Systematics and classification

The modern classification of both fossil and living organisms attempts to organize life-forms in a logical and uniform manner and to illustrate the evolutionary relationships between different groups of animal and plant life. Only 250,000 fossil species have been described, while it has been estimated that some 500 million species have existed over the past 3.4 billion years. Thus, there are enormous numbers of fossil species still awaiting discovery and classification.

Binomial System

The system of classification utilized by biologists and paleontologists to classify both living and fossil organisms is based on the premise that similarities in morphology or structure provide evidence of a biological kinship between organisms. The biological classification employed by biologists and paleontologists not only expresses the similarities and differences between groups but also summarizes or conveys a vast amount of information concerning these groups, especially the general phylogenetic or evolutionary relationships between groups.

“Systematics” is the study of taxonomy, which is concerned with identifying and classifying fossil organisms into morphologically and evolutionarily related groups. These groups or taxonomic categories are referred to as taxa. The classification system used worldwide is the binomial system of nomenclature, proposed by the Swedish naturalist Carolus Linnaeus in the 1700s. The fundamental unit, or taxon, in this classification is the species. A species can be defined as a group of morphologically similar individuals whose ecological demands and physiologic functions are the same and whose members interbreed to produce fertile offspring. Groups of closely related species that share a common ancestry and possess a set of similar morphological characteristics are termed “genera” (singular, genus). Under the binomial system, each plant or animal is given two names: the genus name first, the species second. The human species, for example, is referred to as Homo (genus) sapiens (species). The names of both the genus and the species are printed in italics. Species names are given in Latin, as originally this was the universal language of science in the Western world, and naturalists were looking for a standardized nomenclature recognizable to scientists from all countries. The Latinized names of species have remained universal, recognizable to biologists and paleontologists worldwide.

There are also taxonomic categories above the genus level. Similar genera are grouped into a single family; groups of related families are placed into an order; similar orders form a class; and closely related classes form a phylum. The more than thirty-five phyla are placed into one of five large groups termed kingdoms. These include the Monera (single-celled organisms with no cell nucleus), the Protista (single-celled organisms with a cell nucleus), the multicellular Plantae (organisms that produce their own food via photosynthesis), the multicellular Animalia (organisms that gain nutrients from the ingestion of other organisms), and the Fungi (multicellular organisms that absorb nutrients from their environment). Whereas the species represents a real, natural taxonomic unit, all taxonomic categories above the species level are artificial and subjective. The classification of the carnivorous dinosaur Tyrannosaurus rex would be as follows: kingdom, Animalia; phylum, Chordata; class, Reptilia; order, Saurischia; family, Tyrannosauridae; genus, Tyrannosaurus; species, Tyrannosaurus rex. The basic categories of higher taxa are sometimes supplemented by additional categories with the use of a sub- or super-prefix.

Hierarchical Classification

This type of classification is referred to as a hierarchical system. In such a system, each fossil specimen belongs to one “lower” taxonomic category or species and one of each of the “higher” or superspecific categories. From the highest rank (kingdom) to the lowest rank (species), these categories become progressively narrower in scope. For example, 1.5 million or more species form the kingdom Animalia, but only one species constitutes the species Homo sapiens. The criteria used to differentiate taxonomic groups also become narrower and more trivial as one goes from higher to lower taxa.

The binomial system attempts not only to divide organisms into structurally differentiated groups but also to illustrate the phylogenetic, or evolutionary, relationships between these groups. This classification scheme serves as a phylogenetic outline for the evolution of life on Earth and as an indicator of the diversity of basic body plans available to organisms through time. This is often diagrammatically depicted in the rough form of a branching tree, with the more primitive, structurally simpler organisms at the base and the more advanced, structurally more complex forms in the crown. In such figures, one can see how animals evolved from simple, single-celled protozoans through increasingly more complex multicellular organisms with successively more sophisticated grades of development. These evolutionary pathways are determined from studies of the comparative anatomy of living organisms and by information provided by the fossil record.

The identification of fossils and the integration of fossil organisms into the system of biological classification allows for reconstructing the history of life on earth. The present records just one instant in the long history of life on this planet—a “snapshot” of life as it exists today. The fossil record, however, extending back some 3.4 billion years, provides the whole epic motion picture. Although some frames are missing or blurred by the effects of time, fossils provide the only record of life on Earth and, therefore, offer the sole source for the factual documentation of the evolution and extinction of organisms through time. The systematic study of fossil organisms provides four types of evolutionary information: It allows scientists to reconstruct the phylogenetic relationships that exist between various taxonomic groups; it reveals the times of major evolutionary events, such as major evolutionary radiations and so-called mass extinction events; it gives some idea as to the rates of evolutionary change; and it reveals patterns of evolutionary change.

Fossil Removal and Description

Determining the identity of a particular fossil provides an outline of its biological affinities, evolutionary pedigree, and anatomical complexity and also provides some idea of how the organism lived in its environment. The fossil then becomes more than a curiously shaped stone; it becomes the remains of a once-living organism that, in itself, is a time capsule of information about that interval of geologic time during which the organism lived and died.

The basic tools of the systematic paleontologist traditionally have been the various implements used to remove the fossils from the rock matrix and prepare them for description, calipers to measure morphological dimensions, and an adequate reference library to aid in determining the identity and relationships of the fossils under study and compare them with those already described. Today, computers can store and compare the dimensions of large suites of specimens and statistically analyze large sets of data in order to determine the possible relationships between fossil taxa. Studies of shell microstructure and geochemistry have also provided new approaches to fossil identification and classification beyond the study of the external features of a fossil specimen.

The general approach to fossil identification, however, remains essentially the same as it has been for the past two centuries. A fossil is discovered and collected, with the details of its occurrence—that is, the rock layer or stratum and geographic locality where it was found—being determined and recorded with the fossil. Ideally, associated fossil species should also be noted, as well as the orientation of the fossil specimens in the rock and the nature of their preservation. This may help the paleontologist to infer something about the life habits and habitats of the fossils. A single specimen of a particular fossil taxon is not enough; the larger the number of specimens of the fossil available for collection from the site, the better for the purposes of statistical analysis of the species.

The fossil specimens are removed from the enclosing rock matrix, and rock matrix adhering to the specimens is removed using preparatory equipment ranging from hammers and chisels to air-abrasive equipment and vibra-tools. This preparation is necessary to expose the main morphological features of the shell or bone, features critical in determining the fossil's identity. Paleontologists occasionally use sonic cleaners, weak acids, or strong detergents to remove more stubborn matrix adhering to the fossils. Fortunately, many fossils weather cleanly from soft sedimentary rocks like shales, marls, and poorly cemented sandstones and require little cleaning and preparation.

Following this, a detailed description of the specimen is formulated, including the dimensions of the specimen (measurements of length, width, height, thickness, and so on), the general arrangement of the parts that compose the fossil, and a description of the external and internal morphological features that distinguish the specimen. For certain fossil groups, especially corals, bryozoans, and nautiloid cephalopods, it is necessary to cut open specimens with rock saws, both longitudinally and tangentially, to expose critical internal structures and the microstructure of the shell if it is preserved. If more than one specimen of the fossil type under study were collected, each specimen would be described and then compared with its associates in order to determine any morphological variation present within the fossil population or to determine whether one or more distinct species were present in the collection.

Fossil Classification

To classify a fossil specimen, scientists compared them with fossil forms already described in the scientific literature. The usual process for identifying a fossil specimen is to determine its phylum first and then proceed from the general to the specific. Information to aid in the identification of fossils can be found in a number of reference books, textbooks, and popular field guides on fossils. Invertebrate fossils (remains of animals that lack backbones) constitute the majority of fossils found. These primarily marine animals are also the easiest to identify, largely as a consequence of the possession of hard parts consisting of mineralized external or internal shells that usually are composed of a minimal number of parts that are preserved intact.

The fossil remains of vertebrate animals and plants are much less common and more difficult to identify and classify. This is attributable, in part, to the fact that these organisms are most common in terrestrial environments where the rapid and complete burial necessary for preservation as a fossil is not a common process. The internal skeletons of vertebrates consist of a multitude of individual bones held together by tendons, cartilage, and fleshy tissue. Upon the death of the organism, these soft parts decay, and the skeleton readily disaggregates into a pile of loose bones that, upon transport by stream action or scavengers, are spread over a broad area and are difficult to reassemble into the original skeleton. Plant remains lack mineralized hard parts and consist of multiple parts (roots, stems, leaves, seeds) that also are easily separated from one another in the process of burial.

After comparing the fossil specimens with species that have already been described, one of two conclusions can be made. Either the fossil specimens can be identified as those of an existing species, or they constitute a species new to science. If the specimens are identified as a preexisting species, they should be closely compared with the type specimens of the known species to ascertain if any variation exists between the two suites of specimens. The “type specimens” are the original specimens for which the fossil species was first erected. They are usually preserved for posterity in collections held at major natural history museums. The age of the rock unit and the geographic locality of the collecting site should be compared with that of the existing species to determine whether these new specimens extend the range of the species in time or space. If it is determined that the fossil specimens are sufficiently distinct from existing species, then a new species is created for these specimens.

Description of New Species

The description of a new species must follow a specific format that has been specified in the Code of Zoological Nomenclatureestablished by the International Commission on Zoological Nomenclature in the early 1900sto bring a measure of uniformity to animal taxonomy. A comparable set of procedures has been established for plants. This code specifies the choice of the species name, publication of the name, description of the new species, and the designation of one or more type specimens.

The name given to a new species must be binomial, consisting of the genus and the species name. The name used for the species cannot already be in use with the genus name with which it is to be associated. The names of the species and the genus must be Latin words or words that have been Latinized. Species names can be Latinized place-names, the names of people, or descriptive words. A species cannot be named anonymously, and the author's name is part of the official species name. For example, the official name for the living species of the chambered nautilus from the southwest Pacific would be Nautilus pompilius LinnaeusCarolus Linnaeus being the original author of the species.

In order for a new species to be officially recognized, the name and description of the distinctive features that identify it must be published in an approved medium. That is, it must be in print, published in quantity, and circulated to libraries to ensure that the announcement of the new name is readily available to scientists around the world. The description of a new species should include a diagnosis listing the characteristics distinguishing the new species from similar species as well as a fuller description of the various morphological aspects of the species, along with its geographic locality, geologic occurrence, and a reference to the museum where the designated type specimens for the species are stored. It is also strongly recommended that an illustration of the new species be included in this description.

The International Code also specifies that each new species description must be accompanied by the designation of a type specimen or set of type specimens. These are the actual original fossil specimens upon which the species is based. The code requires that type specimens be clearly labeled as such and that measures be taken to ensure their preservation and accessibility to interested scientists. This usually means that specimens are deposited in a major natural history museum, where such facilities are available. If the new species is based on a single specimen, this specimen is designated the type specimen, or “holotype.” If several specimens serve this purpose, they are designated the “syntypes” for the species.

The naming of new species is best left to a professional paleontologist who is familiar with the rules of zoological nomenclature and the accepted format for publication. Nevertheless, amateur fossil collectors, who typically find the bulk of new species in the field, are often recognized for their contributions by being named a coauthor of the new species or having the new species named for them.

Cladistics

Reconstruction of the phylogenies, or evolutionary family trees, of fossil organisms operates in a manner opposite that of fossil identification and classification. A paleontologist starts at the species level, reconstructing evolutionary relationships between species, and then works up the taxonomic hierarchy, from the narrow to the broad. Phylogenies are reconstructed using a system called “cladistics,” in which organisms are grouped according to the presence of shared characteristics that are particularly informative. This system assumes that as evolution progresses, new, heritable traits emerge and are passed on; two groups of animals sharing such new traits will be more closely related to each other than to groups that only share the original traits. Analyses of these characters are presented as treelike diagrams called “cladograms,” which show the order in which the new characters, and thus the new species, evolved. Each branching point in the diagram reflects the emergence of an ancestor that founded a group that exhibited advanced characters not present in the groups that had developed earlier. Through detailed studies, or “monographs,” of groups of related species and genera, the phylogenies of most of the major animal groups have been mapped out. These reconstructions, however, are constantly undergoing change, primarily as the result of the description of newly discovered fossil taxa and the restudy of older, established species. Furthermore, with the help of artificial intelligence (AI) and computer systems, scientists are able to link new species to previous ones, study timelines more succinctly, and learn, in greater detail, the origins of species in relation to others within their taxa.

Principal Terms

binomial system: the current system of classifying organisms, which gives each organism a dual name consisting of the genus and the species

genus: a group of closely related species that share a common ancestry and similar morphological characteristics

kingdom: one of the five large subdivisions of life, differentiated on the basis of gross body plan (single-celled versus multicellular) and method of obtaining food or nutrients (produces own food or obtains it from other organisms)

nomenclature: the names and terms used in a classification system

phylogeny: the study of the evolutionary relationships between organisms

phylum: a major grouping of organisms, distinguished on the basis of basic body plan, grade of anatomical complexity, and pattern of growth or development

species: a group of actually or potentially interbreeding organisms; the basic taxonomic unit

taxon: one of several systematic groups or categories into which a particular organism may be placed

type specimens: the original fossil specimens upon which a species was first erected

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