Extinctions vary in intensity from normal low-level background to major events where some 50% of the total species of plants and animals disappear. Of the many global restructurings that have been detected, only five qualify as mass extinctions. The best publicized occurred around the Cretaceous-Tertiary (K/T) boundary some 65.5 mya and coincides with the demise of the dinosaurs. The many theories that have been proposed for the cause of these extinctions have broadly divided the scientific community into two camps, the catastrophists and the gradualists, and pivot around the central issue of time span. How fast did the detected extinctions take place? Was the time frame overnight, over centuries, millennia, or millions of years? Unfortunately, the fossil record is fraught with problems and inconsistencies that make it difficult, if not impossible, to interpret the timing and magnitude of extinctions. Some of the more obvious difficulties are listed in appendix C.
What do the catastrophists say? They tend to favor a mass extinction that was instantaneous or occurred over an unspecified but short time, geologically-speaking. Their theory involves the meteor (or comet) impact proposed by Alvarez and his group.336 Ample physical evidence has been collected to indicate that an impact occurred in the sea,9 including an iridium spike in the soil at the K/T boundary, shocked quartz, tektites, and a crater of the correct size at Chicxulub in the Yucatan Peninsula.
The gradualists do not deny that an impact occurred, but they also consider other physical events that occurred around the K/T boundary, including marine regression—a drastic decline in sea level—increased volcanism, and climatic changes. Many feel that it's either too early to say or that the available facts are insufficient to support either an abrupt and/or mass extinction. The gradualist's explanations tend to favor a series of accelerated background extinctions—a random grouping of coincidental events before and at the K/T boundary that had a cumulative, global effect.
Perhaps the best way to illustrate the necessity of reappraising the terminal Cretaceous extinctions would be to examine today's world. Keep in mind that by definition, at least 50% of all species must have perished for a mass extinction to occur. If an extinction event happened right now and killed off all terrestrial and aquatic animals except insects, that loss would only be about 25% of the total animals. If 50% of the vertebrates died out, as was suggested to happen at the K/T boundary, it would represent only about 2% of all animals. To a vertebrate zoologist, the loss would be significant, and the rest of us would probably concur because after all, we are vertebrates, but that would not make it a mass extinction.
How abrupt was the demise of those organisms that appeared to die out at the K/T boundary? The answer depends on the particular scientist's viewpoint. The literature abounds with contradictions centering on who, where, and when. Many of the groups whose populations were examined around the K/T boundary were marine forms, and it has been estimated that approximately 38% of the animal genera in the sea became extinct at the K/T boundary,298 even though most of the major families survived. Genera that disappeared included some marine reptiles, certain lines of bony fish, sponges, snails, clams, cephalopods, sea urchins, and foraminifera, particularly those from shallow tropical seas. However, many of these appear to have already been declining in diversity prior to the K/T boundary. Among the giant marine reptiles, ichthyosaurs disappeared well before the event, ple-siosaurs were decreasing at the time, but the mosasaurs appeared to have been thriving 65.5 mya.299
Many of the marine groups that did disappear, such as the ammonites and the belemnites, both cephalopods, and the microscopic foraminifera, were also declining far before the K/T boundary, indicating that other factors threatened their existence. Thus these extinctions cannot be directly tied to a catastrophic impact at the end of the Cretaceous. If some of these threatened groups were keystone species, then their demise was undoubtedly directly linked to the extinction of other organisms that depended on them for survival. Such a scenario possibly occurred with the demise of rudist clams, the major component in Cretaceous reef architecture.300 The loss of reef habitats due to their decline and disappearance would have eliminated an enormous number of codependent species that depended on such environs for survival. The cause of rudist clam extinctions may have been as simple as a disease intensified by marine regression and accompanied by changes in ocean currents and temperatures.
Similar inconsistencies have been noted with terrestrial plants, the very foundation of the food chain. The data regarding K/T plant extinctions is conflicting but suggests a global pattern. Many scientists agree that there was a latitudinal effect because, based on spore and pollen remains, vegetation in the more southern latitudes (Australia, New Zealand, and Antarctica) was relatively unaltered. The more northern latitudes also indicated fewer floral turnovers than mid-latitudes, and even regional differences were noted. Much of the data reveals variable species turnover before and at the K/T boundary. And it appears that angiosperms were the most affected by events of the K/T transition. The most severe effects on plants have been reported from the Hell Creek Formation in the western interior of the United States. Changes in fossil leaf morphology there were interpreted as evidence of a 79% turnover in plants at the K/T boundary, but this was after a 75% turnover observed prior to that. So plants in that area were already in a state of flux. In contrast, palynoflora (pollen) data from the same area imply only a 30% plant extinction rate.301302 Another pollen study also indicated that species were changing before the K/T boundary, as plant pollen became unusually small. This was interpreted as evidence that ecosystems were transitioning and that plants were probably at the edge of their preferred geographic range as well as stressed from climatic changes, as the community structure went from open to closed habitats.303 In contrast, in the Raton Formation of New Mexico and Colorado, 85% of the plant species are thought to have survived,304 which is similar to the 70% proposed survival rate in Saskatchewan.305
This apparent variability in plant extinctions may be attributable to there being very few fossil sites with plants and dinosaurs that span the Cretaceous-Tertiary boundary. Presently these are confined to the western interior of North America, although sites in South America and Asia hold promise for contributing additional information. Those at the Hell Creek/Lower Tullock formations in eastern Montana have been used extensively to gauge the severity of extinctions at the K/T boundary. One problem with this limited data pool is that the changes noted there might be an anomaly. Since extinctions seen in leaf fossils around the K/T boundary at Hell Creek do not seem to be supported by information gleaned from studies in more southern and northern global latitudes, nor by the pollen record, one is left to wonder if the changes there were related to some short-term localized phenomena.
One of the abiotic causes for regional extinctions could have been drought. Some data has suggested that the Hell Creek area was seasonally much drier than originally thought. To see how catastrophic a drought can be, consider the most well-known one in the United States. It created the infamous Dust Bowl that devastated the Great Plains in the 1930s, lasted more than a decade, and by 1934 had affected 75% of the country and twenty-seven states.306-308 Lack of water was exacerbated by overgrazing and by replacing deep-rooted native plants with shallow-rooted agricultural crops. Hot, dry windstorms picked up the topsoil and created dust clouds so thick that it was dark at noon. The windborne soil was carried over 1,500 miles away. About 90 million acres (140,625 mi2 = 3.64 x 105 km2) of farmland was destroyed or severely damaged. How amazing that this amount of habitat destruction occurred in such a geologically insignificant time frame. Facts have been presented indicating that the drought was caused by anomalous tropical sea-surface temperatures and amplified by interactions between the atmosphere and the land surface.307 So it is easy to image that droughts, dinosaur overgrazing, and a dearth of deep-rooted plants might account for the successive turnovers in fossil vegetation noted at Hell Creek. Other scenarios could also explain what are perceived as drastic localized extinction events, such as periodic incursions and regressions of the inland seas that destroyed old habitats and created new ones. These possibilities underscore the necessity for investigating many other vertebrate and plant fossil sites that span the K/T boundary globally before arriving at definite conclusions about extinctions.
Estimating insect extinctions at the K/T boundary is hindered by gaps in the fossil record throughout the Late Cretaceous and Early Tertiary. There are currently no sites with enough fossils bridging the K/T boundary to determine if and when various groups disappeared. Second, insects were so diverse that to observe a 50% loss of species would be impossible with the available fossils. Knowing that these caveats exist, insects appear to have passed the K/T boundary relatively unscathed.309-312 This is based on the examination of higher taxa, especially families, that persisted into the Tertiary. Since an insect extinction event was observed in the mid-Cretaceous using these taxa levels,35 if one of a similar magnitude occurred at the K/T boundary, it should be detectable. Since insect species represent the majority of animal diversity, why haven't they even been mentioned in most discussions of mass extinctions?
Ninety-six percent of all animal life are invertebrates,309 and this figure does not even include unicellular or microbial organisms like protozoa. A large portion of those are soft-bodied and not amenable to fossilization, so any population changes at the K/T boundary cannot be assessed. Others with a reasonable Tertiary fossil record, such as spiders, are poorly represented in Cre taceous deposits. Those with a poor fossil record, such as the mites and the lesser-known fossil arachnid groups (scorpions, daddy longlegs, wind spiders, whip scorpions, pseudoscorpions), as well as millipedes and centipedes, also have not been considered in estimating losses at the K/T boundary. Thus, aside from the dinosaurs, only a few marine invertebrate groups with hardened parts have been used to evaluate K/T extinctions. Perhaps at this point in time, with the incompleteness and bias of the fossil record, scientists need to reevaluate the data. Since the fossil record of common invertebrate groups is so poorly known, estimating possible extinctions at the K/T boundary has to be based on their present diversity and distribution.320 An example is the nematodes, extremely important organisms in the ecosystem not only as animal parasites but because they are responsible for 50% of the energy flow in the soil, and their feeding can decrease above-ground plant production by 30-60%.66
In numbers of individuals, nematodes are probably one of the most abundant invertebrate groups on the face of the earth, ranging from 3 billion in an acre of soil and 1.5 billion in the upper 20 mm of an acre of marine beach sand313 to 9 million in a square meter of temperate grassland.314 As many as 71 species have been recovered from just 100 grams of grassland soil.315 While only some 20,000 species have been described, estimates of their diversity range from 500,000 to 10 million species.279 316 They occur in soil, fresh water, and the marine environment as well as inside plants and animals. Because of the cosmopolitan distribution of so many genera today, it is obvious that many terrestrial microbotrophic (utilizing microbes as food) as well as plant and animal parasitic nematode lineages317-319 extend back to the period when all continents were united into the supercontinent Pangaea. Yet their fossil record is virtually nonexistant.
There is no doubt that large non-avian dinosaurs died out sometime around the K/T boundary. Debate still rages whether they were already experiencing a loss in diversity by the Late Cretaceous. Some consider that both genera and species declined dramatically in the Maastrichtian by about 40%,321343 while oth ers argue for no loss and perhaps even an increase in diver-sity,322,323,342 and some suggest that dinosaurs were holding their own.324 While the dictum is that all dinosaurs died out at the K/T boundary, there are several reports of dinosaur fossils after the Cretaceous. One well-documented study provides convincing evidence that some fossils date from the Paleocene and were not redeposited. Bones of ornithomimids, dromaeosaurs, sauror-nithoidids, ankylosaurs, nodosaurs and hadrosaurs were recovered in the Ojo Alamo beds of the San Juan Basin in New Mexico, not more than 965 miles from the Chicxulub meteor impact site.325 But how could large dinosaurs survive so close to ground zero? The authors suggest that they endured as buried eggs that hatched after the impact event. But if dinosaurs did survive into the Paleocene, how much longer did they last?
All of the proposed events impacting the planet's ecosystems over a ten-million-year time span around the K/T boundary would have had similar drastic consequences. They resulted in physical changes in the land surface. The marine regression that occurred from 70-60 mya culminated in a 11.2 million mi2 loss of ocean primarily in shallow neritic areas and epicontinental seas, with a concomitant increase in land.9 The Deccan Traps volcan-ism originally resulted in an approximately 772,200 mi2 basaltic flow up to 7,872 ft deep across parts of India that accumulated 69-63 mya.339 And the meteor impact on the Yucatan peninsula right at 65.5 mya carved an underwater crater with an area estimated to range from as little as 2,826 mi2 to as much as 27,300 mi2 and threw debris into the atmosphere.340 341 The volcanic activity and the meteor impact caused some similar consequences: release of heat, air pollution with increases in CO2 and SO2, toxic fallout including acid rain, ozone damage, diminished sunlight, and contaminated water. The difference is that the Deccan Traps volcanism was episodic over 6 million years, while the meteor produced an environmental impact that lasted for a much shorter period.
The three major events surrounding the K/T boundary produced some common repercussions, including changes in ocean temperatures and salinity, as well as in global wind patterns and worldwide terrestrial temperatures. In addition, marine regression was accompanied by changes in ocean currents, wave action, and the addition of 11.2 million mi2 of land.9 The latter brought about shifts in coastlines, the formation of land bridges, and an increase in freshwater lotic systems as the distance that rivers and streams flowed to reach the seas extended.
Some areas may have been more impacted than others, while effects such as climate change would have been worldwide (fig. 39). Many of the results would have been cumulative, with all working in tandem to produce a greater outcome than one event. Obviously the planet was suffering from a series of environmental stresses for an extended period of time, and these would have affected all life forms.
Although most generally accepted theories for the causes of the Cretaceous-Tertiary extinction events center around marine regression, meteor impact, or volcanism, many other condi-tions326 could have added to the stress load of endangered species, thereby decreasing their vigor and making them susceptible to extinction.
Abiotic factors would have had diverse and harmful effects on all life forms. Just as climatic change can be considered a direct abiotic corollary to these three events, disease is certainly a biotic corollary shared by them. Anytime an ecosystem is challenged by harmful physical events, the organisms within it experience stress, virulent pathogens appear, and epidemics run rampant.
We believe that disease played a significant role in dinosaur extinction during the terminal Cretaceous. The importance of diseases in shaping ecosystems should not be overlooked. Animals can be brought to the brink of extinction by disease, and a combination of other factors can institute, contribute to, or finish off the process.
We are not talking about a single disease wiping out all dinosaurs, but rather the cumulative, cascading effects of many diseases working along with major contributing abiotic elements and diverse biotic factors such as those discussed in previous
Figure 39. This map illustrates the probable sites of the confluent events 60-70 millions years ago (mya) that framed the K/T boundary. The configuration of the continents and their positions is based on a comparison of the Maastrichtian and Pale-ocene maps of Smith et al.9 Light gray represents exposed, old land. Dark gray represents newly acquired land due to marine regression from 70 to 60 mya. These same darkened areas then also represent loss of neritic (continental shelf) seas, the primary source of life and energy production in the oceans. The extant of the basaltic floods associated with the Deccan Traps is shown in black.339 The putative site of the meteor impact on the Yucatan Peninsula, the Chicxulub crater, is ringed. This site was apparently submerged from the Aptian (120 mya) to the Late Eocene (37 mya) and therefore at the time of impact. The present-day outlines of the continents are shown. Extensive areas in Africa, Europe, northern India, central North America, and north-central South America were submerged.
chapters. Others have suggested that diseases could have contributed to Late Cretaceous extinctions. Over ten years ago, Robert Desowitz, a medical parasitologist, suggested that an epi demic of reptilian kala azar (an especially lethal form of leishmaniasis) transmitted by sand flies could have caused the extinction of the dinosaurs.327 But there was no evidence of any Cretaceous pathogens at that time. However, amber studies have shown that vertebrate pathogens were indeed present in the distant past and could have been significant causes of mortality.
As we have pointed out, the effects of Leishmania and malaria on dinosaurs could have been quite severe, especially if they already had compromised immune systems. A current example of how pathogens act in concert is Leishmania and HIV coinfections in humans. The synergistic effect of these two maladies causes a very serious new disease complex since the HIV virus weakens the patients' immune system and allows the Leishmania to spread unimpeded into internal tissues.328
In today's world, pathogens have been implicated in the demise of many species, and such parasite-driven host extinctions can occur in populations of any size, especially when host reproduction is significantly reduced.329 The decline and extinction of frogs in tropical American and Australian rain forests are now considered the result of fungal diseases enhanced by a slight rise in global temperatures.330 331 The devastating bluetongue disease of ruminants has spread into northern Europe within the past decade as a result of climatic changes, allowing the virus to be acquired and transmitted by endemic species of biting midges and resulting in the deaths of over one million sheep.165
The recent extinctions of endemic Hawaiian birds by malaria show how damaging a disease can be when introduced, along with its vector, into populations of immunologically naive hosts.332 A land snail in the Pacific Islands was driven to extinction by a microsporidian parasite.333 Examples of introduced pathogens causing the decimation of human populations include measles and mumps in Native Americans, as well as pandemics of plague, typhus, Asian flu, and yellow fever. Pathogens are constantly mutating and producing new strains capable of infecting different host groups. That is why a global pandemic of the avian flu virus is considered likely in wild mammals, birds, and even humans if an especially pathogenic mutant appears.334
The world was changing as the Cretaceous came to an end. The spread of flowering plants pollinated by insects modified the ecosystem, insects competed with dinosaurs for food, and insects and other arthropods transmitted emerging pathogens. At the same time there were periods of temperature change, marine regression, volcanic eruptions, and one or more meteor impacts. Such a confluence of biotic and abiotic events would have been a perfect setting for the spread of diseases.
Although it is difficult to believe that microscopic organisms could render one of the final knockout blows to such powerful giants as the dinosaurs, it nevertheless stands as a feasible scenario when viewed in the overall context of already threatened and debilitated populations. Whether a catastrophist or gradualist, you cannot discount the probability that diseases, especially those vectored by miniscule insects, played an important role in exterminating the dinosaurs.
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