MOLECULAR EVIDENCE ON THE ORIGIN OF TETRAPODS AND THE RELATIONSHIPS OF THE COELACANTH

Coelacanths were believed to have gone extinct more than 80 million years ago - until the sensational rediscovery of one surviving member of this lineage, Latimeria chalumnae, in 1938. Since then, paleontologists and comparative morphologists have argued whether coelacanths or lungfish (two groups of lobe-finned fish) are the living sistergroup of the third extant lineage, the tetrapods. Recent molecular phylogenetic data on this debate tend to favor the hypothesis that lungfish are the closest relatives of land vertebrates. Somewhat surprisingly, the strongest molecular support for this hypothesis stems from mitochondrial rather than nuclear DNA sequences, despite the expectation that the more-slowly evolving nuclear genes should be more appropriate in addressing a phylogenetic issue involving taxonomic groups that diverged around 400 million years ago. This molecular estimate might serve as a framework to test paleontological phylogenies and hypotheses about morphological and physiological innovations and preadaptations that allowed Devonian lobe-finned fish to colonize land.