RESEARCH ARTICLE Skeletal Morphogenesis ofMicrobrachis and Hyloplesion (Tetrapoda: Lepospondyli), and Implications for the Developmental Patterns

The ontogeny of extant amphibians often is used as a model for that of extinct early tetra-pods, despite evidence for a spectrum of developmental modes in temnospondyls and a paucity of ontogenetic data for lepospondyls. I describe the skeletal morphogenesis of the extinct lepospondylsMicrobrachis pelikani and Hyloplesion longicostatum using the largest samples examined for either taxon. Nearly all known specimens were re-examined, allow-ing for substantial anatomical revisions that affect the scoring of characters commonly used in phylogenetic analyses of early tetrapods. The palate of H. longicostatum is re-interpreted and suggested to be more similar to that ofM. pelikani, especially in the nature of the con-tact between the pterygoids. Both taxa possess lateral lines, andM. pelikani additionally ex-hibits branchial plates. However, early and rapid ossification of the postcranial skeleton, including a well-developed pubis and ossified epipodials, suggests that neither taxon meta-morphosed nor were they neotenic in the sense of branchiosaurids and salamanders. Mor-phogenetic patterns in the foot suggest that digit 5 was developmentally delayed and the final digit to ossify inM. pelikani and H. longicostatum. Overall patterns of postcranial ossifi