TAXON 59 (1) • February 2010: 261–268 Cleal & Thomas • Botanical nomenclature and plant fossils NOMENCLATURE Edited by John McNeill, Anthony E. Orchard & John C. David Botanical nomenclature and plant fossils Christopher J. Cleal1 & Barry A. Thomas2 1 Department of Biodiversity & Systematic Biology, Amgueddfa Cymru – National Museum Wales, Cardiff CF10 3NP, U.K. 2 Institute of Biological, Environmental and Rural Sciences, University of Aberystwyth, Llanbadarn Fawr, Aberystwyth SY23 3AL, U.K. Author for correspondence: Christopher J. Cleal, [email protected] Abstract The provisions in the Code for naming plant fossil taxa have changed substantially over the years. The history of these changes reflects the tension between palaeobotanists (including palynologists studying plant microfossils) who need a flexible set of regulations, and the tendency for the Code to include nomenclatural regulations that constrain taxonomic decisions. The current Vienna Code now provides for plant fossils to be named as fossil taxa, which is a flexible taxonomic concept that should suit the needs of most palaeobotanists. However, the Vienna Code also incorporates the more restrictive concept of morphotaxa and most palaeobotanists seem to be under the misapprehension that plant fossils can only be named as morphotaxa. In our view, the concept of morphotaxa is logically flawed and unnecessary in practice, and should be removed from the Code. Keywords form-genera; fossil taxa; morphotaxa; organ-genera; palaeobotany; palynology; satellite taxa INTRODUCTION at least partly lost through fossilisation. They are now biologi- cally inert objects, but originated from living organisms. Since the publication of the Vienna Rules (Briquet, 1906) Fossil plants, in contrast, were the plants from which these it has been accepted that taxonomic nomenclature of plant fossilised remains were derived. They were whole living or- fossils should follow essentially the same procedures as used ganisms, which in principle could be treated in the same way for living plants. However, it soon became evident that the as living plants. But there is the crucial difference that fossil fragmentation of plants prior to and during fossilisation, and plants no longer exist except in the minds of palaeobotanists, the different types of information provided by different modes a view echoed by Bateman & Hilton (2009); nowhere can you of preservation, made it impossible to name taxa of fossils in today see a fossil plant other than as an illustration or model. exactly the same way as taxa of living plants. Thus, various It is of course true that the larger living plants are usually different nomenclatural measures have been proposed over the typified by herbarium specimens representing only parts of years for inclusion within the Code to accommodate fossils, the organism, and these could be regarded as analogous to the most recent being the introduction of “morphotaxa” in the fossil fragments (Stafleu, 1967). Since the nomenclature of St Louis Code (Greuter & al., 2000a) and of “fossil taxa” in whole living plants is based around fragments preserved in the Vienna Code (McNeill & al., 2006). herbaria, why can we not similarly name fossil plants based Many palaeobotanists (including palynologists dealing on our fragmentary plant fossils? The reason is, in our view, with organic-walled plant and “algal” microfossils) remain because there is a fundamental difference between the two uncertain about the current rules for palaeobotanical taxo- situations. With living plants it is still usually possible to go nomic nomenclature and their implications, and we agree that to a whole living organism to check features that may not the position appears somewhat cryptic. However, the current be shown by the type, a procedure that is impossible with Code does in fact meet the needs of palaeobotanists, and in long-extinct fossil plants. In some cases, reproductive organs our view there need only be a few fairly modest changes to and foliar organs have been found in apparent attachment, the provisions covering plant fossils for the situation to be- but except for the smaller fossil plants (e.g., Ash & Tidwell, come clear. 1986: fig. 2) whole articulated organisms are rare, and those that are available do not preserve the full range of their tissue and organs. WHAT ARE WE NAMING? In the current Code (McNeill & al., 2006) the wording is somewhat ambiguous. In the footnote to Preamble 7, the This may seem self-evident, but in our view there has been implication is that the palaeobotanical provisions within the some confusion. Palaeobotanists deal with two concepts: plant Code are dealing with fossil taxa (“… the term ‘fossil’ is ap- fossils and fossil plants. Plant fossils are the physical objects plied to a taxon when its name is based on a fossil type …”) taken out of the ground that reveal evidence of long dead and the term “Fossil taxa” is explicitly used in Art. 1.2. This plants. These remains nearly always represent just fragments term can only be referring to taxa of [plant] fossils and not of of the plants in which aspects of the original tissue have been reconstructed (“whole”) extinct plants. However, in most of 261 Cleal & Thomas • Botanical nomenclature and plant fossils TAXON 59 (1) • February 2010: 261–268 the rest of the Code where palaeobotanical nomenclature is no palaeobotanists, and they had no draft proposals from the dealt with (Arts. 8.5, 11.1, 13.1(f), 36.3, 38.1, 38.2; Rec. 8A.3) Thomas committee. Moreover, the American botanist H.W. the term “fossil plants” is used; only Art. 7.9 mentions “plant Rickett stated that American palaeobotanists had not had an fossils”. This has led to the strange concept of “morphotaxa opportunity to consider the 1935 proposals (which contradicts of fossil plants” (Art. 11.1); if we had available an actual fossil the comments by Thomas 1935: 111 that the British proposals plant, we would surely name it in the same way as we would “have received the general approval of many American palaeo- a living plant rather than using morphotaxa. There can be no botanists”). Consequently, no palaeobotanical provisions were doubt that in most places in the Code where the term “fossil added to the published supplement to the Rules (T.A. Sprague, plants” is used, it really means “plant fossils”. 1950), nor had any been included in the earlier “unofficial Many palaeobotanists seem to focus exclusively on these rules” published in America (Camp & al., 1947). hypothetical “whole” plants, at least partly because they are The taxonomic nomenclature of plant fossils was finally considered necessary for incorporation in phylogenetic (“cla- addressed properly in 1950 at the Stockholm Congress, where distic”) analyses. However, as they no longer exist as realities the International Association for Plant Taxonomy was estab- there are very real practical problems in formally naming ex- lished (Cowan & Stafleu, 1982; Stafleu, 1988), and the Rules tinct plants reconstructed from the fossil record, problems that became the International code of botanical nomenclature. At remain unsolved: a problem noted by Chaloner (1986). Argu- this meeting, H.H. Thomas tabled a report from the Special ably the best practice is to not name fossil plants formally at all, Committee for Palaeobotany (Lanjouw, 1953) that integrated but to refer to them by names such as the “Lepidodendron-tree” the Thomas (1935) and Jongmans & al. (1935) proposals (the outside of a formally codified system of nomenclature. Plant report was published as “Appendix III” in the proceedings fossils, in contrast, are physical objects that can be examined of the Stockholm Congress – Lanjouw, 1953: 547). There is directly and thus more suitable to codified nomenclature. no record of the discussions that produced this compromise wording, nor were the proposals on form- and organ-genera discussed in detail at the Stockholm meeting itself. However, INTRODUCTION OF FORM- AND ORGAN- except for the proposed change to Article 20 (later 23) giving GENERA the starting date for palaeobotanical taxonomic nomenclature, the Thomas report was accepted “almost unanimously” by The Vienna Rules (Briquet, 1906) specifically stated in the Stockholm Congress (Lanjouw, 1953: 534), and became Art. 9 that it covered fossils but few provisions were devoted incorporated into the Code, initially as Appendix IV (Stock- to them. The first serious attempt to address the issue came holm Code; Lanjouw & al., 1952) and then Appendix II (Paris with a meeting of British palaeobotanists held in 1934 at the Code; Lanjouw & al., 1956). British Museum (Natural History), where a series of propos- The original organ-genus definition was flexible: organ- als to emend the Rules were drafted (Thomas, 1935). This genera were to reflect the different parts of the plant being draft was also seen by three palaeobotanists from continental preserved and modes of preservation, but nothing was said Europe (Wilhelmus Jongmans, Thore Halle, Walter Gothan) about the level to which they could be classified. The concept who subsequently published a modified version of the same of form-genus was created with the clear aim of stabilising proposals (Jongmans & al., 1935). Both versions were in es- the use of a limited set of taxa, especially those introduced by sential agreement about the key proposal that there should be Brongniart (1822) that, although being widely recognised as ar- separate taxonomic categories for naming plant fossils: fossils tificial, had a long history of use. Form-genera were regarded as would normally be referred to organ-genera. However, there a small subset of organ-genera: they were “organ genera which would be a subset of organ-genera that would be entirely arti- have been proved to contain species that are not generically ficial (“artificial genera” in the Thomas proposal, “form gen- related in the ordinary taxonomic sense” (Jongmans & al., 1935: era” in the Jongmans & al. proposal) being defined on strictly 4). So, for most purposes, plant fossils were to be assigned to morphological criteria with no implications being made that organ-genera unless one of the Brongniart taxa was to be used.