The evolution of skeletal development in early tetrapods : anatomy and ontogeny of microsaurs (Lepospondyli)
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Because the ancestry of extant amphibians remains highly controversial, under traditional perspectives, amphibians and amniotes often are distinguished by differences in developmental mode rather than their evolutionary relationships. Resolution of relationships is important, however, because phylogeny affects interpretations of biology, including the evolution of development. To address those issues, I documented the growth and development of two extinct lepospondyls, Microbrachis pelikani and Hyloplesion longicostatum, and compared the patterns in those taxa to data from other tetrapods. I quantified allometry in the skeleton using both measurement-based and geometric morphometric analyses. I applied Ontogenetic Sequence Analysis (OSA), a size-independent method, to the reconstruction of ossification sequences based on fossils. I also documented skeletal morphogenesis and used Parsimov Analysis and Parsimov-based Genetic Inference of ossification sequence data to evaluate the three hypotheses of extant amphibian ancestry, the Lepospondyl (LH), Temnospondyl (TH), and Polyphyletic (PH) hypotheses. Skeletal growth in Microbrachis pelikani and Hyloplesion longicostatum is primarily isometric. Comparisons with data from other Paleozoic taxa suggest that isometry was the ancestral pattern of growth in tetrapods. All regression analyses had a linear fit indicating lack of an abrupt metamorphosis. Absence of metamorphosis is also supported by the possession of lateral lines in both taxa throughout ontogeny, and Microbrachis pelikani additionally retained gills. However, ossification of the skeleton was completed at small body size. The greatest resolution in ossification sequence reconstruction was achieved with OSA, but results from all reconstruction methods indicated advanced ossification of the pubis and delayed ossification of the scapula in the lepospondyls. In terms of total number of sequence shifts optimized across each hypothesis of amphibian relationships, the TH had the shortest tree length. However, the values for the three hypotheses did not differ significantly, demonstrating that none was supported strongly. Based on my synthesis of new developmental data, I propose that Microbrachis pelikani and Hyloplesion longicostatum expressed a mosaic pattern of skeletal development. That pattern included a gradual transition to an adult morphology, and a lack of an amphibian-like metamorphosis. A similar pattern is common to most early tetrapods and Eusthenopteron, supporting the hypothesis that metamorphosis is not ancestral for Tetrapoda.