Did an Impact Kill Off the Dinosaurs

This is still the most widely accepted notion, with polls revealing that a majority of geoscientists and astronomers believing it to be true. Largely because of the intense publicity associated with the Alvarez et al. (1980) discovery and its aftermath, it has entered the consciousness of many scientists who do not keep up with the details of the paleontological and stratigraphic literature, or attend professional meetings that discuss the latest developments in this topic. In their favor, the K/T impact is the only one that has produced a crater of the right age, Chicxulub in the Yucatan Peninsula of Mexico.

But throughout the entire K/T impact debate over the past 28 years, there has always been a strong resistance to overly simplistic ideas about the K/T extinction. At first, skeptics challenged the reality of the impact, but even after

Chicxulub crater was found in 1990, this did not solve the entire problem. As I reviewed in my recent book (Prothero, 2006, Chapter 2), understanding the K/T extinction has always been complicated by the fact that at least two other significant events are also occurring at the end of the Cretaceous. One is the eruption of the Deccan traps in eastern India and Pakistan, one of the largest eruptions in earth history. The Deccan eruptions yielded over 10,000 km2 of lava with thousands of flows totaling over 2.4 km in thickness. Such eruptions undoubtedly produced huge amounts of volcanic gases and ash that could have had catastrophic effects on the environment. The other is a major drop in sea level, causing huge inland seas to regress and dry up, which could have had major effects on many kinds of marine life.

An even more serious complication is that the evidence of the fossil record must be consistent with the predictions of the impact hypothesis. Most non-paleontologists have heard about the K/T impact model, and know that the dinosaurs vanished about this time, and have no further interest in finding out whether the evidence actually fits the impact model. But from the very beginning, paleontologists were the principal skeptics of the K/T impact hypothesis, largely because they did not see evidence in the fossil record that all the K/T victims died out abruptly right at the iridium horizon. Instead, they found evidence that the extinction was much longer and more protracted than could be explained by a single impact. Although some of this has since been explained as an artifact of the incompleteness of the fossil record and the Signor-Lipps effect (Signor and Lipps, 1982), not all of it can be so explained. As a distinguished panel of British paleontologists (MacLeod et al., 1997) showed, most of the marine organisms that were alive in the later Cretaceous were either in decline and extinct long before impact, or sailed right through the events of the K/T boundary with little or no effect. Only the coccolithophorid algae and possibly the planktonic foraminifera show much of an effect consistent with the impact model. By contrast, the benthic foraminifera, diatoms, dinoflagellates, and radiolaria show almost no effect, so the extinction in the microfossils was minimal. More importantly, the major victims of the Late Cretaceous (such as inoceramid and rudistid bivalves) were long gone before the end of the Maastrichtian, and even the ammonites were in a long-term decline, with only a few taxa surviving to near the end of the Cretaceous. By contrast, corals, brachiopods, nautiloids, echinoids, and most bivalves and gastropod showed only a minor extinction, or none at all, at the end of the Cretaceous (MacLeod et al., 1997). The marine reptiles were also apparently declining through most of the Campanian and Maastrichtian, so there is no direct evidence that mosasaurs or plesiosaurs actually witnessed the impact. Teleost fish, on the other hand, showed a 90% survival across the K/T boundary.

Even more revealing is the pattern of land communities. Although there is a change in the pollen flora and a "fern spike" right at the iridium anomaly on land, most other land organisms were little affected (Archibald, 1996). The non-avian dinosaurs did vanish, but there is still much debate as to whether their absence from the final 3 m of section below the iridium horizon is clear evidence as to whether they actually vanished before the impact, or survived to the end of the Cretaceous but are not fossilized. Much more striking is the evidence of nearly every other land vertebrate taxon. Most groups, including the crocodilians, champ-sosaurs, turtles, amphibians, birds, bony fish, insects, and mammals, sailed through the K/T boundary with minimal extinction or no effect whatsoever. This evidence alone falsifies most of the more extreme K/T impact scenarios, such as the "acid rain" hypothesis, which argues that the world was bathed in sulfuric acid created when the Chicxulub asteroid vaporized a lot of buried sulfate from gypsum in the Yucatan subsurface. As Weil (1984) pointed out, this notion can easily be ruled out by the survival of nearly all Cretaceous frog and salamander species, which are very sensitive to even slight changes in the acidity of their environment and even now are declining due to human-induced acid rain. The survival of crocodilians as large or larger than many non-avian dinosaurs rules out the idea that small creatures could hide from the impact, but the larger ones were wiped out. Impact advocates have argued that most of the survivors could have hid from the harsh conditions by aquatic adaptations or by living underground (Robertson et al., 2004). But this still does not adequately address the problem, since Late Cretaceous sharks were wiped out, while bony fish did just fine. In addition, the largest crocodilians were too large to be protected underground or underwater and still survive in a world as hellish as the K/T impact advocates propose.

Thus, the public perceptions of the causes of the K/T extinction are out of touch with the last decade of research on the topic. Most of the recent books (Archibald, 1996; Keller and MacLeod, 1996; Officer and Page, 1996; Hallam and Wignall, 1997; Dingus and Rowe, 1998; Hallam, 2004) on the topic are critical of the K/T impact as the sole or even most important explanation. Even more revealing is the response of paleontologists after 27 years of the controversy. Brysse (2004) polled the vertebrate paleontological community, the scientists who know the terrestrial evidence the best. Of those surveyed, 72% felt that the K/T extinctions were caused by gradual processes followed by an impact. Only 20% felt that an impact was the sole cause.

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