Paleohistology of Micropholis stowi (Dissorophoidea) and Lydekkerina huxleyi (Lydekkerinidae) humeri from the Karoo Basin of South Africa, and implications for bone microstructure evolution in temnospondyl amphibians

ABSTRACT

Temnospondyl amphibians are a large and diverse group of early tetrapods, whose paleohistology has been incompletely studied. Here, humeri of Micropholis stowi and Lydekkerina huxleyi from the Karoo Basin of South Africa (Katberg Formation) were thin–sectioned for paleohistological analysis. Diaphyseal bone histology of both taxa exhibits a convergence to fibrolamellar tissue and an absence of lines of arrested growth; additionally, medullary cavities free of trabeculae support terrestrial lifestyles in both Micropholis and Lydekkerina. The presence of azonal tissue in Micropholis is unlike that of other dissorophoids or extant caudatans, suggesting an adaptation to local conditions in the Early Triassic of the Karoo Basin, as well as a complicated and incompletely studied pattern of histological evolution in dissorophoids. Additionally, the propodial histology of these and 12 other taxa were assessed through different broad–scale phylogenetic hypotheses for Temnospondyli. Results reveal convergence towards sustained, non–cyclical growth and an absence of lines of arrested growth in the diaphyses of Early Triassic temnospondyls. The optimization of histological traits on to existing phylogenetic hypotheses is equally parsimonious between the different topologies. Homoplasy among histological characters suggests that evolutionary history in this group is overshadowed by developmental plasticity in bone microstructure, potentially due to environmental and biomechanical constraints. However, the interpretation of these data is limited by small sample size, and increased sampling is required to validate the patterns revealed in this study.

http://zoobank.org/urn:lsid:zoobank.org:pub:7CF4CD42–3C8B–43F3–83A1–7C611C5CDBB8

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ACKNOWLEDGMENTS

I am very grateful to C. A. Brochu, H. J. Sims, J. M. Adrain, D. W. Houston, J. S. Anderson, The University of Iowa vertebrate paleontology discussion group, A. Weil, and I. Browne for their comments on the manuscript. Thank you to A. Huttenlocker for helpful discussions. I especially thank S. Kaal, R. M. H. Smith, the Iziko South African Museum, and the South African Heritage Resources Agency for their gracious loan of Karoo specimens, without which this project would have not been possible. Thank you to The University of Iowa Thin Section Laboratory and I. Peate for services rendered and the use of equipment. And I thank the editor, an anonymous reviewer, and S. Sanchez for their helpful critiques. Funding for this project was provided by The University of Iowa Graduate College.