Molecular studies suggest that cartilaginous fishes have a terminal position in the piscine tree

Abstract

The Chondrichthyes (cartilaginous fishes) are commonly accepted as being sister group to the other extant Gnathostomata (jawed vertebrates). To clarify gnathostome relationships and to aid in resolving and dating the major piscine divergences, we have sequenced the complete mtDNA of the starry skate and have included it in phylogenetic analysis along with three squalomorph chondrichthyans-the common dogfish, the spiny dogfish, and the star spotted dogfish-and a number of bony fishes and amniotes. The direction of evolution within the gnathostome tree was established by rooting it with the most closely related nongnathostome outgroup, the sea lamprey, as well as with some more distantly related taxa. The analyses placed the chondrichthyans in a terminal position in the piscine tree. These findings, which also suggest that the origin of the amniote lineage is older than the age of the oldest extant bony fishes (the lungfishes), challenge the evolutionary direction of several morphological characters that have been used in reconstructing gnathostome relationships. Applying as a calibration point the age of the oldest lungfish fossils, 400 million years, the molecular estimate placed the squalomorph͞ batomorph divergence at Ϸ190 million years before present. This dating is consistent with the occurrence of the earliest batomorph (skates and rays) fossils in the paleontological record. The split between gnathostome fishes and the amniote lineage was dated at Ϸ420 million years before present.The relationship between gnathostomous fishes and their terrestrial relatives is of fundamental importance for the understanding of vertebrate evolution. Molecular analyses of this relationship have addressed in particular the question of whether, among extant fishes, the lungfishes or the coelacanth are the sister group to terrestrial vertebrates. However, although these analyses have differed with respect to the taxa included, a teleostean (1-4) or chondrichthyan (5) rooting of the gnathostome tree has been a common characteristic, and these studies have, in general, supported a sister group relationship between lungfishes and amniotes (or tetrapods). Because the application of rooting automatically gives evolutionary direction to a tree, it is essential that rooting is performed by using an outgroup that is unambiguously positioned without the ingroup taxa. The commonly applied teleostean rooting of the vertebrate tree is incompatible with piscine paleontology (6, 7) whereas the chondrichthyan rooting is subjective in the sense that it assumes a priori that chondrichthyans are the sister group of all other extant gnathostomes. Therefore, the application of either the teleostean or chondrichthyan rooting is inconsistent with the criterion that unequivocal outgroups should be used to establish the polarity of phylogenetic trees.The conclusions based on the teleostean and chondrichthyan rooting have been challenged in two recent molecular studies (8, 9) in which the gnathostome tree was rooted by using non-gnathostome tax...

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