The Microfossil Record of Early Land Plants: Advances in Understanding of Early Terrestrialization, 1970-1984
The plant spore record indicates two major phases of adaptive radiation of land plants before the Devonian; these can be interpreted to correspond to different reproductive, vegetative, and ecophysiological strategies of these early terrestrial plants. The first major adaptive radiation by plants onto land occurred in the mid Ordovician. These early plants are represented by abundant obligate spore tetrads; this assemblage persists from the mid Ordovician to about the mid-late Early Silurian. The fossil spore records indicate that these primary producers were widespread by the end of the Ordovician and the beginning of the Silurian. The close similarity of the fossil tetrads with obligate spore tetrads produced by some hepatics and mosses suggests a non-vascular vegetative grade of organization for plants of this interval. The second major adaptive radiation begins with the replacement of the monotonous spore tetrad assemblage by single trilete spores in the mid-late Early Silurian. These trilete spores find morphological counterpart with spores produced by vascular cryptogams; they suggest a vegetative grade of organization at the vascular level for plants of this interval. The initially smooth-walled trilete spores of this radiation are followed by diverse assemblages of trilete spores with variously ornamented spore coat patterns and varied laesurae morphologies by the mid-Late Silurian. The interval from the mid Ordovician to the mid-late Early Silurian is hypothesized on the basis of the abundant and widespread spore records to be one of rapid colonization by founder populations with limited genetic diversity and with life-history strategies that included an ecophysiological tolerance to desiccation and a short vegetative life cycle. The interval from the mid-late Early Silurian to the Pridoli largely coincides with the appearance of vascular plant megafossils. It is hypothesized on the basis of the spore assemblages to be one of major establishment of large populations of genetically diverse plants exploiting a broad spectrum of ecological sites that have escaped representation in the meagre megafossil record.