Klein, Nicole Houssaye, Alexandra Neenan, James M. Scheyer, Torsten M. <jats:p>Placodontia, an enigmatic group of durophagous and in part heavily armoured animals, were members of Sauropterygia, the most diverse and successful group of Mesozoic marine reptiles. Microanatomy and histology of long bones of several armoured and non-armoured Placodontia were studied, covering most of their taxonomic breadth, to elucidate the paleoecology, physiology, and lifestyle of its members. Results reveal an unexpected and not phylogenetically or stratigraphically related disparity of microanatomical and histological features for the group. The non-armoured <jats:italic>Paraplacodus</jats:italic> and the heavily armoured <jats:italic>Psephoderma</jats:italic> grew with lamellar-zonal bone tissue type, which is typical for modern sauropsids. In the former, the tissue is nearly avascular surrounding a compacted medullary region, whereas in the latter, the lamellar-zonal bone tissue is vascularized framing a large open medullary cavity and a perimedullary region. Armoured <jats:italic>Henodus</jats:italic> and Placodontia indet. aff. <jats:italic>Cyamodus</jats:italic> as well as non-armoured <jats:italic>Placodus</jats:italic> exhibit a reduced medullary cavity and grew with highly vascularized plexiform to radiating fibro-lamellar bone. Several long bones of Placodontia indet. show circumferential fibro-lamellar bone and can be distinguished into two groups on the basis of microanatomical features. In addition, all bones that grew with fibro-lamellar bone show locally primary spongeous-like architecture and had secondarily widened primary osteons throughout the cortex, resulting in a secondarily spongeous tissue. The highly vascularized fibro-lamellar bone of these Placodontia indicates growth rates comparable to that of open marine ichthyosaurs. Differences in microanatomy and bone histology as expressed by a principal component analysis, thus clearly indicate different paleoecologies, including differences in lifestyle and swimming modes and capabilities in Placodontia. This would have reduced competition in the shallow marine environments of the Tethys and might be a key to their success and diversity. A certain developmental plasticity among the studied placodonts is interpreted as response to different environmental conditions as is obvious from inter- and intraspecific histological variation. Most striking is the difference in life history strategy in armoured <jats:italic>Psephoderma</jats:italic> and non-armoured <jats:italic>Paraplacodus</jats:italic> when compared to armoured <jats:italic>Henodus</jats:italic>, Placodontia indet. aff. <jats:italic>Cyamodus</jats:italic>, non-armoured <jats:italic>Placodus</jats:italic>, and Placodontia indet. Bone tissue of <jats:italic>Psephoderma</jats:italic> and <jats:italic>Paraplacodus</jats:italic> indicates low growth rates and a low basal metabolic rate, as many modern sauropsids have such as the marine iguana, whereas the others grew with extremely fast growth rates, more typical for birds and mammals, indicating an increased basal metabolic rate.</jats:p> application/pdf https://sonar.ch/global/documents/71981 eng info:eu-repo/semantics/altIdentifier/doi/10.1163/18759866-08401005 info:eu-repo/semantics/altIdentifier/issn/1383-4517 info:eu-repo/semantics/openAccess Contributions to Zoology. - Brill. - 2015, vol. 84, no. 1, p. 59-S15 Animal Science and Zoology Ecology, Evolution, Behavior and Systematics Long bone histology and microanatomy of Placodontia (Diapsida: Sauropterygia) http://purl.org/coar/resource_type/c_6501