Foliar physiognomy in Cathaysian gigantopterids and the potential to track Palaeozoic climates using an extinct plant group

Abstract

Foliage of Late Palaeozoic Cathaysian gigantopterids encompasses a large diversity of forms, both inter- and intraspecific, and includes many with striking resemblance to the leaves of extant woody dicotyledons. Similarities include leaf size, shape, margin and venation. Other vegetative similarities between angiosperms and gigantopterids include the presence of water conducting vessels with the potential for maintaining similar vascular physiologies. In combination, these features suggest that Late Palaeozoic gigantopterids can be regarded as vegetative analogues for flowering plants. This convergence in vegetative morphology probably reflects a comparable response to environmental conditions. In this paper, we document the morphological diversity of a broad taxonomic sample of gigantopterids. These gigantopterids are described and illustrated both photographically and as line drawings, these data forming a significant record of gigantopterid physiognomic diversity. Rather than using standard taxonomic schemes a morphotype approach is adopted in order to accurately and, where possible, quantitatively characterise foliar physiognomic variation using a standard terminology throughout. In doing this, we highlight physiognomic features shared with extant angiosperms, allowing us to consider the reasons for this similarity. Quantitative techniques based on foliar physiognomy in angiosperms are applied exclusively to gigantopterids for the first time, including Leaf Margin Analysis (LMA) and Climate Leaf Analysis Multivariate Program (CLAMP). Results suggest that the striking similarity in foliar physiognomy between gigantopterids and angiosperms has the potential to track Late Palaeozoic climates, although limitations of the currently available collections have prevented us from doing so in a robust manner. Foremost of these limitation is an absence of sufficient gigantopterid taxa from individual beds, floral assemblages and geological formations, restricting the application of angiosperm based techniques that rely on over 20 taxa for statistical confidence. Other limitations relate to the incompleteness of many of the specimens for which architectural characters cannot be adequately discerned. Although quantitative physiognomic methods have been limited by the data available, the problems identified can be overcome in the future by adequate and systematic collecting. This detailed foliar physiognomic treatment of Cathaysian gigantopterids should also form a substantive basis for future systematic revision of the group.