, Eugene Schumacher, who was very interested in cratering by atomic bombs, compiled an exhaustive list of asteroids with potentially Earth-crossing orbits. He pointed out that our planet should be impacted so as to produce a 10-km diameter crater every 110,000 years, and a 100-km one every 50 mil- lion years, but did not connect this with the then poorly-known pattern of global extinction events. In 1980, Nobel laureate Louis Alvarez and three colleagues put out a paper arguing from a layer of clayey “crud” (including shocked Figure 38

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Figure 39 Figure 40 Maurasse had studied catastrophic deposits on southern Haiti with blocks of rock the size of collecting to have, in fact, survived until the houses and city blocks, and composed of ma- boundary. Westermann was happy to admit terial from a vast spectrum of environments that he had been wrong. Peter Sheahan from deep ocean abyssal sediments to non- (mainly a brachiopod worker) from the Milwau- marine material. He argued that these were kee Museum, organized a similar massive at- generated by enormous tsunamis caused by a tack on a richly fossiliferous non-marine rock dramatically large impact. The age of these sequence in Wyoming collecting dinosaurs, spectacular deposits is 65 million years— fish, mammals, plants and invertebrates. The almost exactly on the Cretaceous–Cainozoic only dinosaur materials found to be younger boundary. Likewise, spectacular boulder de- that the Cretaceous-Cainozoic boundary were posits were deposited by enormous tidal waves a very, very few battered fragments obviously sweeping up river valleys, such as the Rio derived by erosion from a Cretaceous rock se- Grande, for distances of up to 200 km from the quence—presumably sluiced out by stream ac- Caribbean Sea at the same time.And the age— tivity. Ruth Mawson met Hickey at a botanical approximating the Cretaceous-Cainozoic conference in the mid-1980s, by which stage boundary (about 65 million years)—was also he had had the benefit of Peter Sheahan's appropriate. Chicxulub was therefore identified enormous, accurately located collections which by the geologic community as the “smoking showed him very convincingly that not only was gun” (Figs 35–37). But is the evidence strong there a major cutback in plant diversity during enough to pinpoint the site of a bolide impact the critical interval, but that it had taken place and does Chicxulub have precisely the right very sharply at the event. He was happy to ad- age to be the “smoking gun”? mit in a public lecture that he had been wrong. Reservations were expressed by sev- Thus, the opposition seemed to fade eral palaeontologists who thought that no dra- away. That the end-Cretaceous extinction matic event took place at the K-T boundary, event had been generated by a bolide and that among them three leading experts: ammonoid the point of impact was on the Yucatan penin- worker Gerd Westermann who, like his fellow sula became dogma, especially for by the main ammonoid workers, believed the ammonoids mass of non-scientists. It has been taken for were dying out through the Late Cretaceous; granted in much popular scientific literature and mammal worker Richard Tedford who believed seemingly all television documentaries about that the a few dinosaurs had survived into the extinctions in the deep past. Any who would early Cainozoic; and Leo Hickey who felt that question the dogma have been attacked, the plant record implied no dramatic change at sometimes savagely, for instance by the late the boundary. Louis Alvarez. Westermann, with the help of what I Gerder Keller and Norm McLeod, two think was an Earthwatch team, laboriously col- of the most meticulous workers on Late Creta- lected an ammonoid-rich sequence beautifully ceous-Early Cainozoic foraminifers (Fig. 38), exposed on the Basque coast of Spain. The the most important group for providing high- resultant huge database demonstrated that precision ages through that period of time, many species (especially the rare ones) that have called for caution. Their refined study of were thought to have disappeared well before the foraminiferal sequence occurring as a man- the K-T boundary were shown, from intensive tle over the Chicxulub body, presumed to have

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been a bolide, has shown that it must have im- criminated, commencing very late in the Per- pacted a million years or more before the end mian and punctuating all of Early Triassic time. of the Cretaceous period. Add to that is what I Nevertheless, such a suite of sub-events could believe to be compelling evidence for a much well include one (or more) very major sub- bigger bolide having generated a much bigger events dwarfing others in scale, as seems to crater, the Shiva Crater, in the Arabian Sea off have occurred in this case. Mumbai, India, and for this to have occurred right on the Cretaceous-Cainozoic boundary Final comments (65.5 ± 0.3 million years ago). Part of the cra- What is so important about extinctions (Fig. ter, consisting of suavite—a peculiar rock type 39)? Firstly a global extinction (or ecosphere generated by impacts—forms the arcuate Bom- collapse), no matter what the cause, must have bay High offshore from Mumbai (formerly Bom- been important in the process of natural selec- bay). The Cainozoic sequence draped over this tion—and thus very important, as a recurrent part of the crater provides most of India’s petro- major element of randomness, in the history of leum from numerous bores, several of which life, and how we happen to be here.... have penetrated into the underlying suavite— Not all species are wiped out during the name is derived from Swabia, southern extinction events. Generally a short time after Germany, where the well-documented Ries or during an extinction event opportunistic spe- Crater (with the town of Nördlingen at its cen- cies flourish, taking advantage of niches va- tre) resulted from a Miocene impact. The rest cated by species that have gone into extinction. of the Shiva Crater is an arcuate block of conti- This is exemplified by the Lau Event, following nental rock (granite) making up the Seychelles. which Cyanobacteria flourished briefly, chang- The two halves of the 600 km x 450 km crater ing bottom-water conditions in many areas (oblong because of oblique impact), the largest from well oxygenated to anoxic as a conse- impact crater in the Phanerozoic history of the quence of their role in decomposition of organic Earth, are separated by a sea-floor spreading matter. “conveyor belt” with magnetic stripes showing it Normally during extinction events a was initiated at the Cretaceous-Cainozoic sudden change in the environment or a shift in boundary. The Shiva Crater bolide is estimated the balance can disrupt communities. General- to have been about 40 km in diameter (cf.10 ists are able to cope with a wide variety of envi- km for the Chicxulub bolide). Sadly, the propo- ronmental conditions and can survive changes, nents of this scenario, Shanker Chatterjee and specialists are assumed to be less able to re- D.K. Rudra, published their long paper ob- spond to environmental change, especially scurely in a volume of conference papers: sudden fluctuations in conditions. Examples of Memoirs of the Queensland Museum, 1996, v. specialists include species that have symbiotic 39 (3): 489–532. relations, eat a highly specific food, and/or thrive in very specific environments. Short and sharp or with sub-events Recovery from these catastrophes was There has been a temptation to assume that never swift. In the case of reef systems, recov- global extinction events were short and sharp ery from each major event, exemplified by the (as seems to have been the case with the end- latest Permian-Early Triassic event(s), took 10– Cretaceous Event) but, as will be apparent 15 million years before some sort of equilibrium from the selection of events discussed above, was re-established—often with different groups nothing could further from the truth for most of organisms taking on the role of frame- global extinction events. Many, perhaps all, of builders before “ecological saturation” was re- the discernible extinction events, major and mi- established. nor, global or not, took a significant interval of Obviously it's very important to know time―represented in stratigraphic sequences more about the pattern of extinctions through by very definite extinction-sequences (such as time as the backdrop to what is apparently a the half-dozen sub-events for the Lau Event) developing major global extinction event, re- and by differing patterns of change in ocean ferred to by Jared Diamond as “The Sixth Ex- chemistry from event to event. Similar se- tinction” (Fig. 40). quences of change (“excursions”) in isotopic Because of the relentless process of composition of sea water, globally, may have natural selection, species (reproductively iso- taken thousands or even a million years or two lated life-forms) rarely survive more than one or for completion of an entire extinction cycle. two million years. All is change; all is in the This is particularly the case with the latest Per- process of becoming—as Heraclitus observed mian–Early Triassic extinction event (or events) 2500 years ago—but sometimes there are sig- in which several sub-events are now being dis- nificant, even major jolts in the saga.

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