Causes of the Biodiversity Crisis

The principal cause of the Late Devonian extinction appears to have been rapid global cooling. The Frasnian world was quite hot, with extensive shallow seas covering much of the continental landmasses. In these warm seas the great Devonian reefs, and other high-diversity marine ecosystems, proliferated. The

Figure 1

Temporal Pattern of Biotic Diversity Loss in the Late Devonian Extinction

Source: McGhee, G. R., Jr. 2001. "Late Devonian Extinction," in Briggs, D. E. G., and P. R. Crowther, eds., Palaeobiology II. Oxford: Blackwell Science. figure 2.4.3.1, p. 224. (Reprinted with permission)

Source: McGhee, G. R., Jr. 2001. "Late Devonian Extinction," in Briggs, D. E. G., and P. R. Crowther, eds., Palaeobiology II. Oxford: Blackwell Science. figure 2.4.3.1, p. 224. (Reprinted with permission)

Note: Five pulses of extinction (numbered in the right margin of the figure) occurred in latest part of the Frasnian stage and the earliest part of the Famennian stage within the Late Devonian.

Zone

earth's first forests evolved in the Middle Devonian and spread across the continents in the hot climates of the Frasnian, from tropical to almost subpolar regions. The first true woody trees, the Archaeopteris progymnosperms (ancient relatives of modern nonflowering evergreens), towered some 30 m high in these early forest landscapes. They were accompanied by strange sphenopsid trees, which looked like gigantic reeds, and lycopsid trees 10 to 20 m tall, towering trees that are survived today only by tiny club mosses.

In contrast to Frasnian climates, the Early Famennian world appears to have been cold and arid. The great forests shrank and were confined to low-latitude equatorial regions. The uplands and high-latitude regions of the earth supported only a very sparse vegetation in conditions that were not only very cold but also very dry, without perennial snow. In the oceans, huge reef tracts died out all over the world during the latest Frasnian. The few reef organisms that managed to survive into the Famennian were confined to what few warm waters still existed on the earth, in restricted regions along the equator.

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