manipulate the plants on which they fed. In 2016, Markus Lambertz and colleagues questioned the terrestrial lifestyle of large caseids like Cotylorhynchus. These authors showed that the bone microstructure of the humerus, femur and ribs of adult and immature specimens of Cotylorhynchus romeri resembled that of aquatic animals more than that of terrestrial animals, the bones having a very spongy structure, an extremely thin cortex, and having no distinct medullary canal. This low bone density would have been a handicap for animals weighing several hundred kilos with a strictly terrestrial lifestyle. Lambertz et al. also argued that the joints between the vertebrae and the dorsal ribs allowed only small ranges of motion of the rib cage, thus limiting costal ventilation. To overcome this, they proposed that a proto-diaphragm was present to facilitate breathing, especially in aquatic environment. These authors also consider that the arid paleoclimates to which the caseid deposits correspond are not incompatible with an aquatic lifestyle of these animals. These paleoenvironments in fact included a significant number of water bodies (rivers, lakes and lagoons). The arid conditions could have been the reason why the animals would sometimes congregate and eventually die. In addition, arid environments have a low density of plants, which would require even more locomotor effort to find food. Thus, for Lambertz et al., large caseids like Cotylorhynchus must have been mainly aquatic animals that only came on dry land for the purposes of reproduction or thermoregulation. This hypothesis is however disputed by Kenneth Angielczyk and Christian Kammerer as well as by Robert Reisz and colleagues based on paleontological and taphonomic data combined with the absence in these large caseids of morphological adaptations to an aquatic lifestyle. According to Angielczyk and Kammerer, the low bone density of caseids identified by Lambertz et al. does not resemble that of semiaquatic animals, which tend to have a more strongly ossified skeleton to provide passive buoyancy control and increased stability against current and wave action. Cotylorhynchus bone microstructure is more similar to what is seen in animals living in the open ocean, such as cetaceans and pinnipeds, which emphasize high maneuverability, rapid acceleration and hydrodynamic control of buoyancy. However, the caseid morphology was totally incompatible with a pelagic lifestyle. Thus, due to these unusual data, Angielczyk and Kammerer consider that the available evidence is still insufficient to question the more widely assumed terrestrial lifestyle of caseids. Robert Reisz and