Terrestrial record

For better or worse, virtually all of what we know of the K/T boundary on land also comes from the Western Interior of North America (Figure 15.11). There, several well-studied, complete sections have provided the best insights available into the dynamics of the extinction.

Eort Union Formation

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Hell Creek Formation

Figure 15.11. The K/T boundary in eastern Montana, USA. The boundary is midway up the butte, right at the dotted line. Below is the dinosaur-bearing latest Cretaceous Hell Creek Formation; above is the Tertiary Fort Union Formation. No dinosaurs have ever been found in the Fort Union Formation.

Plants. The plant fossil record in the Western Interior has two major components, a palyno-flora (spores and pollen) and a megaflora (the visible remains of plants, especially leaves; Figure 15.12). After 15 years of intensive scrutiny, both records agree nicely with each other and both records indicate that a major extinction occurred geologically instantaneously at the K/T boundary.

Interestingly, pollen that is typical of early Paleocene time does not immediately follow the extinction of the Cretaceous pollen. Instead there is a high concentration of fern spores just after the iridium anomaly, suggesting that, immediately after the extinction of the Cretaceous plants, there was a "bloom" of fern growth, interpreted to be a pioneer community growing on a devastated post-impact landscape. In time, the fern flora gave way to a more diverse angiosperm flora characteristic of the early Paleocene.

Outside North America, an interesting southern high-latitude flora is known from New Zealand. There, an abrupt pollen and spore extinction as well as the fern spike are also known. In short, the pollen record suggests that the terrestrial K/T boundary was characterized by global deforestation.

The megafloral record based upon 25,000 plant specimens for the Western Interior of North America shows that while some environmental changes caused extinctions earlier than the K/T boundary, a major extinction took place precisely at the boundary, exactly correlated with the pollen extinction and iridium anomaly. The extinction of 79% of the known angiosperms suggested that, as suspected, the fern "bloom" may have been a response to the absence of flowering plants that would normally have occupied the ecosystem.

Vertebrates. Some clear patterns of survivorship, that is who survived and who did not, can be extracted from the K/T vertebrate fossil record of the U.S. Western Interior. Organisms that lived in aquatic environments (that is, rivers and lakes) showed up to 90% survival, whereas organisms living on land showed as little as a 10% survivorship. Thus the extinction

Figure 15.12. Plant fossils. (a) Late Cretaceous leaf. The leaf is from an angiosperm that became extinct at the K/T boundary. The specimen is from just outside Marmarth, North Dakota, USA. (b) Pollen grains belonging to the genera Proteacidites (1) and AqUiapoi-iemtes (2), both important genera in measuring the moment of the terrestrial K/T extinction. Proteacidites is about 30 pm across; AqmiapoWemtes is about 50 pm.

Figure 15.12. Plant fossils. (a) Late Cretaceous leaf. The leaf is from an angiosperm that became extinct at the K/T boundary. The specimen is from just outside Marmarth, North Dakota, USA. (b) Pollen grains belonging to the genera Proteacidites (1) and AqUiapoi-iemtes (2), both important genera in measuring the moment of the terrestrial K/T extinction. Proteacidites is about 30 pm across; AqmiapoWemtes is about 50 pm.

seems not to have drastically affected aquatic organisms such as fish, turtles, crocodiles, and amphibians, but apparently wreaked havoc among terrestrial organisms such as mammals and, of course, dinosaurs. Several other survivorship patterns, not as statistically robust, also appear in the data: small vertebrates are favored over large vertebrates, ectotherms over endotherms, and non-amniotes over amniotes (Figure 15.13).

| Sheehan & Fastovsky (1992) | Archibald & Bryant (1990); Archibald (1996)

Figure 15.13. Patterns of survivorship at the K/T boundary as reconstructed by J. D. Archibald. The study suggests that aquatic habitat, ectothermy, small size, and the absence of an amnion were qualities that statistically facilitated survival across the K/T boundary. Of these, aquatic habitat may have been the most important; in a separate publication, D. E. Fastovsky and P. M. Sheehan reconstructed the aquatic and land-dwelling survivorship pattern as even more extreme than that proposed by Archibald, with land-dwelling organisms showing only a 12% survivorship, but aquatic organisms showing 90% survivorship. (Data from: Archibald, J. D. 1996. Dinosaur Extinction and the End of an Era. Columbia University Press, New York, 237pp.; Sheehan, P. M. and Fastovsky, D. E. 1992. Major extinctions of land-dwelling vertebrates at the Cretaceous-Tertiary boundary, eastern Montana. Geology, 20, 556-560; Archibald, J. D. and Bryant, L. 1990. Differential Cretaceous-Tertiary extinctions of non-marine vertebrates: evidence from northeastern Montana. In Sharpton, V. L. and Ward, P. D. (eds.), Global Catastrophes in Earth History: An Interdisciplinary Conference on Impacts, Vokanism, and Mass Mortality. Geological Society of America, Special Paper no. 247, pp. 549-562.)

| Sheehan & Fastovsky (1992) | Archibald & Bryant (1990); Archibald (1996)

Figure 15.13. Patterns of survivorship at the K/T boundary as reconstructed by J. D. Archibald. The study suggests that aquatic habitat, ectothermy, small size, and the absence of an amnion were qualities that statistically facilitated survival across the K/T boundary. Of these, aquatic habitat may have been the most important; in a separate publication, D. E. Fastovsky and P. M. Sheehan reconstructed the aquatic and land-dwelling survivorship pattern as even more extreme than that proposed by Archibald, with land-dwelling organisms showing only a 12% survivorship, but aquatic organisms showing 90% survivorship. (Data from: Archibald, J. D. 1996. Dinosaur Extinction and the End of an Era. Columbia University Press, New York, 237pp.; Sheehan, P. M. and Fastovsky, D. E. 1992. Major extinctions of land-dwelling vertebrates at the Cretaceous-Tertiary boundary, eastern Montana. Geology, 20, 556-560; Archibald, J. D. and Bryant, L. 1990. Differential Cretaceous-Tertiary extinctions of non-marine vertebrates: evidence from northeastern Montana. In Sharpton, V. L. and Ward, P. D. (eds.), Global Catastrophes in Earth History: An Interdisciplinary Conference on Impacts, Vokanism, and Mass Mortality. Geological Society of America, Special Paper no. 247, pp. 549-562.)

Figure 15.14. Paleogeography of the Western Interior of the USA, as it would have looked during Late Cretaceous time.

Figure 15.14. Paleogeography of the Western Interior of the USA, as it would have looked during Late Cretaceous time.

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