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Extinction


Abstract

A number of extinction concepts are discussed and loosely defined, followed by a survey of some important extinction and "extinction-like" events, primarily from the Phanerozoic.

Keywords: extinction, mass extinction, Cambrian, Ordovician, Devonian, Permian, Permo-Triassic, Triassic, Cretaceous, Cretaceous-Tertiary

Introduction

The idea of individual species becoming extinct is quite familiar; indeed it is a rather sad indictment of our stewardship of the planet that we are all too familiar with extinction. But, in fact, extinction is a rather complex phenomenon. At one end of the continuum we have the notion of a population of organisms evolving into something else. Here, the disappearance of the original phenotype might be accomplished by nothing more than natural turn-over of the generations (anagenesis).

At the opposite end of the spectrum, we have the mass extinctions, where huge proportions of the earth’s biota disappear more or less simultaneously, within an interval that is, in some sense, short. At least some of the more sensational explanations for these phenomena require the wholesale killing of individual organisms.

Between these two extremes we have a range of possibilities, further complicated by the vagaries of the fossil record and our imperfect interpretation thereof. And always, even in the case of the KT event which cannot be “explained away” in its entirety by meteorite impact, there is the enigma of underlying cause.

Working Definitions

For our purposes, extinction of a single taxon – whether a species or higher level taxon – is accomplished when the last representative of that taxon dies. Of course we could also distinguish the point at which the organism was no longer able to reproduce (e.g. when the population density of a dioecious species drops below its reproductive threshold) but any such subtlety is pointless: The fossil record is not a good witness to the fate of individuals, so our notions of extinction are necessarily approximate. In practice, to the paleontologist, extinction is the last (most recent) occurrence of an identifiable fossil.

We have more difficulty with the concept of mass extinctions. Ward 2000 (pp. 6-7) offers the definition that mass extinction events are geologically short intervals of intense species extinction. However, this definition admits events such as the decimation of the South American marsupial fauna following the establishment of a land bridge with North America in the late Pliocene, which is almost certainly not his intention. Of course such events are also interesting extinction phenomena. But to properly capture the idea of Mass Extinction, it also seems necessary that a mass extinction should be global in extent and involve ‘participants’ from widely diverse taxonomic groups.

Species Transitions

At one end – perhaps we might call it the “gentle end” – of the extinction continuum we have anagenesis: the notion of a population of organisms evolving into something else. As indicated above, here the disappearance of the original phenotype might be accomplished by nothing more than natural turn-over of the generations. This is sometimes known as phyletic extinction or pseudoextinction.

Creationists like to believe that this gentle transition of one species into another is poorly documented. But in fact it is rather common, particularly among microfossils, of which huge numbers can be recovered from continuous rock sequences, allowing statistical study of morphological trends. One of the best documented of these species transitions I’ve seen is Hornibrook’s (1968) “Orbulina Bioseries” (Fig. 1).

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Fig. 1. Hornibrook’s “Orbulina Bioseries” (The Geology of New Zealand, v.2, fig. 7.10, after Hornibrook 1968).

Abrupt Extinctions

However, many species do not leave (identifiable) descendants; at some point they simply cease to be. The same is true of higher level taxa, though of course it always comes down to the taxon’s last surviving representative species in the end.

Considered over whole lineages and long periods of time, we also notice a tendency for certain groups of animals (for example, trilobites and ammonoids, the Graptoloidea) to experience severe, and probably abrupt, extinction of many of their component clades, repeatedly.

Leaving aside discussion of the controversial dinosaur-bird semi-clade for another time, we can be sure there was a “last graptolite,” a “last ammonite,” and a “last australopithecine” – which, with their death, effected the extinction of their lines. These are the extinctions with which we are already intimately familiar; and the almost inevitable fate awaiting gorillas, tigers, and black rhinoceros today.

We well understand the reason for many contemporary extinctions – it is us – but those of the past are more mysterious. The youngest graptolites are known to us from Carboniferous sediments. Their disappearance from the fossil record was not assisted by any known meteor collision or other deus ex machina; nor was it associated with other phenomena leading us to infer a mass extinction (as we have defined it). Late Ordovician graptolite extinctions appear to be linked closely with changes in oceanic oxygen minimum zones that are, in turn, linked with climate changes, so perhaps it was something similar which finally extinguished the group. Whatever the merits of this argument, this degree of uncertainty is far more typical of our understanding of prehistoric abrupt extinction, particularly of individual taxa.
Although the main group of organisms widely recognised, even by amateurs, as “graptolites” (order Graptoloidea) went extinct in the Devonian, so-called dendroid graptolites (order Dendroidea) persisted through to the Carboniferous.

Where multiple taxa are involved, or we can demonstrate coincidence with other major (e.g. tectonic) events, we may be able to speculate more confidently. For example, the decimation of the endemic South American marsupial and primitive placental faunas is confidently attributed to the establishment of a land bridge with North America in the late Pliocene and the consequent invasion of more advanced placentals from the north.

Patterns of Extinction

A survey of extinction intensities over time (fig. 2) reveals an on-going ‘background’ level of extinction, punctuated by a number of very high peaks of intense extinction: the so-called mass extinctions.

The background extinction data displays two patterns: First there is an apparent decline in extinction intensity values through the Phanerozoic; very high rates of extinction seemed to be ‘normal’ for the early Paleozoic, gradually dropping away to levels around 5 to 10% from about the Jurassic onwards.


Fig. 2: Marine genus-level extinction data aggregated at the stage level of geological time for the Phanerozoic. US = Upper Solvan; Do = Dolgellian; Ag = Ashgillian; Fr = Frasnian; Ta = Tatarian; No = Norian; Ma = Maastrichtian. (After MacLeod 2002.)
The second pattern cannot be discerned at a casual glance, and was first reported by Raup (1991, fig. 4-4): when all time intervals from the Phanerozoic are ranked in order of extinction intensity, the intensities are found to be distributed as a continuous exponential series grading smoothly from low background extinction levels to the greatest mass extinction. There is no marked discontinuity separating mass extinctions from the range of other events when the Phanerozoic is considered as a whole. However, more recently it has been noted that the six highest peaks do appear to form a discrete set if the very high background levels of the Cambrian and Early Ordovician are excluded from the data (Bambach & Knoll 2001).

Mass Extinctions

Owing to the incompleteness of our understanding of the fossil record, mass extinctions are harder to pin down than it might seem, and the task becomes more difficult the farther one goes back in time. Very ancient rocks are poorly represented today, so we cannot say with surety than a given assemblage went extinct within a geologically short interval or not; the critical horizon may simply not be available for sampling.

So it is not presently known for sure how many mass extinctions have occurred throughout the history of life on earth, and different authors offer varied interpretations. There is good evidence available for most of Phanerozoic, however, and nearly every recent publication will list the following events as being of the greatest severity:

  1. mid Cambrian (Upper Solvan)
  2. at the close of the Cambrian (Dolgellian)
  3. at the close of the Ordovician (Ashgillian)
  4. near the end of the Devonian (Frasnian-Famennian)
  5. at the Permian-Triassic boundary (Tatarian)
  6. at the Triassic-Jurassic boundary (Norian)
  7. at the Cretaceous-Tertiary boundary (Maastrichtian)

Although the Cretaceous-Tertiary event is by far the most widely familiar – on account of its association with the ever-popular dinosaurs and Alvarez’ exciting platinum/iridium evidence for the role of a major meteor impact – the event at the end of the Permian was by far the most profound. At this time something like 60% of all species (and perhaps 90% of all marine forms) went extinct.

Next, in terms of severity, was probably the Ashgillian (latest Ordovician) event when perhaps 50% of marine species went extinct.

Third, the Triassic-Jurassic event claimed something like 45% of species each, followed closely by the mid-Cambrian, Cretaceous-Tertiary and other events, all in the 40-45% range.

(These estimates of species mortality after Raup 1991, fig. 4-4, and MacLeod 2002.)

Some of the characteristics of mass extinctions seem to be:

  1. More intense extinction of tropical forms.
  2. Extinction strikes in both the land and the sea, though higher rates are generally cited among marine forms.
  3. On the land, while animals suffer repeatedly, plants seem to be more resistant to mass extinctions.
  4. There has been a suspicion that fish might also be more than usually resistent to mass extinction events, but this idea has been rejected as an artefact of preservation (see below).

However, species mortality alone may not tell the full story. Ecology may be important too: in other words, not all taxa are equal. Mary Droser of UC Riverside and her colleagues have compared the late Devonian and end-Ordovician extinctions. The Ordovician extinction was significantly the greater in terms of number of taxa killed, but did not disrupt global ecology to the extent of the end-Devonian extinction (Droser et al. 2000).

An unresolved question is whether mass extinction events represent a vastly increased rate of natural selection, in which the least well-adapted organisms are killed preferentially, or whether they are the result of catastrophic change that randomly eliminates taxa regardless of adaptation (Brenchley 2002).

Causal Mechanisms

“[T]hree particular mechanisms have emerged as being the most likely to cause such large-scale changes in the earth’s environment that they might plausibly be considered possible ‘mass extinction’ mechanisms: sea level change, continental flood-basalt volcanism, and asteroid/comet impact” (MacLeod 2002).

A postulated equal spacing, or periodicity in geological time, occurring about every 26 million years, offered the intriguing prospect of an astronomical explanation for all mass extinction phenomena. However, subsequent investigation has not demonstrated any real periodicity (Stigler & Wagner 1987).

“The Phanerozoic asteroid/comet impact record exhibits a strikingly low correspondence with the Phanerozoic stage-level extinction record. Although some type of asteroid/comet impact occurs in virtually all of the Permian–Recent stages that contain a ‘mass extinction’, neither impact size nor impact number exhibits a statistically significant association with extinction magnitude.

“Sea level change exhibits a mixed association with Phanerozoic extinction history. Of the 14 major global sea level falls, seven occur in stages containing an elevated extinction intensity peak. While this is not a statistically significant result, it is interesting to note that the Palaeozoic portion of this distribution exhibits a considerably higher association (five out of seven) than the Mesozoic and Cenozoic records. Nevertheless, global sea level falls are associated with each of the three largest stage-level extinction events of the last 250 million years. These observations suggest that sea level plays (at the very least) a substantial contributory role in accentuating stage-level extinction intensities.

“Unlike the bolide impact–extinction intensity comparison, large continental flood-basalt volcanic events exhibit a near-perfect stage-level association with marked increases in Mesozoic and Cenozoic extinction intensity. Ten out of the 11 major flood-basalt eruption events of the last 250 million years occur during a stage that contains a local extinction peak (i.e. mass extinction). It is presently unknown whether this association is scale-dependent or extends to smaller events as well” (MacLeod 2002; also see Wignall et al. 2009).

Isotope Studies

Most major extinction events are associated with pronounced isotopic excursions. The platinum/iridium anomaly at the KT boundary is so famous now that every school child of a certain age knows about it, but it is the carbon and oxygen isotopes which provide us with most of our information. It is the large negative d13C excursion (e.g., see Fig. 3) which normally suggests to us “mass extinction” and d18O provides us with paleotemperature data.

“This isotopic evidence stems from the fact that the carbon atom has two stable isotopes, carbon-12 and carbon-13. The 12C/13C ratio in abiotic mineral compounds is 89. In biological syntheses, the processing of carbon CO2 and carbonates gives a preference to the lighter carbon isotope and raises the ratio to about 92. Consequently, the carbon residues of previously living matter may be identified by this enrichment in 12C. A compilation has been made of the carbon isotopic composition of over 1600 samples of fossil kerogen (a complex organic macromolecule produced from the debris of biological matter) and compared with that from carbonates in the same sedimentary rocks. This showed that biosynthesis by photosynthetic organisms was involved in all the sediments studied. In fact, this enrichment is now taken to be one of the most powerful indications that life on Earth was active nearly 3.9 billion years ago because the sample suite encompasses specimens right across the geological time scale” (Brack 2001).

A Selective Survey

The following is a brief review of some extinction and "extinction-like" events. It is not comprehensive, nor should inclusion in the following survey be equated with "importance" in any sense; events have been chosen just because good information was available or because they seemed interesting.

Proterozoic

“[B]etween about 900 Ma and perhaps 675 Ma ago, the world’s planktonic micro-algal flora experienced a major collapse – the ‘wane’ in Proterozoic eukaryotic history. Although the occurrence of such a decline is well-documented, the reasons for the collapse, and even its exact timing, are somewhat uncertain. These are new questions ... and the data are not all in. ... [H]owever, during this period there appears to have been a decrease in global atmospheric carbon dioxide, a decline possibly responsible for the onset of widespread continental glaciation.... Moreover, this decrease in atmospheric CO2 appears to have been coupled with an increase in the concentration of atmospheric oxygen (brought on by rapid burial at this time of photosynthetically produced organic matter). As it turns out, these two gases, CO2 and O2, are both able to react with ... the carboxylase/oxygenase enzyme that drives photosynthesis. Experiments with populations of living micro-algae show that if the concentration of CO2 is decreased while that of O2 is increased, photosynthetic activity rapidly diminishes, ultimately reaching a point at which the micro-algae completely cease to grow. Perhaps this is what occurred, on a global scale, during the late Proterozoic” (Schopf 1992b, p. 58).

Ediacaran-Cambrian

Although some Ediacaran taxa are now known to have persisted, and others may have evolved into different forms, most of them simply vanish from the fossil record near the beginning of the Cambrian. Some believe this is evidence of a mass extinction.

In the past few years, evidence has mounted for a strong (7 to 9 per mil) but short-lived negative perturbation in the carbon cycle close to the Vendian-Cambrian boundary. The causes and meaning of this event remain uncertain, but more recent, comparable perturbations coincide with widespread extinction. For example, the Permo-Triassic mass extinction is marked by a similar though smaller excursion (Knoll & Carroll 1999).

One school of thought holds that Ediacarans may have been largely wiped out by a supposed nutrient crisis – ‘Kotlin Crisis,’ see Brasier 1992 – immediately prior to the Vendian-Cambrian boundary.

However, other researchers observe that a mass extinction event is not necessary to explain the disappearance of the Ediacarans from the fossil record; conditions may simply have ceased to be favourable to their preservation with the arrival of more numerous and more diverse scavenging and bioturbating organisms.

It must be emphasised that whatever took place during the Vendian-Cambrian transition is not known to have been a mass extinction. “We cannot tell how abruptly the Ediacaran Faunas became extinct, but only a very small number are represented by possible survivors...” (Briggs et al. 1994, p. 46).

Late Cambrian (Dolgellian, Marjuman-Steptoean)

“Three significant paleoceanographic events are juxtaposed in Upper Cambrian (Steptoean Stage) sequences of Laurentia:

  1. a mass extinction of trilobites that marks the base of the Steptoean (Marjumiid-Pteroceaphliid biomere);
  2. a large positive excursion in carbon isotope values that spans much of the Steptoean (SPICE event); and
  3. an imprecisely dated craton-wide drop in sea level (Sauk II – Sauk III event) that was terminated by widespread flooding in the Late Steptoean (mid-late Elvinia Zone time).

The first two events are clearly global in scope, but the scale and timing of the sea level drop is not known in detail outside Laurentia. We have recently undertaken a project aimed at evaluating whether there are demonstrable cause and effect relationships among these events in widely separated Laurentian sections.

“New carbon isotope data from sequences in western Newfoundland and northern Utah indicate that the acme of the Sauk II – Sauk III regression and the peak of the SPICE excursion are essentially synchronous events. This pattern is identical to that seen in the Upper Mississippi Valley, as determined previously by two of us working independently on a core in central Iowa. The simplest explanation is glaciation, though we continue to explore alternatives. The precise relationship between the basal Steptoean extinction event and the beginning of the SPICE are more problematic, although they also appear to correlate closely in time. The rapid mixing of sub-thermoclinal waters onto the shelf remains a viable hypothesis. Perhaps the most challenging question relates to the nature of sea level changes during the period of time spanning the base of the Steptoean Stage and prior to the mid-Steptoean acme of regression. Our preliminary analysis suggests that a brief sea level rise marks the basal Steptoean interval, and that the more protracted Sauk II – Sauk III fall in sea level begins relatively early in the Steptoean” (Saltzman & Runkel 2001).

Ordovician

The early to mid-Ordovician radiation of Paleozoic faunas established complex suspension-feeding communities, characterized by brachiopods, bryozoans, crinoids and corals, which reached an equilibrium generic diversity by the start of the Late Ordovician. This equilibrium was ended “by the Late Ordovician mass extinction at ~439 Ma. The extinction occurred in two main phases, the first at the start of the Hirnantian Stage and the second in the mid-part of that stage ..., an estimated 0.5 to 1 million years apart” (Brenchley 2002).

“The Late Ordovician mass extinction (LOME), one of the five largest Phanerozoic biodiversity depletions, occurred in two pulses associated with the expansion and contraction of ice sheets on Gondwana during the Hirnantian Age. It is widely recognized that environmental disruptions associated with changing glacial conditions contributed to the extinctions, but neither the kill mechanisms nor the causes of glacial expansion are well understood. Here we report anomalously high Hg [mercury] concentrations in marine strata from south China and Laurentia deposited immediately before, during, and after the Hirnantian glacial maximum that we interpret to reflect the emplacement of a large igneous province (LIP). An initial Hg enrichment occurs in the late Katian Age, while a second enrichment occurs immediately below the Katian-Hirnantian boundary, which marks the first pulse of extinction. Further Hg enrichment occurs in strata deposited during glacioeustatic sea-level fall and the glacial maximum. We propose that these Hg enrichments are products of multiple phases of LIP volcanism” (Jones et al. 2017, Abstract).

Permian-Triassic

Of all Phanerozoic mass extinctions, the event at the end of the Permian was by far the most profound. At this time something like 60% of all species (and perhaps 90% of all marine forms) went extinct. The last few trilobites disappeared during this event although, as a class, they were already a very impoverished group by then.

No clear explanation is available and it is possible that the extinction was an emergent consequence of many factors which, individually, may have been too subtle to leave much evidence. However, “[w]idespread basaltic volcanism occurred in the region of the West Siberian Basin in central Russia during Permo-Triassic times. New 40Ar/39Ar age determinations on plagioclase grains from deep boreholes in the basin reveal that the basalts were erupted 249.4 ± 0.5 million years ago. This is synchronous with the bulk of the Siberian Traps, erupted further east on the Siberian Platform. The age and geochemical data confirm that the West Siberian Basin basalts are part of the Siberian Traps and at least double the confirmed area of the volcanic province as a whole. The larger area of volcanism strengthens the link between the volcanism and the end-Permian mass extinction” (Reichow et al. 2002, p. 1846, Abstract).

“Fundamental to understanding its cause is determining the tempo and duration of the extinction. Uranium/lead zircon data from Late Permian and Early Triassic rocks from south China place the Permian-Triassic boundary at 251.4 ± 0.3 million years ago. Biostratigraphic controls from strata intercalated with ash beds below the boundary indicate that the Changhsingian pulse of the end-Permian extinction, corresponding to the disappearance of about 85 percent of marine species, lasted less than 1 million years. At Meishan, a negative excursion in d13C [see Fig. 3] at the boundary had a duration of 165,000 years or less, suggesting a catastrophic addition of light carbon” (Bowring et al. 1998, p. 1039).

“The marine sediments of Jameson Land, East Greenland, preserve a unique record of the Permian-Triassic extinction event. High rates of sedimentation, controlled by active faulting, have produced a greatly expanded succession compared to other sections worldwide. In addition, the sediments have suffered remarkably little burial and thermal alteration and contain well preserved marine organisms and terrestrially derived plant remains. For the first time, it is possible to compare the terrestrial and marine fossil records of this extinction event using the same samples from the same sections. These studies have importance for palaeoecology as well as for correlation between marine and terrestrial sections worldwide. Marine ecosystem collapse is signalled by a sharp reduction in bioturbation, a disappearance of Permian taxa, a sharp decrease in the 13C curve (interpreted as an indication of productivity collapse) and the appearance of widespread oxygen restriction. This all occurs within approx. 50 cm at the top of the Schuchert Dal Formation. During the same interval, there is also a turnover in the terrestrial ecosystem from gymnosperm to pteridosperm dominated floras. Studies of sections in northern Italy also suggest that a similar synchronicity exists in the timing of ecosystem recovery in both marine and terrestrial realms during the late Lower Triassic” (Looy & Twitchett 1999).

“There is a widespread belief that ‘throughout their history fish have proved virtually immune to mass extinction events’” (Hallam & Wignall 1997, p. 71). During the Permian-Triassic extinction event fish diversity seems to have increased, whereas all other marine groups declined sharply. This presents an ecological problem: how can organisms at the top of a food chain survive a widespread collapse in productivity and biomass at lower trophic levels? Good Permian-Early Triassic fish faunas are known only from a few areas worldwide: Madagascar, East Greenland and Spitsbergen. One possibility is that in these areas the extinction was less severe than elsewhere and thus the food chain remained relatively intact. However, palaeoecological data from East Greenland show that conditions were just as bad here as elsewhere. Instead, it appears that fish were better preserved in the Lower Triassic than in the latest Permian. This is due to widespread benthic anoxia, high sedimentation rates and rapid concretion formation in the narrow, fault controlled basin of East Greenland. In contrast, benthic invertebrate groups show much worse preservation in the Lower Triassic than in the Late Permian (due to a dearth of silicified faunas). When the number of Lazarus taxa are taken into account, fish diversity shows a similar pattern through the Permian-Triassic interval as all other marine groups” (Twitchett 1999).

An analysis of “global Permian and Triassic plant data in a paleogeographic context show that the scale and timing of effects [on plants] varied markedly between regions” and that “the patterns are best explained by differences in geography, climate, and fossil preservation, not by catastrophic events” (Rees 2002, p. 827).


Fig. 3. Carbon isotope curve at Meishan section; from Bowring et al. 1998.

Triassic-Jurassic

“The Triassic period came to a close with a wave of extinctions. Most of the protomammals disappeared, as did the crocodile-like phytosaurs, all large amphibians, and some of the seagoing reptiles. In the sea the ... ammonites nearly came to an end.... The coral reefs of the time disappeared from the earth. This event remains shrouded in mystery. Its record is found at only a few places on earth” (Ward 2000, p. 99).

Cretaceous-Paleogene

Surely the most widely reported – and in the lay media, at least, the most widely misreported – of all extinction events is that which brought the age of the dinosaurs to an end at the close of the Maastrichtian: the “KT” extinction.

The exact timing and nature of the end-Cretaceous mass-extinction event is famously contentious, so let us be clear about two things right from the outset: First, a bolide did strike the Earth at the end of the period. In fact, the Cretaceous-Paleogene boundary is essentially defined by this event [→ sidebar], so any suggestion that the impact occurred before or after the end of the Cretaceous is simply nonsensical. But, second, the almost unimaginably vast Deccan Traps volcanism was in full swing at the same time. Both of these events inevitably influenced the climate, the atmosphere, and life, to a great degree. One may reasonably argue about which event had the most influence over a particular group of organisms at a particular place and time, but to adopt one or other phenomenon as a complete explanation for the mass extinction, to the outright exclusion of the other, strikes me as ideological. That is not science.

“The idea of mass extinctions of life, traditionally by great floods, still has a strong hold on western imagination. Everyone has a favourite theory for major extinctions, all united by the common theme of attributing dominant importance to physical factors, and playing down the importance of normal biological mechanisms. Our own work strongly favours the diversification of both mammals and birds at least 30 million years before the extinction of dinosaurs – there must be ecological consequences for small dinosaurs from this early diversification” (Penny 2001). Fossil evidence also supports a progressive change in the composition of mammal communities across the K-T boundary, although dating uncertainties have complicated any simple interpretation of this data (e.g. Lofgren 1995).

“Recent field and laboratory investigations have established that the latest Cretaceous (i.e. Campanian-Maastrichtian) sedimentary succession exposed within the James Ross Basin, Antarctica is in excess of 2 km in total thickness. Comprising essentially fine-grained, shallow-water, volcaniclastic rocks that are in places intensely fossiliferous, it represents one of the best opportunities to investigate palaeobiological and palaeoenvironmental changes leading up to the K-T boundary anywhere in the southern hemisphere. The exceptionally early extinction patterns of the inoceramid bivalves and belemnites can be confirmed, but it is apparent that other key groups such as the ammonites [also see Witts et al. 2018] and trigoniid bivalves go right up to the boundary itself. Studies throughout the 1000 m thick Maastrichtian sequence indicate that, although molluscan assemblages are abundant, they are never particularly diverse. The benthic element has a distinctly temperate aspect and there is both sedimentological and palaeontological evidence to suggest that it was subjected to periodic intervals of reduced oxygen levels. The comparatively small, but nevertheless still abrupt, extinction event at the end of the Cretaceous in Antarctica may well have been buffered to some extent by both the high-latitude position and unusual sedimentological setting of the basin” (Crame 1999).

A number of authorities have reported multiple iridium spikes in the vicinity of the Cretaceous-Tertiary boundary (Ganapathy et al. 1981; Donovan et al. 1988; Graup & Spettel 1989; Bhandari et al. 1995, 1996; Zhao et al. 2002). These observations do not fit comfortably with the theory of a single, massive bolide impact being largely responsible for numerous end-Cretaceous phenomena. In the Nanxiong Basin, China, the evidence suggests that “the K/T event was not marked by an instantaneous geochemical environmental change, but stretched out over a considerable time” (Zhao et al. 2002, p. 10).

The end of the Cretaceous, which is of course the same as the base of the next overlying unit, the Paleogene, and also the lowermost subunit within that, the Danian, is defined by the presence of ejecta from the extraterrestrial impact which occurred approximately 66 million years ago. (The current best estimate is 66.04 Ma.)

As described by Gradstein et al. 2012, p. 859, the “Global Boundary Stratotype Section and Point” (GSSP) for this horizon “has been fixed in a section of Oued Djerfane, 8 km west of El Kef (168 km southwest of Tunis), in Tunisia ... and ratified by IUGS in 1991. Details of the GSSP section and a summary of the studies since the original definition have been published by Molina et al. (2006). The GSSP level has been defined at the rusty colored base of a 50 cm thick boundary clay. A similar layer occurs in many K/Pg sections worldwide and it includes an iridium anomaly, microtektites, Ni-rich spinel crystals and shocked quartz. ... The bulk of this layer was deposited during a few days and the level is considered isochronous all over the world, in marine as well as in continental sections. ... The GSSP level represents the moment of the extraterrestrial impact, implying that all sediments generated by the impact are already Danian in age.”

Gradstein et al. 2012 further notes some of the many extinctions which appear to occur at more or less exactly the same time, but these two ideas should not be conflated. The coincidence of the impact ejecta with the base of the Paleogene is a matter of definition whereas the exact timing of many extinctions is still poorly resolved and contentious, and the precise causes even more so.

Modern

What’s it like to be in the middle of a mass extinction? Well, nobody knows for sure, but some people believe we are experiencing one right now.

However, a major misconception about the “modern” mass extinction, embodied in the very name, is that it is just now beginning. The first of the Pleistocene ice ages commenced some 2.5 million years ago and, as thick ice sheets began to cover much of northern America and Europe, the sea level dropped, and both marine and terrestrial species began to die off.

Man’s role in the modern mass extinction is therefore somewhat problematic; we can be sure that predation and ecological disruption due to pre-human activities were inconsequential 2.5 Ma ago, when the extinction began, yet we can be equally sure that human activities are a major force today.

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