EVOLUTIONARY RELATIONSHIPS OF THE COELACANTH, LUNGFISHES, AND TETRAPODS BASED ON THE 28S RIBOSOMAL-RNA GENE

The origin of land vertebrates was one of the major transitions in the history of vertebrates. Yet, despite many studies that are based on e ither morphology or molecules, the phylogenetic relationships among te trapods and the other two living groups of lobe-finned fishes, the coe lacanth and the lungfishes, are still unresolved and debated. Knowledg e of the relationships among these lineages, which originated back in the Devonian, has profound implications for the reconstruction of the evolutionary scenario of the conquest of land. We collected the larges t molecular data set on this issue so far, about 3,500 base pairs from seven species of the large 28S nuclear ribosomal gene. All phylogenet ic analyses (maximum parsimony, neighbor-joining, and maximum likeliho od) point toward the hypothesis that lungfishes and coelacanths form a monophyletic group and are equally closely related to land vertebrate s. This evolutionary hypothesis complicates the identification of morp hological or physiological preadaptations that might have permitted th e common ancestor of tetrapods to colonize land. This is because the r econstruction of its ancestral conditions would be hindered by the dif ficulty to separate uniquely derived characters from shared derived ch aracters in the coelacanth/lungfish and tetrapod lineages. This molecu lar phylogeny aids in the reconstruction of morphological evolutionary steps by providing a framework; however, only paleontological evidenc e can determine the sequence of morphological acquisitions that allowe d lobe-finned fishes to colonize land.