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S. Blair Hedges, Ronald A. Nussbaum, Linda R. Maxson, Caecilian Phylogeny and Biogeography Inferred from Mitochondrial DNA Sequences of the 12S rRNA and 16S rRNA Genes (Amphibia: Gymnophiona), Herpetological Monographs, Vol. 7 (1993), pp. 64-76
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Hedges et al. (1993) studied this question.
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