Palaeobotanical systematics for the phylogenetic age: applying organspecies, form­species and phylogenetic species concepts in a framework of reconstructed fossil and extant whole­plants

Abstract

AbstractA typical vascular land­plant consists of ten to twelve definable organs. Most plant fossils have been disarticulated into their component organs, which must therefore be correlated if the fossil plant is to be understood holistically and compared with its presumed descendants in the extant flora. The resulting conceptually reconstructed whole­plants are the crown jewels of palaeobotany, as they permit full morphological comparison with extant plants and provide templates that guide through reciprocal illumination further attempts at reconstruction. Each of the three lines of evidence facilitating whole­plant reconstruction (association/dissociation, morphological similarity and organic connection) yields only a probability statement that the organs in question have been successfully correlated. Disarticulation means (1) that phenotypic variation can be studied only at the level of individual organs, and (2) that in order to be distinguished from all other kinds of the same organ, an organ must bear a unique morphological character state (autapomorphy). In our terminology, each definable kind of organ is an organ­species. The few organ­species perceived as possessing autapomorphies (and thus as unique) are termed autapospecies, whereas the remaining organ­species characterise more than one whole­plant species and hence are termed form­species. The distinction between autapospecies and form­species is dependent on the range of taxa sampled and is wholly character­based; the age of the fossils under comparison is irrelevant, and the state of preservation is relevant only through its influence on the range of characters that can realistically be scored. None of the many other species concepts recognised by (palaeo)biologists is applicable to fossil plants. Our ability to apply this phylogenetically­inspired approach and terminology to formal taxonomy has been increasingly compromised by modifications to successive editions of the International Code of Botanical Nomenclature, culminating in the 2001 and 2006 Codes which reduced palaeobotanical provision to the fatally over­generalised 'morphotaxon' concept. A more conceptually rigorous, palaeobotanically informed revision of the Code, placing nomenclature more clearly in the service of taxonomy, would strengthen the crucial roles of reconstructed plants within palaeobotany and of palaeobotany within 21st Century science.