Types of characters in fish systematics
2.2.1 Types of characters in fish systematics
For fishes, osteological and external characters have dominated historically, at least in part be- cause they are most easily studied, particularly in fossil specimens. It is therefore not surprising that most higher taxa are diagnosed mainly or entirely by such characters. The modern discovery and survey of osteological characters has been greatly enhanced by the relatively recent development of improved methods for preparing specimens (Taylor 1967; Dingerkus and Uhler 1977; Taylor and Van Dyke 1985).
Muscles, particularly those operating the jaws, have received a reasonable level of investigation for character analyses, and have also played a role in determining topographical relationships of os- teological features, an important step in refining our definition of such characters. Other systems associated with soft anatomy, such as neural anatomy, heart and blood vessel anatomy, have not been as comprehensively studied, though they are of considerable potential, both as characters in their own right and for refining our understanding of characters for osteological and other studies. For example, Parenti and Song (1996) studied innerva- tion patterns in an attempt to define more care- fully the character ‘pelvic-fin position’, which has had a long history in systematic ichthyology. Other morphological systems that have been important sources of characters include egg mor- phology (e.g. White et al. 1984; Mooi 1990; Britz 1997), scale ultrastructure (e.g. Roberts 1993) and spermatozoan morphology (e.g. Jamieson 1991). Behavioural characters has also been em- ployed in phylogenetic studies. For example, the Ostariophysi are distinctive in possessing an
Phylogeny and Systematics
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alarm reaction to alarm substance (see below) and McLennan et al. (1988) employed behavioural characters to investigate relationships among gasterosteids.
Finally, recent years have seen a strong bias to- wards the use of biochemical characters, such as isozyme and sequence data. Despite tremendous technological advances in methods of, for exam- ple, DNA extraction and sequencing and their demonstrated utility at lower taxonomic levels, biochemical characters have yet to contribute in a significant way to our understanding of the higher relationships of fishes. This, in part, reflects our poor understanding of biochemical characters, their distribution and homology, in short the lack of an appropriately comprehensive historical basis. Studies of higher relationship in which one or a few exemplars from huge clades are surveyed, which is the current norm in molecular studies of higher relationships of fishes, are unlikely to pro- duce a realistic understanding of characters, their distribution, or the relationships they imply. For example, Rasmussen and Arnason’s (1999) conclu- sion from an analysis of mitochondrial DNA se- quences that cartilaginous fishes are nested within bony fishes is flawed, because the eight fish species surveyed do not meaningfully represent 25 000 or so fish species. Similar concerns about taxonomic sampling were expressed by Hennig (1966, p. 103) in his discussion of early studies of serum char- acters. Nevertheless, it is likely that this situ- ation will improve as our historical basis for biochemical characters improves and more taxa are surveyed. As these challenges are overcome, molecular characters will provide fish systema- tists with valuable information that, along with continued analysis of morphological data, will significantly advance higher-level phylogeny construction.