A group of molecular biologists at the Department of Genetics at Lund University in Sweden published some new results which cast a new light on tetrapod origins. The results of their research on the mitochondrial DNA of Myxine glutinosa, an extant Agnatha, and the comparisons with representants for other major vertebrate groups do not conform to the general phylogenetic scheme, with amphibians evolving from fishes and reptiles from amphibians. The results indicate a basal divergence between gnathostome fishes and the branch leading to birds and mammals. Results also suggest that amphibians are inside the piscine tree.
The results of the Lund group suggest a tetrapod polyphyly, with amphibians being closely related to Osteichthyes (fishes) and reptiles/birds/mammals (Amniota) representing another branch, diverging from the basal vertebrate stock at a very early stage. The idea of tetrapod polyphyly was earlier advocated on morphological grounds by e.g. Jarvik.
Rasmussen, Ann-Sofie; Janke, Axel and Arnason, Ulfur, 1998: The mitochondrial DNA Molecule of the Hagfish (Myxine glutinosa) and Vertebrate Phylogeny. J. Molec. Evol. 46: 382-388
Jarvik, E., 1980: Basic structures and evolution of vertebrates 1 and 2. London.