The so-called “Paleophytic–Mesophytic” transition in equatorial Pangea — Multiple biomes and vegetational tracking of climate change through geological time

Abstract

The Paleophytic, Mesophytic and Cenophytic were originally conceptualized as chronostratigraphic intervals (not specific floras) and were used principally in that way for about 100 years. They supposedly reflected massive, irreversible biotic changes on land that were thought to have happened about half a geological period prior to the changes in marine faunas that mark the more traditional geological era boundaries. The terms were applied initially only to terrestrial rocks from the Euramerican portions of equatorial Pangea, especially parts of central Europe, where the boundary between the Paleophytic and Mesophytic, the “eras” of interest here, was placed at the top of the German Rotliegends. More recently, the terms have been coopted to describe vegetation types (not chronostratigraphic units), even to the point where specific Linnean ordinal or class level clades (evolutionary lineages) of plants have been attributed solely to one vegetation type or the other at a global level. Furthermore, the “Paleophytic–Mesophytic” transition has been recognized in the largely taxonomically distinct vegetation types of temperate as well as the equatorial paleolatitudes. The result of this conceptual coopting of terms that already were of questionable value, is a blurring of the concepts almost to the point of meaninglessness. A reading of the literature indicates confusion in terms of what represents the “Paleophytic” flora vs. the “Mesophytic” flora, the botanical basis for the floras and the taxonomic scale at which they can be recognized, the geographic pattern and temporal correlation of the supposed transition, and the evolutionary and ecological significance of the vegetational changes these terms supposedly capture. We propose that these terms be completely and utterly abandoned as confusing and, worse yet, misleading. Evidence suggests that Paleozoic vegetation tracked climate, which should not come as a surprise in light of what is known of plant dynamics in the Holocene and Recent. Furthermore, the evidence for a global “Paleophytic” vs. “Mesophytic” “vegetation” is simply unsubstantiated by the fossil record. Rather, there appears to have been a complex of global species pools reflecting climate at many spatio-temporal scales. The vegetational changes occurring in the late Paleozoic thus can be understood best when examined as spatial–temporal changes in biome-scale species pools responding to major global climate changes, locally and regionally manifested.