Oceans

Continental seas and shelves. Because the shallow seas that covered large expanses of the continents receded before the K/T boundary, very few shallow marine deposits are preserved that record the last 2-3 million years of the Cretaceous. And because many groups of organisms lived and died in shallow continental seas and shelves, we lack data for such groups.

How well or badly fish and sharks fared remains largely conjectural, although it is apparent that, as a group, they did not suffer the kind of wholesale decimation seen in other groups.

The whale- and dolphin-like marine diapsids called ichthyosaurs (Figure 15.9a) are known to have disappeared well before the K/T boundary. Not so in the case of marine-adapted lizards called mosasaurs (Figure 15.9b). Recent work suggests that these went extinct geologically abruptly, at the end of the Cretaceous. More equivocal is the record of plesiosaurs, the long-necked, Loch Ness-type, fish-eating diapsids of the Jurassic and Cretaceous (Figure 15.9c), for whom there are, Loch Ness notwithstanding, no credible post K/T records.

Among fossil invertebrates, perhaps the most famous group are the ammonites (Figure 15.9d). Ammonites lived right up to the K/T boundary, before finally going extinct.

Tsunami deposits

Impact crater

Tsunami deposits

Lowlands

Area of ejecta blanket

Impact crater

Coastline , Coastal shelf

Chicxulub

Area of ejecta blanket

Figure 15.6. Paleogeographic map of the ground-zero region for the K/T asteroid, Yucatán Peninsula, Mexico. The geography of the region as we know it today is superimposed over the geography of the region at 65.5 Ma.

Figure 15.7. Three-dimensional geophysical reconstruction of the remnants of the Chicxulub crater. A gravimeter measures subsurface changes in gravitational attraction of rocks under the town of Chicxulub. These variations in gravitational attraction show a large-scale bullseye pattern of concentric rings, diagnostic of a meteor impact. North is toward the top of the page.

Figure 15.7. Three-dimensional geophysical reconstruction of the remnants of the Chicxulub crater. A gravimeter measures subsurface changes in gravitational attraction of rocks under the town of Chicxulub. These variations in gravitational attraction show a large-scale bullseye pattern of concentric rings, diagnostic of a meteor impact. North is toward the top of the page.

Figure 15.8. Reconstruction of an asteroid impact with Earth. Planetary geologist P. H. Schultz and geobiologist S. L. D'Hondt suggest that the asteroid struck Earth at an angle of about 30°, coming from the southeast.

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