Tree ferns, seed ferns, and even some of the more archaic plants from the coal ages, such as lycopsids, characterized the cool climate of the early Permian. But as time progressed, conifers, ginkgos, cycads, and other seed plants replaced these floras. This change seems to reflect adaptations to heating and drying, the two climatic trends of the latter part of the Permian. So how severe was the heating? In a study published in 2002, University of Chicago paleobotanist Peter McAllister Rees compiled lists of fossil plant recoveries from around the world during the Permian. This was a Herculean effort of data collections, but its payoff was substantial. Rees showed that during the latter part of the Permian there was a marked shift of high-diversity floras toward higher latitudes—just the sort of pattern that would be expected by a slow but significant global warming. Eventually the tropical latitudes became too hot for most plants, and there would have been giant regions of the globe essentially barren of plant life at the end of the Permian.

This change culminated with the extinction itself. While the death toll varies from place to place, well over half of plant species may have gone extinct, and in the southern hemisphere the total extinction of what has been called the Glossopteris flora (glossopterids were a type of woody seed fern that formed forests akin to conifer forests but were lower-growing forms) seemingly all went extinct.

Another curious aspect of the extinction has been the finding of abundant fossils, presumably from fungi at the Permian-Triassic boundary itself, and so global and pervasive is this layer that it has been used for correlation of the boundary. While it was eventually found that this so-called fungal layer was in reality several layers rich in fungal and algae remains, its presence is further evidence that the catastrophic plant extinction that killed plants did not faze them, and with plants gone, the low-growing fungi and algae had no competition for light and nutrients.

RESULTS OF THE PERMIAN EXTINCTION

While intense controversy still exists about the cause or causes of the Permian extinction, on one aspect of that time interval there is agree-



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