Contributions to Zoology, 74 (1/2) (2005)S. Grosjean: The choice of external morphological characters and developmental stages for tadpole-based anuran taxonomy: a case study in Rana (Sylvirana) nigrovittata(Blyth, 1855) (Amphibia, Anura, Ranidae)
Discussion and conclusions

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According to the results presented herein (Table 3), a R. nigrovittata tadpole bearing all its potentiallyuseful characters is a tadpole at stage 32 or later, accordingto the relative length of keratodont rows,which is a valuable character in taxonomy especiallyin morphologically homogenous groups suchas the tadpoles of the genera Bufo or Hyla (Annandaleand Rao, 1918; Gosner and Black, 1957; Limbaughand Volpe, 1957; Zweifel, 1961). The importantmodifications occurring in the buccopharyngeal cavityfrom stage 43 onwards (Wassersug, 1976b) – thedegeneration of the mouthparts beginning at stages40 or 41, independently of the genus (Limbaugh andVolpe, 1957; Volpe, 1957; Gosner, 1960; Sedra andMichael, 1961; Zweifel, 1961), and the resorption oftail and cloacal tail piece (stage 41; Gosner, 1960),linked to metamorphosis – prevent the use of tadpolesof these late stages for taxonomic purposes. Therefore,tadpoles in developmental stages from 32 to 40possess the complete set of characters and only tadpolesin this range should be used for taxonomic andsystematic purposes. Earlier stages of tadpoles mustbe compared only with specimens in the same stageof development.

A taxonomically reliable character can be defined as one that is stable during a part of ontogeny, i.e. showing low variation and being suitable for taxonomic comparisons. Not all such characters are necessarily species-specific. It is the comparative study itself that will establish, among the set of available characters, those that are species-specific for the taxa studied and that are taxonomically or phylogenetically informative. The present study should be useful in pointing to characters and stages that are likely to bear such information. However, it is important to emphasize that all characters are potentially suitable, in the sense that they can be used in certain cases. Also, the variable morphometric characters should be mentioned in the description of a tadpole in order to describe the shape of the animal.