Ben Brothers, 1996
ABSTRACT: What caused the extinction of the dinosaurs? An examination of the geologic record strongly suggests a meteor impact at the end of the Cretaceous Period. However, this hypothesis does not adequately explain the fossil record of flora and fauna near the K-T boundary, which shows step-wise extinctions. A multiple impact theory is compatible with both the geologic and the biological records, and is further supported by examinations of other mass extinction events, especially in the last 250 million years.
Dinosaurs ruled the Earth for almost 180 million years, emerging from the ruins of the devastating Permian-Triassic mass extinction, during which perhaps 99% of living species perished, to fill almost every evolutionary niche in the world. They ranged in size by over five orders of magnitude; the Triassic predator Lagosuchus weighed less than four ounces while the giant sauropods of the Cretaceous weighed in at over thirty tons (Bakker, 1980).
Yet at the end of the Late Cretaceous, the dinosaurs vanished, leaving to us the task of finding out why. The extinction of the dinosaurs was part of a greater occurrence known as the Cretaceous-Tertiary mass extinction. By deciphering the evidence given us by nature, and by determining the reasons for this mass extinction, we may be able to foresee and prevent future mass extinctions.
Mass extinctions have occurred throughout history. Indeed, they are often used by geologists to mark geologic boundaries. In the last 250 million years, where the geologic record is generally more precise, a period of 26-30 million years is observed between mass extinctions (Gore, 1989). They are not, relatively speaking, uncommon. It is not known for certain if different extinctions are related or similar, nor is it known why they occur. Theories that attempt to describe mass extinction abound; some are more credible than others, but it must be remembered that all are just theories. Yet certain deductions can be made, and certain trends can be noted, and the theory which is most credible must be able to explain such observations, and even more importantly, must be supported by them. And so it is hoped that examination of the Cretaceous-Tertiary boundary will yield some clues as to the nature of mass extinction events in general.
It is perhaps fitting that the best evidence is available concerning the Cretaceous-Tertiary mass extinction, because this event, and the concomitant death of the dinosaurs which is by far the most storied and best-known of all extinction events. Before an analysis can be made, it must be noted that mass extinctions are not related to normal, so-called "background extinctions" which occur continuously throughout history. Mass extinctions place totally different pressures on species. Traits which have proven themselves advantageous are not necessarily so, and may actually prove disastrous (Jablonski, 1986).
The dinosaurs had been so successful for so long that by the Late Cretaceous, they had evolved to function in specific roles in specific ecosystems. They had in essence evolved themselves into a corner. Widespread species are quite naturally more likely to survive a mass extinction event than are those endemic to small areas. Thus, when tropical climates were afflicted with high extinction rates, the dinosaurs, many of whom occupied small tropical niches, were affected disproportionately (Gore, 1989). During mass extinction events, the rules are changed.
FINDINGS: What caused the extinctions across the Creataceous-Tertiary boundary? There is clear evidence of a "cataclysmic event" which occurred 65 million years ago (Dayton, 1990). In addition, evidence for a meteor impact off of Mexico's Yucatan Peninsula is well-documented. Rock formations near the boundary show shocked quartz, which is formed under enormous pressure. Such pressure can only be easily accounted for by the pressure created by an impact. The energy released by the resultant explosion also caused small globules called microtektites to be formed from the heating and subsequent cooling of silica-rich material (Fastovsky and Weishampel, 1996). Most telling, however, is the presence in boundary strata of large quantities of iridium, a element which is extremely rare on the earth, but far more common in extraterrestrial sources, including meteors. The average level of iridium in Cretaceous rock is approximately 0.076 parts per billion. However, at the Tertiary boundary, the concentration jumps to 114 parts per billion, an increase not easily explainable by known terrestrial sources (Wolbach et al., 1988). In addition smaller, but still anomalous, spikes consisting of iridium concentrations of 0.3 and 0.7 parts per billion can be detected in the 2.5 million years preceding the Cretaceous-Tertiary boundary (Sutherland, 1994).
A meteor impact would also create a large crater on the scale of several miles. Indeed, the search for a sufficiently sized crater with an age of approximately 65 million years has yielded intriguing possibilities. As I mentioned above, a suitable crater in the Yucatan has been dated at 65.0 million years, ±0.2, and is generally considered the source of the meteor that ushered in the Tertiary Period. A similar crater near Manson, Iowa is 22 miles wide, and considered too small to have caused the extinction of the dinosaurs. However, it has been dated at 65.4 million years, ±0.4. In addition, two Siberian craters, which have not been accurately dated, are roughly estimated to be at or near the Cretaceous-Tertiary boundary (Hecht, 1993).
Further field analyses have suggested that the destructive impact of the meteor was extremely large at low latitudes, and less disastrous at higher ones (Keller et al., 1993). Unusually large concentrations of soot were also detected along with the iridium layer at spots ranging around the world (Gore, 1989). That suggests widespread fires immediately after the impact; moreover, the carbon found there is isotopically uniform, suggesting that the fire was a global one. Based on the positioning of the soot, we can conclude that the fire was essentially immediate, taking place before the debris from the impact had settled (Wolbach et al., 1988).
The impact hypothesis should ideally mesh with the fossilized biological record, yet the disappearance of the dinosaurs does not appear to have been instantaneous, as one would expect from an extraterrestrial collision. Although the biological record is far more difficult to read than the physical, it does hold additional clues concerning when and how the dinosaurs and other species perished.
Various studies of fauna across the Cretaceous-Tertiary boundary have reached similar results; results that are not necessarily compatible with a simple impact scenario. "The extinction pattern shows the early disappearance of large, complex, and ornamented tropical species followed by smaller, less-ornamented species, and the survivorship of small cosmopolitan species into the Tertiary (Keller et al., 1993)." Such a scenario would point more to relatively long-term oceanic and environmental instability, and not to an instantaneous extinction event. An analysis of 300 species of flora show step-wise extinctions which accelerated rapidly between 300,000 and 400,000 years before the end of the Cretaceous. There was also an "impoverishment of flora" immediately before the boundary, which indicates fluctuations in precipitation levels and temperature, along with increases in atmospheric content of CO2 (Dayton, 1990).
The fossil record of the dinosaurs in the Great Plains of North America indicated extinctions that began in the last 7 million years of the Cretaceous, and which became particularly apparent in the last 300,000 years before the K-T boundary. The fossil record also shows decreasing density and variety among dinosaur remaints. There were 30 distinct genera 10 million years before the boundary, but only 12-13 are observed to have survived to the bitter end of the Cretaceous (Sloan et al., 1986). Such a record would point to a long-term decline among dinosaur populations, with a death-blow delivered by a meteor impact at the end of the Cretaceous.
Part of the decline may be attributed to a phenomenon known as the Signor-Lipps Effect, which quite simply states that, as a boundary approaches, there is less rock by which a specimen can be preserved. Thus, a plot of fossil finds implies a gradual decline, when in fact no such decline occurred (Fastovsky and Weishampel, 1996). However, the careful observer will point out that the Signor-Lipps Effect still fails to account for the step-wise nature of the noted decline. Therefore, while we must conclude that a step-wise decline did indeed occur, although it may be less dramatic than implied by the fossil record.
We must also ask ourselves: How did a physical event bring about a biological event? What is the correlation between the two? Studies in North America seem to show a lowering of temperature during the last 15 million years of the Cretaceous. There was also a significant drop in sea level, and a sizable increase in seasonal temperature fluctuation (Sloan et al., 1986). There was also considerable oceanic instability, which began around 100,000 years before the Cretaceous-Tertiary boundary, and which did not subside until 300,000 years into the Tertiary (Keller et al., 1993). In low and mid-latitudes, there was also a "sudden and dramatic" decrease in surface water productivity, leading to the extinction of marine organisms and planktic foraminiferal species (Barrera and Keller, 1994). On land, floral records indicate high survival among deciduous plants, and low survival among those evergreen species which are sensitive to the low temperatures which seem to have been endemic to the K-T boundary (Wolfe, 1986).
Thus, although the biological record is by nature incomplete and somewhat unreliable, there is strong evidence for high levels of extinction in low latitudes, as well as major ecological devastation, large increases in precipitation, and temperature decreases in accordance with "impact winter" scenarios (Wolfe and Upchurch, 1986). Unfortunately, no substantive conclusions can be drawn concerning the timeline of dinosaur extinction. The answer must rely, in the final analysis, on inferences and a good deal of guesswork. Studies of animal remains are imprecise, and can only be dated roughly. Little or no differentiation can be made between a decline which occurred over 200,000-300,000 years, and one really bad week. Both events appear geologically instantaneous (Fastovsky and Weishampel, 1996).
DISCUSSION: Any theory that would seek to explain the extinction of the dinosaurs must combine all of the detected physical evidence, or at least adequately explain it. It must also agree with the fossil record. Failure to do this would necessitate the acceptance of random coincidence, which is certainly not the ideal scientific solution. This is where two of the more common theories, namely volcanic activity and climate change, fall short. There was large volcanic activity near the end of the Cretaceous, which created the Deccan basalt flows on the Indian subcontinent. However, the volcanism produced flood basalts, and not the pyroclastic events necessary to propel enormous amounts of debris into the atmosphere (Fastovsky and Weishampel, 1996). Similarly, it does not explain the anomalous iridium concentrations present at the K-T boundary.
The idea of climate change is, although probably technically correct, quite incomplete. The observations made are valid, yet it explains only the effects, and not the cause. The theory begs the question: What caused the climate change? The answer must be sought elsewhere.
It is likely, I submit, that several comets or comet fragments struck the earth near the end of the Cretaceous. Smaller meteors could generate chaos during the last 2 million years of the Cretaceous. There are several craters of the right date, and others on the sea floor may yet be unaccounted for. There is evidence of a small maritime impact preceding the K-T boundary (Gore, 1989). This could, by churning the water, disrupt marine ecosystems and sea life in general. It also would lower sea levels through vaporization and induce climatic changes similar to those evidenced in the record. This would also explain the smaller iridium peaks detected prior to the terminal event. The low iridium count is not unexpected; after all, the meteor was not as large as the Yucatan specimen, and a oceanic impact is less likely to distribute iridium as widely as a terrestrial shot. It was then the grand finale of the Cretaceous which finished off the declining world of the Mesozoic. The heat of an impact such as the one in the Yucatan would cause fires to erupt spontaneously world-wide, an event witnessed in the carbon content. By comparing the amount of debris to that released in volcanic events such as the eruption of Mt. Saint Helen's, it has been estimated that sunlight would be completely blocked for upwards of three months, ensuring the death of many flora which rely upon the energy of the sun for photosynthesis (Fastovsky and Weishampel, 1996). A large amount of the biomass surely perished as a result of global wildfires. Such a drastic disruption of the food chain would inevitably lead to collapse of large numbers of ecosystems. As noted above, dinosaurs would have been unusually susceptible to such disruptions. The impact could also have given rise to acid rain, as sulfuric and nitric acid is released in large quantities (Dayton, 1990); indeed, this has been detected on a small scale in nuclear tests. Furthermore, a land-based impact would wreck far more havoc on land than in the oceans. This corresponds to studies which indicate a 78% survival rate in aquatic species, but only a 28% survival rate in terrestrial organisms (Archibald and Bryant, 1990).
A multiple impact scenario, as described above, does make progress in explaining the record of the Cretaceous-Tertiary boundary. I can specify three areas in which the multiple impact theory exceeds the traditional single impact view. First, multiple craters can only be adequately explained by multiple impacts. Second, multiple impacts would cause the step-wise extinctions that are observed in fossil records. If the decline of the dinosaurs predated the Yucatan impact by 300,000 years, there are likely to be other contributing factors. Third, the multiple impact theory could explain the 26-30 million year periodicity of mass extinction events.
A timetable of 26 million years requires a celestial origin, as no known earthly cycle is so slow. A comet "shower" would explain the periodicity, as well as the time passage. The Sun passes up and down through the spiral arm of the galaxy about every 30 million years. It seems plausible that the gravitational field through which our solar system passes on this journey is non-uniform. These fluctuations could disturb icy comets in the Oort Cloud, causing some of them to alter their orbits and enter the solar system. Once there, there is a chance that they would collide with the planets, including Earth. Some astronomers, basing their measurements on observed interstellar densities, calculate that such storms would have a duration of one to three million years, a period which would coincide accurately with the disarray at the K-T boundary. Furthermore, it is calculated that there would be an average of 2.5 impacts per storm, with an 8% chance of zero impacts, and a 21% chance of one impact (Muller, 1986). This makes the likelihood of multiple impacts very good.
If such a theory is true, there should be evidence of similar impacts at other extinction events. Indeed, iridium anomalies have been found at the Frasnian-Famennian, Callovian-Oxfordian, and Permain-Triassic boundaries. All three boundaries represent known extinction events (Hoffman, 1989). There have also been two detected iridium peaks near the Eocene-Oligocene boundary, the site of a moderate mass extinction (Muller, 1986). The recurrence of multiple iridium boundaries would also correspond to the recurrence of multiple impacts. Furthermore, three craters larger than 6 miles have been dated in the last 3.5 million years, a time frame which would correspond to a 30 million year cycle, being one event removed from the K-T event (Gore, 1989).
CONCLUSION: Although the multiple impact theory successfully explains large amounts of evidence, it cannot be considered a panacea. There remains no known cause of the 26-30 million year periodicity. Thus the idea of comet clouds remains speculation. The scenario may fit the facts, but the scenario is unsubstantiated. All that can confidently concluded, in my opinion, is that there was a extraterrestrial impact at the end of the Cretaceous, and that it exacerbated a decline which may or may not have been instigated by previous impact events.
That being said, however, the numbers are intriguingly supportive of a multiple impact theory. And in any event, absence of proof is not proof of absence. It remains a matter into which we must look further. One test may come from our interplanetary spacecraft. If a significant number of comets have their orbits altered by our movement through the galaxy, there should be evidence of similarly dated impacts on other planets as well. It seems likely that, as our technology improves, these areas will be open to scientific investigation as well. If the theory is true, it may explain not only the extinction of the dinosaurs, but also many other mass extinction events which have occurred throughout geological time.
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