Systematics And Classification

This book emphasizes the origin, evolution, and diversity of the major groups of plants based on the fossil record, and their relationships through geologic time; floristic changes through time are discussed to a lesser extent. To do this, we need to address the systematics of plants. The field of system-atics is concerned with classifying, naming, and determining the evolutionary relationships of taxa. Taxon (pl. taxa) is a general name to indicate any level of organization (i.e., a species, a genus, a family, etc.). Within systematics, taxonomy is the process of describing and classifying organisms into natural groups and nomenclature is the process of naming taxa. In this book, we will use Linnean nomenclature, in which each plant has a two-part name, sometimes called a binomial, which consists of a genus name and a species name (specific epithet). The rules of naming plants are complex and are encoded in the International Code of Botanical Nomenclature (ICBN) (McNeill et al., 2006), which is refined every 4 years.

In spite of long study and continued refinements, naming plants still represents a highly subjective exercise. Generally, all classification systems are based on the same type of evidence: shared features. Shared features allow one to recognize genera, families, and other higher categories of a classification scheme (Funk and Brooks, 1990). Such features may fall into two general categories. In one group are the primitive features (plesiomorphies) that evolved relatively early in the evolution of a group of organisms, such as the vascular tissue present in most terrestrial plants. Features of this type may be regarded as evolutionary holdovers that have persisted, but tell us little about the relationships among members of the group, because every member of the group has the same feature. The other group of characters is believed to have evolved more recently. These advanced or specialized features (apomorphies) can be used to identify organisms that have a common ancestry. The cladistic or phylogenetic system of systematics has the goal to produce a hierarchical organization of taxa based on shared, derived features (synapomorphies) that reflect the evolution of particular groups of organisms (Duncan and Stuessy, 1984). Classifications that group organisms based on the overall similarity of characters, whether both primitive or derived, are termed phenetic systems.

Another system that has been proposed for classifying living organisms, including plants, is the Phylocode (Cantino and de Queiroz, 2006). This system is very controversial, but is meant to reflect phylogenetic systematics more than Linnean taxonomy (Nixon et al., 2003). Monophyletic groups, that is those consisting of a single common ancestor and all descendants of that ancestor (clade), are defined solely by their position on the tree of life. Clades may have any rank, but the rank is added after nomenclature is completed. It will be interesting to see if this system gains recognition within the plant systematic community since many plant taxa are now thought to be paraphyletic (Rieseberg and Brouillet, 1994); paraphyletic groups include the ancestor and some, but not all, of its descendants). It would appear reasonable to assume that a classification scheme like the Phylocode, which includes only taxa that fit into mono-phyletic groups, will not be an accurate and useful tool for arranging the enormous biological diversity represented in the fossil record (Briggs and Crowther, 2001). Moreover, it is difficult to envision how fossils would be treated in the phylogenetic nomenclature of this classification system.

the plants are represented in the fossil record as disarticulated parts. This has resulted in the establishment of a special system of nomenclature for parts of fossil plants. As in other areas of botany, fossil plants are named according to the rules in the ICBN, but in paleobotany each disarticulated part is given a separate generic and specific name. In the past, paleobotanists had two types of names for parts of fossil plants. An organ genus was designated when there was enough information to assign a plant part to a family. For example, Lepidodendron, Stigmaria, and Lepidostrobus (Brack-Hanes and Thomas, 1983) (FIG. 1.77) are generic names used to designate parts (stem, roots, and cones) belonging to a particular type of Carboniferous lycopsid. The form genus was used for fossil plant parts that could not be assigned to a family, for example a piece of wood that could be assigned to the gymnosperms, but not to any particular group of gymnosperms. Originally, an organ genus was considered to represent a more natural (i.e., phylogenetic) taxon than a form genus, but confusion arose because names have been given to the same plant parts in different states of preservation or development. Today, the term morphotaxon has replaced the designations form and organ genera in paleobot-any. A morphotaxon is a fossil taxon which, for nomenclatu-ral purposes, comprises only the parts, life history stages, or preservational states represented by the corresponding nomenclatural type (Chaloner, 2004). The nomenclatural type is the plant fossil on which the name is based.

Why do paleobotanists give names to different parts of the same plant? The first reason for naming parts is so that the fossils can be studied and referred to in publications ph

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