The Origin of Higher Clades. Osteology, Myology, Phylogeny and Evolution of Bony Fishes and the Rise of Tetrapods. Rui Diogo.

This book is intended for those thoroughly convinced of the effectiveness of computer-driven phylogenetic systematics in establishing relationships at all taxonomic levels, and willing to accept significant changes in the usage of long-accepted names of major groups of organisms. This is immediately evident in the use of the term “tetrapod” in the subtitle, which I assumed to mean four legged, primarily terrestrial vertebrates, but which I soon discovered was extended to include all their antecedents, back to a basal dichotomy among very early bony fish. Even the author occasionally slipped back to the common usage in referring to Ichthyostega and Acanthostega (oddly not in the index) as early tetrapods. Another major problem, common to many large-scale analyses using PAUP, is to concentrate on living representatives of long-surviving taxonomic groups, necessary when dealing with molecular data and aspects of the soft anatomy, but failing to consider the obvious importance of fossils, which may much better represent the characteristics of the actual antecedents. For example, Diogo asks whether lungfish or coelacanths are the sister-taxon of tetrapods, to which a paleontologist would answer, “neither,” but refer to another group, known only from the fossil record, the rhipidistians that include the most primitive vertebrates possessing an internal nares, otherwise a synapomorphy of tetrapods. These are the only known assemblage providing evidence of the evolutionary origin of terrestrial vertebrates. The bias toward comparison among modern vertebrates is further exemplified by a character matrix incorporating 73 extant taxa, but only 7 fossil genera. Aside from osteological characters common to all clades, emphasis was placed on cranial and pectoral musculature and the Weberian apparatus. These features may provide an effective means for establishing probable relationships among the living members of particular lineages, but inherently limit comparison with clades known only from the fossil record. The rate of turnover at the level of species, genera, and families (as indicated by the very good record of fossil mammals) demonstrates that there were probably far more fish taxa that have become extinct than are living today, making it improbable that any phylogenetic analysis restricted to modern species will provide an accurate image of the inter-relationships of clades that originated hundreds of millions of years ago. Large-scale analyses of other taxonomic groups such as amphibians (Frost et al.: “The Amphibian Tree of Life”, 2006, Bulletin of the American Museum of Natural History, 297: 1–370) and many examples described in Systematic Biology clearly demonstrate that apparent patterns of relationships based on living species are highly dependent on the particular taxa studied and the specific characters or genes selected. The most informative analysis in this book was a contrast of results gained from myological versus osteological characters in comparison with those characters combined in a single phylogenetic reconstruction. This demonstrated that muscular features were more “conservative,” making for much less resolution at lower taxonomic levels, and resulting in many polytomies. Individual bones were more informative, providing for an average of 1.8 informative characters per bone, compared with 1.4 for muscles. Muscles, of course, were of no help in the placement of fossil groups, but in combination with osteological characters resulted in an overall phylogeny closer to that produced by other analyses. Phylogenetic analyses of long-surviving groups based on molecular data and knowledge of the soft anatomy will continue to provide evidence of putative relationships, but it should be recognized that without inclusion of available data from the fossil record, these approaches will provide only an incomplete understanding of the nature of prior evolutionary events such as phylogenetic divergence, progressive modifications in anatomy, and adaptation to different environments.

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