Metasequoia in the Oligocene Bridge Creek Flora of Western North America: Ecological Implications and the History of Research

Abstract

The early Oligocene Bridge Creek flora of the John Day Formation in Oregon, USA, is an example of the broad-leaved deciduous vegetation that became widespread in the Northern Hemisphere following the Eocene-Oligocene climatic cooling, and demonstrates the significant participation of Metasequoia Miki in newly-developing communities analogous to the modem Mixed Mesophytic Forest of China. Before the discovery of living Metasequoia glyptostroboides Hu et Cheng in China, the Bridge Creek fossils were identified as Sequoia Endlicher, which led to erroneous ecological and community composition interpretations of the Bridge Creek flora based upon comparisons with the modem redwood forest of California. Soon after the discovery of living M. glyptostroboides, palcobotanist Ralph W. Chaney visited the modem M. glyptostroboides forest in China in 1948, and this had a major influence on his concepts about the composition and biogeographic history of North American fossil forests. Many of the North American fossils that had previously been assigned to Sequoia or Taxodium Richard were subsequently reassigned by Chaney to Metasequoia. Chaney postulated that the Oligocene deciduous forests such as Bridge Creek had reached the middle latitudes by the ``migration{''} of an intact Arcto-Tertiary geoflora from the higher latitudes, but this concept is no longer well supported. Instead, the Mixed Mesophytic Forest apparently originated through various responses of individual species during the Eocene-Oligocene transition, including 1) extinction or extirpation, 2) pre-adaptation or evolution in place, 3) dispersal from higher elevations, or 4) dispersal from higher latitudes. Extant M. glyptostroboides is endemic to a very restricted area in central China, and survival of the few remaining natural stands is doubtful unless new measures are taken to conserve the Mixed Mesophytic Forest as an entire community.