The classical phylogenetic tree depicting relationships among living tetrapods groups birds and crocodilians as each others closest genetic relatives. Recent molecular studies in our laboratory and some analyses of morphological and molecular data of others has suggested that birds may be more closely related to mammals than to crocodilians. The slow-evolving 18S and 28S nuclear ribosomal RNa (rRNA) genes are excellent probes for examining evolutionary relationships among living species, including the tetrapods. Collecting rRNA sequence data from additional species of amniotes will permit alternative hypotheses of relationships to be tested. The living amphibians are placed in three groups: frogs, salamanders, and caecilians. The caecilians are limbless animals which are found in tropical areas throughout the world. Nucleotide sequence data from the 28S nuclear rRNA gene and two mitochondrial genes (12S rRNA and cytochrome b) will be gathered from caecilians in order to examine evolutionary relationships within this group. Caecilians are ideal organisms for study biogeography and therefore knowledge of their relationships, needed for understanding past distributions, will be especially important.
