The evolutionary significance of the earliest cloacal opening in Synapsida

The cloaca is a fundamental structure of modern terrestrial vertebrates, including the terminal parts of the urogenital and digestive systems that converge in a single channel and opening (vent). Although it is widespread in amphibians and reptiles (including birds) and occurs in mammals, the fossil record of this structure is scarce. Here we present an exceptional specimen from the early Permian Bromacker locality (294 Ma), which includes a tail impression with two rows of elevated scales separated by a vertical slit, that we interpret as cloacal lips and vent. Because of its morphology and association with the footprint Dimetropus , we attribute this impression to caseid synapsids. This represents the first occurrence of a cloaca in the fossil record of stem mammals. The comparison with modern and fossil specimens allows us to hypothesize a vertical vent orientation for early crown tetrapods, that subsequently changed orientation at least three times during tetrapod evolution.