Dominance-diversity architecture of a mixed hygromorphic-to-xeromorphic flora from a botanically rich locality in western equatorial Pangea (lower Permian Emily Irish site, Texas, USA

Abstract

Here we describe the Leonardian-age flora from “Emily Irish” (Russell Ranch), a small oxbow lake deposit, which is believed to be one of the most extensively collected, single-excavation floras of this age from any place in the world, including ca. 5200 hand specimens housed at the US National Museum of Natural History. The flora is dominated by a mixture of xeromorphic and meso-hygromorphic elements. The xeromorphs include conifers, taeniopterids, cordaitaleans, and noeggerathialeans. The mesomorphic-to-hygromorphic elements include marattialean tree ferns, sphenopsids, lycopsids, callipterids, and medullosan pteridosperms. This co-dominance of hygromorphic and xeromorphic elements is made especially conspicuous by the intimate co-mingling of plant remains from each group on the same hand specimens, and by the relative preservation-state of the macrofossils, which does not differ between the two groups. These observations suggest that they inhabited the same landscape immediately surrounding the depositional environment, and were likely only differentiated by microhabitat. We suggest that the Emily Irish flora grew under seasonally dry climate, but that areas adjacent to the lake and feeder streams retained sufficient moisture to support patches of taxa requiring consistently high soil moisture. These patches, thus, were embedded within a landscape of periodic moisture deficit that supported vegetation composed mostly of xeromorphic elements. This is more likely than requiring the xeromorphic taxa to be transported to the site from extrabasinal habitats, the nearest of which were hundreds of kilometers away at the time the deposit formed.