Warm spell did little for Eocene flora. (Earth Science: from Boston, at the annual meeting of the Geological Society of America).

Global warming kicked off the Eocene epoch about 55 million years ago, but new research shows that this greenhouse phase did little for North America's plants. In contrast, warming that began 18,000 years ago caused glaciers to retreat and brought rapid shifts in the continent's flora and fauna.

The transition from the Paleocene epoch to the Eocene was marked by a rise in average temperatures of 4 [degrees] to 8 [degrees] C in just 10,000 to 20,000 years. That initiated the so-called Paleocene-Eocene Thermal Maximum (PETM), which lasted 200,000 years.

North America's menagerie changed dramatically during the PETM, yet plant life during the Paleocene and Eocene remained similar. However, a gap in the floral record during the PETM has left open the possibility that a transient greenhouse effect introduced a dramatic but short-lived flux in plant life.

New data on prehistoric plant diversity at Powder River Basin, Wyo., fill key blanks in the missing interval--and suggest it doesn't hide any surprises. Although the relative abundance of different plants shifted somewhat, the local array of flora changed minimally during the PETM, reports Scott L. Wing, a paleobiologist at the National Museum of Natural History in Washington, D.C. From fossilized leaves and pollen, he and his colleagues have determined that the only new plants known to have migrated into the region appeared after the PETM.

During the PETM, Wing suggests, expansive forest cover may have inhibited the immigration of plants. In contrast, he notes, the glacial retreat that accompanied the warming that began 18,000 years ago opened up virgin ground that plants colonized quickly.

--B.H.