Linnaean Classification of Organisms

Although the Linnaean classification scheme enjoyed a long history, for all practical purposes it has been replaced by cladistics (Chapter 1). One of the problems with the Linnaean system was that organisms with characters that did not fit into standard grades (phylum, class, and so on) were sometimes placed in between them, with the addition of an appropriate designated prefix. For example, superfamily includes more than one family but does not constitute an order, infraclass is within a class but not specific enough to warrant being called a superorder or order, and subphylum is not quite at the level of a phylum. Such splitting of categories presents difficulties on how to delimit them, as well as how to justify the differences between artificial subdivisions such as a superfamily and a suborder. Thus, one of the unwieldy facets of the Linnaean system is evident: where does one draw the line between its hierarchical levels?

Another criticism of the Linnaean system is that the justification for grades is made regardless of new evidence indicating the evolutionary relatedness of descendants, especially when it is applied to fossil organisms. For example, recall that dinosaurs classified in this system belonged to the following categories:

Phylum Chordata

Subphylum Vertebrata Class Reptilia

Subclass Diapsida

Infraclass Archosauria Superorder Dinosauria Order Saurischia Order Ornithischia Class Aves

Unfortunately, the hypothesized relatedness of theropods (in order Saurischia) and birds (in class Aves) is not indicated explicitly here. In this system, a paleontologist might want to challenge the separation of these two grades by saying that anatomical, physiological, and paleontological evidence supports the descent of birds from saurischian dinosaurs. Consequently, birds actually belong within class Reptilia and order Saurischia. Strict adherence to the Linnaean classification would not allow any appeals to new evidence that would place a formerly separate taxonomic group within the group where it shares an ancestry. Under a Linnaean classification, a reptile is a reptile, and a bird is a bird, on the basis of how they look today and how closely a fossil form might anatomically fit the appearance of a common ancestor. Such anatomical similarities are ideally determined by how closely two or more fossil forms are evolutionarily related, which the Linnaean classification approaches but does not quite achieve.

The purpose of this discussion, however, is not to belittle the Linnaean classification scheme, but to learn about it. The main justification for this is its long tradition, which led to its subsequent familiarity and widespread use in the scientific literature of the eighteenth, nineteenth, and most of the twentieth centuries. Hence, ignoring its use in biology and paleontology is akin to omitting the historical context and basis for what has constituted scientific discovery in those sciences during those times (Chapters 2 and 3). Knowledge of the Linnaean classification also helps in understanding and communicating with the biologists and paleontologists who still use it. Finally, some of the original names proposed for phyla, classes, and other Linnaean categories still form the basis of taxonomic categories applied to groups of organisms (not just dinosaurs) that share certain anatomical traits. This illustrates the difference between taxonomy (naming) and classification (sorting the names). Dinosaurs were surrounded by an abundance and diversity of organisms that resembled those of modern ecosystems. Consequently, knowing the general names based on previous classifications for life forms other than dinosaurs is also important for communicating about them. For the sake of simplicity, general groupings that correspond to conventional Linnaean classifications, such as arthropods, mol-lusks, and amphibians, are used throughout this text, although different names have been given in recent years to such groups on the basis of evolutionary relatedness.

This nearly complete change in taxonomic methods highlights some disadvantage of the Linnaean classification system. It does not address adequately the evolutionary relationships of organisms on the basis of shared anatomical characters in both ancestors and descendants. This disadvantage inspired the use of cladistics, which recognized the derivation of certain groups from ancestral groups on the basis of shared traits.

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