T He E Ffects Of Impact On Li F E

prediction ' Prior to the K-T boundary, most species were not already going extinct for some other reason. Their extinction was sudden and right at the boundary.

We can summarize this prediction by saying that the K-T impact will not have been "anticipated." That is, most species that did not survive the boundary were not already going extinct for some other reason; instead, their extinction was delayed until the impact and was caused by it. Critics of the theory argue that just the opposite is true. They say that the dinosaurs, for example, were already on the way out, and being replaced by mammals, well before the end of the Cretaceous. In their interpretation, impact may have happened, but if so it only finished off a few doddering stragglers. Thus meteorite impact has no appreciable effect on life and can be ignored as a factor in evolution. To test this first prediction, geologists need to find and to trace fossils of key species from levels well down in the Cretaceous right up to the boundary.

Of course, as paleontologists have known for over a century, some species did become extinct late in the Cretaceous, but before the boundary, as happens in any geologic period. Conversely, some made their first appearance then. The question is whether this was the case with the most prominent fossil families that we know did not survive the boundary, including the four that are the focus of this chapter: the ammonites, plants, forams, and dinosaurs. Were any of them already well on the way out long before the K-T meteorite made its appearance?

PREDICTION 2: Except where reworking has occurred, species that became extinct at the K - T boundary will not be found above the iridium horizon.

The Alvarez theory maintains that the primary lethal effects of the impact were immediate and that the secondary ones lasted for at most a few hundred or a few thousand years. Since on a geologic time scale these are instantaneous, the time of impact as located by the iridium horizon and the time of the mass extinction are the same. In effect, this prediction holds that the mass extinction, the iridium horizon, the clay layer, and the K-T boundary all are synchronous. Fossils of the major groups that became extinct at the K-T boundary, such as the ammonites and dinosaurs, or that experienced a major species turnover then, such as the foraminifera and plants, will not be found above the iridium level (unless they were brought there by bioturbation or reworking).

But suppose that a few species thought to have gone extinct at the boundary were to turn up above the iridium layer, in confirmed Tertiary rocks: Would that falsify this prediction? In fact, dinosaur fossils have been claimed from Tertiary rocks in China and Montana, though the claim naturally depends on exactly where scientists placed the K-T boundary in each locale and on the assumption that the fossils there are in their original geological sites. We know from their absence in the subsequent fossil record that dinosaur survival into the Tertiary would at best have been rare and temporary. The finding that a few species made it through, only to become extinct a short while into the Tertiary, would have no bearing on whether impact caused the mass extinction and would be nearly immaterial to broad earth history. If however, the main extinction horizon for taxa (a catchall name for an unspecified taxonomic group: species, genus, family, order, etc.) that have always been regarded as having gone extinct at the K-T boundary—say the ammonites and dinosaurs and most plants and forams—were actually found to lie above the iridium, the case for impact-induced, large-scale extinction would be weakened. If all were consistently found above the iridium, this prediction would have failed and this half of the theory would be undone. We would know that a giant impact occurred at the end of the

Cretaceous and that something else caused the mass extinction; we would be back at square one.


It is hard to think of four more diverse groups of organisms than the ammonites, plants, and forams plus the dinosaurs. The ammonites and forams both made their lives in the sea, but in completely different ways; both are fundamentally different from the plants. These four have now been studied sufficiently to allow us to use them to test the extinction half of the Alvarez theory.

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