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Diadectids are Early-Permian herbivores that have been found in North America and Europe. They aregenerally seen as the sister group of the amniotes, but their phylogeny is debated. The diadectid humerus(MNG-14473) investigated in this study can be used to reconstruct the lifestyle and life history of aspecimen through fossil bone histology. Using microtomography, high-resolution scans of the humerushave been made, which were used to make 3D models and to study the microstructures andmicroanatomy of the bone. BoneProfileR by Girondot & Laurin (2003) has been used to obtainparameters for bone compactness.

Calcified cartilage likely points to the specimen not being fully-grown yet, however, no LAGs havebeen observed due to taphonomical or environmental reasons. Therefore it is impossible to tell thediadectid's age. The high density of vascular canals and the erosion of the cortical bone point toward arelatively active metabolism for the diadectid. The thin cortex and a medullary cavity filled with spongybone usually imply an aquatic to amphibious lifestyle, but the bone morphology and localities wherediadectids have been found, suggest a terrestrial environment. Terrestrial testudines, heavy mammalssuch as rhinos and elephants, and extinct terrestrially interpreted seymouriamorphs and pareiasaurs showa similar bone microanatomy. Therefore diadectids do not necessarily need to be interpreted as aquaticnor amphibious relatively to their spongy bone microanatomy. This suggests that the bone compactnessmodel provided by Girondot & Laurin (2003) might not apply to stem amniotes and all early crownamniotes. Future studies on bone microanatomy of other stem amniotes might support or weaken theidea that the bone histology of those animals does not comply with the general bone compactness model.