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Taxa (species, genera, etc) of plant fossils are conceptually different from taxa of living plants. Taxa of living plants are groups of whole organisms in all stages of their life-history. However, palaeobotanists rarely have the remains of whole organisms to study; even small herbaceous plants are rarely preserved complete. Also, plant fossils can be preserved in a variety of ways, each revealing different features of the original parent plant: a compression fossil can yield extensive detail of the morphology of the plant part but little of the anatomy, whereas a petrifaction may yield fine detail of the plant's cellular anatomy but the overall morphology can be difficult to determine.

Consequently, palaeobotanists usually assign different taxonomic names to the remains of different parts of the plant in different modes of preservation. For instance, in the subarborescent Palaeozoic sphenophytes, an impression of a leaf might be assigned to the genus Annularia, a compression of a cone assigned to Palaeostachya, and the stem assigned to either Calamites or Arthroxylon depending on whether it is preserved as a cast or a petrifaction. All of these fossils may have originated from the same parent plant but they are each given their own taxonomic name. In the current edition of the International Code of Botanical Nomenclature such taxa are referred to as fossil-taxa. Although non-palaeobotanist may find it a confusing system, it is generally regarded as the most practical way to overcome the special taphonomic difficulties encountered with plant fossils, and has been in use in one way or another for nearly two centuries.

Early history

This approach of assigning different taxonomic names to fossils of different plant parts originated with the work of the pioneering French palaeobotanist Adolphe-Théodore Brongniart and most palaeobotanists adopted the general approach. However, like most other aspects of botanical nomenclature during the 19th century, there were disagreements as to the detailed approach. For instance, palaeobotanists such as Robert Kidston would name a fossil Pecopteris (Asterotheca) miltonii Artis for a fern whose fronds had morphology similar to the type of Pecopteris and spore-bearing structures of the Asterotheca-type.

During the early 20th century, a set of internationally accepted rules was established for how plant taxa should be named - the International Rules of Botanical Nomenclature. According to these Rules, taxa of plant fossils were to be named in the same way as taxa of living plants and they took no account of the problems of taphonomy and preservation that palaeobotanists encountered with fossils. It is true that these were essentially taxonomic problems and so arguably were not the concern of the Rules that dealt with nomenclature. However, it became evident that some concensus was needed about the nature of plant fossil taxa and that the Rules were probably the best place where this could be resolved.

Organ- and form-genera

The breakthrough came at the 1930 International Botanical Congress held at Cambridge, which was attended by a large number of palaeobotanists (it can be no coincidence that the conference was organised by the leading palaeobotanist Albert Charles Seward). The result was the publication of two sets of proposals by British and continental palaeobotanists as to how plant fossil taxa should be named, the essence of which was introduced into the first International Code of Botanical Nomenclature. These early provisions allowed fossils representing particular parts of plants in a particular state of preservation to be referred to organ-genera. In addition, a small subset of organ-genera, to be known as form-genera, were recognised based on the artificial taxa exemplified by those established by Brongniart mainly for foliage fossils.

Nine years later, at the Montreal International Botanical Congress, the concepts of form- and organ-genera were changed, with the aim of giving them more rigorous defintions. There were now two rigidly defined and mutually exclusive types of genus that were to be used for plant fossils: an organ-genus was for fossils that could be assigned to a family, but fossils that could not be assigned to a family had to be placed in a form-genus. This led to a long dispute essentially centering on the problem that now the International Code of Botanical Nomenclature was impinging on matters that were essentially taxonomic. Various solutions were proposed but none met with universal acceptance (see Cleal & Thomas for a summary of this dispute). Eventually, at the 1975 Lenningrad International Botanical Congress it was decided to simplify matters and all plant fossils were now to be assigned to form-genera, which could not be assigned to a taxonomic family.

Morphotaxa

The resulting situation, whereby plant fossils could not be assigned to a taxonomic family, was totally unacceptable. However, palaeobotanists seem to have lost interest in the whole dispute and many simply ignored the Code. It was another 20 years before any significant attempts were made to improve the regulations for naming palaeobotanical taxa. After a series of proposals were discussed during the late 1990s the concept of morphotaxa was introduced in the 2000 St Louis Code for the naming of all plant fossils (unless they could be assigned to a taxon of living plants).

Unlike organ- and form-genera, morphotaxa were not limited to the generic rank. This gave them far more flexibility. However, they were at the same time very rigid concepts as a morphotaxon could only be used for fossils that represent the same plant part and life history stage as the type specimen, and were preserved in the same way. This restriction was introduced to try to avoid the problem where, for instance, the taxonomic name of a pollen grain pre-dated that of the flower that produced that pollen grain; a strict application of priority, it was argued, would result in the pollen name taking precedence, and this was ragarded as undesirable. However, it had the drawback that, yet again, the nomenclature Code was impinging on taxonomic decisions.

Fossil-taxa

The difficulties caused by the rigid definition of morphotaxa soon became evident and the following Vienna Code introduced a more flexible concept of fossil-taxa. A fossil-taxon is simply a taxon whose type is a plant fossil. Such taxa can refer to a particular part of a plant preserved in a particular way, as defined in the diagnosis of that taxon. Otherwise, the names of fossil-taxa are subject to essentially the same regulations as control the nomenclature of living plants, notably that a name is fixed to a type specimen, and that competing names are chosen mainly on the basis of chronological priority of first publication. Although the name is always fixed to the type specimen, the circumscription (i.e. range of specimens that may be included within the taxon) is defined by the diagnosis and can be changed by formal emendation. Such emendation could result in an expansion of the range of plant parts and/or preservation states that can be incorporated within the taxon. For instance, a fossil-genus originally based on compressions of ovules could be emended so that it also included the multi-ovulate cupules within which the ovules were originally borne. A complication can arise if, in this case, there was an already named fossil genus for these cupules. If palaeobotanists were confident that the type of the ovule fossil-genus and of the cupule fossil-genus could be included within the newly emended genus, then the two names would compete as to being the correct one for the newly emended genus. However, this only happens if the actual type specimens (and not just specimens that are similar to the types) can be included within the newly revised taxon.

Morphotaxa were retained with their original definition but, unlike in the St Louis Code, not all taxa used for plant fossils were to be regarded as morphotaxa; they were in effect a sub-set of fossil-taxa. However, nowhere in the Code did it explain why some fossil-taxa should be regarded as morphotaxa and others not.

Current situation

The aim of morphotaxa was to fix names to fossils of a particular plant part, preservation state and life history stage, in order to ensure the stability of those names. However, it soon became evident that that aim could not be achieved through morphotaxa. It was totally legitimate within the Code to emend the diagnosis of a morphotaxon so that it included other plant parts, preservation states and/or life history stages, whereby it no longer was a morphotaxon. During the 2011 Melbourne International Botanical Congress, therefore, it was decided to remove morphotaxa from the Code. Any taxon whose types is a plant fossil is to be simply referred to as fossil-taxon, and it can represent any plant part or parts in a particular stage of its life history, and preserved in a particular way or ways, as defined in its diagnosis. This in effect is reverting to the original concept of organ-genera as defined in the first International Code of Botanical Nomenclature. except that they are no longer restricted in rank to genus.

Other regulations

Other than the fact that fossil-taxa relate to just parts of plants at particular life history stages and preserved in particular ways (as specified in the diagnosis) their nomenclature does not differ substantially from that of living plants. The only exceptions are as follows.

• The starting point for the publication of plant fossil-taxa is taken as the 31st December 1820, with the publication of Kaspar von Sternberg's Flora der Vorwelt. This was with the specific intention of excluding the species names introduced in Schlotheim's Die Petrefactenkunde, which were regarded as taxonomically problematic.
• From 1912, the publication of a new name of fossil-species or lower ranked taxon has to be accompanied by illustrations or reference to previously published illustrations of the types, and since 2001 one illustration has to be designated as a holotype.