The Kt Event

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Often the only question people ask about the dinosaurs is why they died out. Paraphrasing the words of Malcolm in Macbeth, 'nothing in [their] life became [them] like the leaving it'. Over the years, hundreds of theories for this disappearance 65 Myr ago at the

Cretaceous-Tertiary (KT) boundary have been proposed. It might seem odd that there is still so much debate: after all, the KT boundary is the most studied point in geological time. Despite all this study, however, many key questions remain about the timing of the event, the patterns of what died out and what survived, and the precise nature of the physical environmental crisis.

8.12.1 What died out?

Among terrestrial tetrapods, the dinosaurs and

Fossil Range And Relative Abundance

Fig. 8.39 Phylogenetic tree of the diapsid reptiles and turtles, showing postulated phylogenetic relationships of the main groups (dashed lines), known fossil record of each (vertical time dimension) and their relative abundance through time (horizontal axis). Mass extinctions, and possible mass extinctions, are noted with asterisks on the left.

Fig. 8.39 Phylogenetic tree of the diapsid reptiles and turtles, showing postulated phylogenetic relationships of the main groups (dashed lines), known fossil record of each (vertical time dimension) and their relative abundance through time (horizontal axis). Mass extinctions, and possible mass extinctions, are noted with asterisks on the left.

pterosaurs disappeared, as well as several families of birds and marsupial mammals. In the sea, plesiosaurs, mosasaurs and some families of sharks and teleost fishes disappeared. The ichthyosaurs had dwindled and died out 30 Myr earlier. Among non-vertebrates, many important Mesozoic groups disappeared: the ammonites, belemnites, rudist bivalves and various plankton groups. Many groups, such as diatoms, radio-laria, benthic foraminifera, brachiopods, gastropods, some fishes, amphibians, turtles, lizards and terrestrial plants, were apparently little affected (MacLeod et al., 1997).

It is hard to separate the survivors and non-survivors into simple ecological categories. Most of the land animals that survived were small, except for certain crocodilians. Most of the marine forms that died out were free-swimming or surface forms (plankton, ammonites, belemnites), but of course many open-water fishes survived. Among forms that lived on the sea-bed, it was mainly the filter-feeders like corals, bryozoans and crinoids that suffered extinction (possibly by loss of plankton food?), whereas forms that fed on detritus were little affected.

Are there any convincing ecological correlates of extinction risk and survival? Jablonski and Raup (1995) looked at how marine bivalves fared across the KT boundary, and found that the only factor that promoted survival was the geographical range of genera. In other words, a species of bivalve could insure itself against the risk of a mass extinction by belonging to a genus that was distributed over several faunal provinces. Diet, body size, mode of reproduction, water depth inhabited, ability to burrow, latitudinal distribution (tropical versus temperate) and other factors made no difference whatever to survival chances. There is little evidence for selectivity during the KT extinction event, so that any explanation has to account for an essentially random set ofdisappearances.

8.12.2 How long did it take?

Some geologists assume that all major extinctions occurred essentially instantaneously, in as little as one week or one year. Others posit a 'sudden' event, but allow several thousands or tens of thousands ofyears. At the level of discrimination that is possible, there is no way of distinguishing such time spans because both appear to be the same in the geological record.

Radiometric dating of KT boundary volcanic rocks using the 40Ar/39Ar technique can give dates in Myr with uncertainties of as little as ±0.1 Myr. This technique works well when such igneous rocks are available, but the majority of fossil-bearing KT sequences are not associated with volcanics. Another technique is to measure the polarity of magnetization of rocks. Every few Myr, the Earth's north and south magnetic poles flip over, and all iron-bearing minerals in rocks that are just being formed acquire the relevant magnetization. In the latest Cretaceous, Earth's polarity changed eleven times, the KT boundary lying in polarity band 29R (i.e. reversed), which lasted as little as 0.5Myr. Magnetostratigraphic techniques can identify the likely age of particular geological formations, but the precision is still too poor for a decision on the exact duration of the KT event.

Classic sedimentology and stratigraphy, however, suggest that the KT event was effectively instantaneous, even though an exact age date cannot be assigned. Some two hundred KT boundary sections have been recorded from all over the world, in sediments deposited under the sea and on land, and they all show the same succession (Smit, 1999).The KT boundary is defined formally at the base of the boundary layer, and then follows an ejecta layer and a clay enriched in iridium.

Not only was the KT event rapid, but it happened in early June! Wolfe (1991) examined sediments in a lily pond less than 2 m deep that straddles the KT boundary. He tracked a series of catastrophic events in the pond, including freezing of the fossil lily leaves, which is shown by irregular folds on the surface, for a period of less than 2 months. In all, the sequence of catastrophic events, he argues, lasted from 3 to 4 months.

8.12.3 The pattern of extinction

Did all the plant and animal groups that died out near the end of the Cretaceous do so essentially at the same time (catastrophic event) or over a span of several million years (gradual event)? The evidence suggests that some groups disappeared catastrophically right at the KT boundary, whereas others were in long decline before the end ofthe Cretaceous.

The ichthyosaurs, for example, disappeared 30 Myr earlier than the KT boundary, and the ostracods, bry-ozoa, ammonites, bivalves, plesiosaurs and pterosaurs had apparently dwindled to low diversities (MacLeod et al., 1997). Such claims require careful checking because of the quality of the record. An apparently gradual pattern of extinction may arise if all the last fossils have not been collected. The first studies of ammonite distributions in the north of Spain suggested that the group had dwindled to very low diversities by the very end of the Cretaceous, but more intensive collecting (Ward, 1990) showed that several species survived right up to the boundary. What was a gradual pattern became catastrophic after more intensive collecting. Likewise, an apparently catastrophic pattern can arise if there is a gap in sedimentation: many species apparently disappear at one level, but that is simply because a long interval of time is missing above.

What of the dinosaurs and other vertebrates? The stratigraphic ranges of vertebrates across the KT interval certainly indicate a mass extinction: one estimate (Benton, 1989a) showed that 28 out of 89 families of terrestrial tetrapods died out at that time, a percentage loss of 31%. Revised results calculated (Table 8.1) from chapters in Benton (1993a), indicate an overall loss of 64 out of 210 families of vertebrates, a drop of 30%. This is made up (Figure 8.40) from the extinction of 14 out of 94 families of fishes (15% loss) and 50 out of 115 families of tetrapods (43% loss). The highest extinction rates, inevitably, are for dinosaurs, pterosaurs and ple-siosaurs (all 100%), with high rates also for birds and marsupials (both 75%). Among other groups, crocodil-ians (36%) and turtles (27%) lost more than a quarter of families, but extinction rates for fishes, amphibians, lizards and snakes, basal mammals and placental mammals are all lower than 15%, and hence not different from normal, or 'background', extinction rates.

Table 8.1 Data on the rates of extinction of vertebrates at the KT boundary. Figures are based on the numbers of families extant during the Maastrichtian Stage and the numbers that died out some time during that time interval. All data are taken from chapters in Benton (1993a).

Group Families extant Families extinct Extinction rate %

Table 8.1 Data on the rates of extinction of vertebrates at the KT boundary. Figures are based on the numbers of families extant during the Maastrichtian Stage and the numbers that died out some time during that time interval. All data are taken from chapters in Benton (1993a).

Chondrichthyes

44

8

18

Bony fishes

50

6

12

Amphibians

11

0

0

Reptiles

71

36

51

Turtles

15

4

27

Lizards and snakes

16

1

6

Crocodilians

14

5

36

Pterosaurs

2

2

100

Dinosaurs

21

21

100

Plesiosaurs

3

3

100

Birds

12

9

75

Mammals

22

5

23

Basal groups

11

1

9

Marsupials

4

3

75

Placentals

7

1

14

All vertebrates

210

64

30

Fishes

94

14

15

Tetrapods

116

50

43

Amniotes

105

50

48

These results confirm that reptiles as a whole, and dinosaurs in particular, suffered a devastating loss of diversity at the end of the Cretaceous. The figures are based on documentation at the level of the stratigra-phic stage, and the last stage of the Cretaceous, the Maastrichtian, was probably 5-8 Myr long. It is hard to correlate precisely the ages of rocks from Asia to Europe to North America, so it is not clear whether all the dinosaurs, pterosaurs, plesiosaurs, mosasaurs and other groups died out right at the end of the Maastrichtian, or scattered through the whole time span.

Local studies are required. The richest terminal Cretaceous dinosaur beds are in western North America, the Hell Creek Formation of Montana and the Lancian (Ferris Formation) ofWyoming,but detailed collecting has thrown up controversial results. Early studies of the Hell Creek Formation suggested that there had been a long-term decline among dinosaurs and other tetrapods through the last 5 Myr of the Cretaceous. In more detailed work, Archibald and Bryant (1990) sur veyed collections of 150,000 specimens, representing 111 species of fishes, amphibians, reptiles and mammals from latest Cretaceous and earliest Tertiary beds of north-east Montana. They found that 36-47% of commoner species died out across the KT boundary,but apparently over a long span of time.

On-the-ground collecting, however, shows no long-term decline. Sheehan et al. (1991) summarized 15,000 hours of fieldwork by scores of volunteers who marched across the Hell Creek Formation picking up anything that did not move. The thousands of bones collected were plotted against time and the majority of extinctions appeared to fall right at the KT boundary. Hurlbert and Archibald (1995) argued that Sheehan and colleagues had over-interpreted their data,whereas Sheehan and Fastovsky (1992) reinterpreted the Archibald-Bryant data base to show that, although freshwater fishes and tetrapods (amphibians, turtles, crocodiles) showed a species extinction rate of only 10%, 88% of the fully terrestrial species died out at the

Amphibian Extinction Rates

Fig. 8.40 The proportions of different vertebrate families that became extinct during the KT event, based on data from Benton (1993a), listed in Table 8.1.Note the 100% extinction of pterosaurs, dinosaurs and plesiosaurs, but 0% extinction of amphibians, and extinction rates of less than 20% for fishes, lizards and snakes, basal mammals and placental mammals.

Fig. 8.40 The proportions of different vertebrate families that became extinct during the KT event, based on data from Benton (1993a), listed in Table 8.1.Note the 100% extinction of pterosaurs, dinosaurs and plesiosaurs, but 0% extinction of amphibians, and extinction rates of less than 20% for fishes, lizards and snakes, basal mammals and placental mammals.

KT boundary, hence making it a catastrophic event. Sheehan et al. (2000) went on to show that a long-held belief, that there were no dinosaurs in the last 3 m of the Hell Creek Formation, immediately below the KT boundary, was false: they found as many bones in that interval as in any other 3 m unit and concluded that dinosaur extinction was abrupt. This result was confirmed by Lillegraven and Eberle (1999) in the Lancian of Wyoming, who found no evidence for a decline among dinosaur species, but rather a geologically sudden disappareance. They note, however, that there is a zone of uncertain stratigraphy 8m thick spanning the KT boundary, so cannot rule out the possibility of a decline over thousands, or tens of thousands, of years. Contrary to some previous statements, mammal fossils are rare in the dinosaur-bearing beds and mammals appear in some diversity and abundance only after the disappearance ofthe dinosaurs.

In Montana and Wyoming, then, several dinosaur families lasted right to the end of the Cretaceous: the tyrannosaurids, ornithomimids and dromaeosaurids among theropods, the nodosaurid and ankylosaurid ankylosaurs, the hypsilophodontid and hadrosaurid ornithopods, the pachycephalosaurids and the proto-ceratopsid and ceratopsid ceratopsians. The latest Cretaceous of western North America teemed with familiar, and highly successful, dinosaurs such as Ankylosaurus, Triceratops and Tyrannosaurus, and their disappearance was abrupt.

8.12.4 Theories of extinction

Over the years, more than a hundred hypotheses have been presented for the extinction of the dinosaurs (Benton, 1990b). A common view in the latter half of the nineteenth century and in the first three decades of the twentieth was that the dinosaurs simply died out because their time had come—they were described by many palaeontologists as prime victims of racial senility—their genetic potential was exhausted, they exhibited giantism (if not acromegaly), excessive spin-osity and a loss of the ability to adapt. From about 1920, dozens of hypotheses were put forward, ranging from the physiological (slipped discs, excessive hormone production, loss of interest in sex) to the ecological (competition with mammals, change in plant food), from the climatic (too hot, too cold, too wet) to the terrestrial catastrophic (vulcanism, magnetic reversal), from the topographic (marine regression, mountain building) to the extraterrestrial (sunspots, cometary impact). Many of these explanations were little more than whims, and most were hard to couch in terms that would allow them to be tested. Present hypotheses are more 'scientific'.

There are three current models to explain the KT event.

1 The gradualist ecological succession model sees a decline caused by long-term climatic changes in which the subtropical lush dinosaurian habitats gave way to the strongly seasonal temperate conifer-dominated mammalian habitats. The evidence for this hypothesis is mainly palaeontological and stratigraphic.

2 The catastrophist vulcanological model explains the geochemical data in the boundary rocks by means of a major volcanic eruption that caused abrupt extinction.

3 The catastrophist extraterrestrial model explains the extinction as a result of the after-effects of a major extraterrestrial impact on the Earth from geochemical and astrophysical evidence.

A catastrophist would argue that the main extinction event lasted less than a year, or perhaps as much as a few hundred or thousand years, whereas a gradualist would argue for a longer-term decline lasting for 1Myr or more.

The gradualist model sees declines in many groups of organisms (Archibald, 1996a; MacLeod et al., 1997) caused by long-term climatic changes in which the subtropical lush dinosaurian habitats gave way to the strongly seasonal temperate conifer-dominated mammalian habitats. The gradualist scenario has been extended to cover all aspects of the KT events on land and in the sea, with evidence from the gradual declines of many groups through the Late Cretaceous. Climatic changes on land are linked to changes in sea level and in the area of warm shallow-water seas, and the impact and volcanism are either discounted or seen as the coup de grâce. This position is disputed by those who claim the extinctions were rapid and the apparent long-term declines are artefacts ofincomplete collecting.

The second school of thought has focused on explaining the KT event by volcanic activity (Courtillot, 1999). The Deccan Traps in India represent a vast outpouring of lava that occurred over the 2-3 Myr spanning the KT boundary. In some interpretations, the volcanic model explains instantaneous catastrophic extinction, whereas in others it allows a span of 3 Myr or so, for a more gradual dying off caused by successive eruption episodes. Petrologists and geochemists argue that the shocked quartz and iridium spike could not be produced by any known kind of volcano, that the geochemistry of the glassy spherules indicates a source from rocks lying below a postulated impact site and that they do not have a volcanic signature (see below).

The impact hypothesis was presented in 1980, when Luis Alvarez and colleagues published their view that the extinctions had been caused by the impact of a 10 km diameter asteroid on Earth. The impact caused massive extinctions by throwing up a vast dust cloud that blocked out the sun and prevented photosynthesis, and caused freezing, and hence plants died off, followed by herbivores and then carnivores. There are four key pieces of evidence for the impact hypothesis.

1 An iridium anomaly worldwide. Iridium is a platinum-group element that is rare on Earth's crust and reaches Earth from space in meteorites at a low average rate of accretion. At the KT boundary, that rate increased dramatically, giving an iridium spike (Figure 8.41).

2 Shocked quartz has been found in many sections, especially close to the impact site (Smit, 1999). These are grains of quartz bearing criss-crossing lines produced by the pressure of an impact.

3 Glassy spherules also occur abundantly at the base of the boundary clays from sites close to the impact site. These were produced by melting of the rock beneath the crater and were then thrown through the air in the aftershock.

4 A fern spike (Figure 8.39) is found in many terrestrial KT boundary sections, indicating an abrupt shift in pollen ratios from angiosperm-dominated to ferndominated. This indicates the aftermath of a catastrophic ash fall: ferns recover first and colonize the new surface, followed eventually by the angiosperms after soils begin to develop. This interpretation has been made by analogy with observed floral changes after major volcanic eruptions.

Fig. 8.41 A typical iridium spike (left) and fern spike (right) from a core taken through the KT boundary in freshwater coal swamp deposits in York Canyon, New Mexico, USA. Note that both the iridium abundances, measured in parts per trillion (ppt), and the ratios of angiosperm-pollen:fern-spores are plotted on logarithmic scales. (Courtesy of Carl Orth.)

Fig. 8.41 A typical iridium spike (left) and fern spike (right) from a core taken through the KT boundary in freshwater coal swamp deposits in York Canyon, New Mexico, USA. Note that both the iridium abundances, measured in parts per trillion (ppt), and the ratios of angiosperm-pollen:fern-spores are plotted on logarithmic scales. (Courtesy of Carl Orth.)

North America

"XZ7

0 500 km

Sea shore at the end ot the Cretaceous

Atlantic Ocean

North America

"XZ7

0 500 km

Sea shore at the end ot the Cretaceous

Atlantic Ocean

Cretaceous-Tertiary boundary deposits

Marine with evidence of large waves

South America

/\ Nonimarine

Fig. 8.42 Evidence for the impact site: (a) location ofChicxulub Crater,on the Yucatán Peninsula,Mexico,as well as the end-Cretaceous coastline of the proto-Caribbean Sea and sites indicating activity of tsunamis (tidal waves); (b) the KT boundary section at Beloc, showing the sequence of arrivals of airfall debris and tsunamis (a, spherule layer; b, layer with smaller spherules; c, spherule-bearing marl lens; d, sandy marl and micrite; e, chalk lens; f, sandy marl with lenses of coarse spherules; g, fine clay with iridium spike; h, limestone); airborne melt spherules arrived first (a, b), then the tsunamis (b—f) and finally the dust-borne iridium (g), before a return to normal marine deposition (h). [Figure (a) from various sources; (b) modified from Florentin etal., 1991.]

Cretaceous-Tertiary boundary deposits

Marine with evidence of large waves

South America

/\ Nonimarine e Rare o Few

• Common to abundant

Danian

Maastrichfian (b)

Fig. 8.42 Evidence for the impact site: (a) location ofChicxulub Crater,on the Yucatán Peninsula,Mexico,as well as the end-Cretaceous coastline of the proto-Caribbean Sea and sites indicating activity of tsunamis (tidal waves); (b) the KT boundary section at Beloc, showing the sequence of arrivals of airfall debris and tsunamis (a, spherule layer; b, layer with smaller spherules; c, spherule-bearing marl lens; d, sandy marl and micrite; e, chalk lens; f, sandy marl with lenses of coarse spherules; g, fine clay with iridium spike; h, limestone); airborne melt spherules arrived first (a, b), then the tsunamis (b—f) and finally the dust-borne iridium (g), before a return to normal marine deposition (h). [Figure (a) from various sources; (b) modified from Florentin etal., 1991.]

The reality of impact was debated through the 1980s, but the discovery of the crater in 1990 convinced most doubters. The Chicxulub Crater, on the Yucatán Peninsula, Mexico (Figure 8.42(a)), is 195 km in diameter, with inner rings at 130 km and 80 km, and is filled with Tertiary sediments (Morgan and Warner, 1999). A ring of coeval coastline deposits shows evidence for tsunami (massive tidal wave) activity, presumably set off by a vast impact into the proto-Caribbean (Smit, 1999). Further, the KT boundary clays ringing the site also yield abundant shocked quartz and glassy spherules that match geochemically the bedrock under the crater site. Further afield, the boundary layer is thinner, there are no tsunami deposits, spherules are smaller or absent and shocked quartz is less abundant.

Detailed studies of KT boundary sections around the proto-Caribbean have allowed geologists to reconstruct what happened. The famous section at Beloc on

Haiti (Figure 8.42(b)), a boundary clay that is 0.7m thick (Florentin et al., 1991), documents a three-phase process.

1 The spherule layers, the lower 0.5 m of the section, are two bands of glassy spherules that have two geochemi-cal compositions, some indicating a source from melting of basement rocks and the others indicating a source from evaporites and limestones (the rock underlying Chicxulub). The glassy spherules were melted and thrown up by the impact and came hurtling through the air, and were scattered throughout the proto-Caribbean.

2 The tsunami beds, 0.2 m thick, consist of marls and clays with large limestone clasts and are capped by a thin clay layer. The tsunami followed, moving rapidly over hundreds of kilometres of sea, but more slowly than the airborne spherules, and churning up the limestones and other sediments in the area.

3 The iridium spike and the shocked quartz occur in a fine clay band about 0.1m from the top of the section. Several hours or days later, the iridium and fine dusty material fell from the upper atmosphere, long after the heavier spherules had been deposited. Deposition finally reverted to normal limestone, as it had been before the impact. This three-phase pattern is seen in all other KT boundary layers throughout the world (Smit, 1999), although the tsunami layers are omitted outside the proto-Caribbean. For example, Wolfe (1991) noted the arrival of coarser debris first, combined with freezing, and then the airborne dust and iridium some time later 3000 km away in Wyoming.

There is little doubt that there was a major impact on Mexico 65 Myr ago. But much of the palaeontological data indicates longer-term extinction over 1—2 Myr. Key research questions are whether the long-term dying-off is a genuine pattern, or whether it is partly an artefact of incomplete fossil collecting, and, if the impact occurred, how it actually caused the extinction. Available killing models are either biologically unlikely, or too catastrophic: recall that a killing scenario must take account of the fact that 70—75% of families survived the KT event, many of them seemingly entirely unaffected. Whether the two models can be combined so that the long-term declines are explained by gradual changes in sea-level and climate and the final disappearances at the KT boundary were the result of impact-induced stresses is hard to tell.

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