as Sakmarian or Artinskian, turns out to be older with an early or middle Asselian age. It is thus the oldest known Permian caseid, only surpassed in age by the Carboniferous taxa Eocasea and Datheosaurus. Paleoecology The Millery Formation is 250 m thick and consists of dark gray oil shales deposited in a lacustrine environment. The volcanic ash, altered into tonsteins, preserved in these deposits is linked to an aerial explosive volcanism. The exact origin of these volcanic ash is uncertain but the closest active volcanoes to the Autun basin during the Carboniferous-Permian boundary were in the north of the Massif Central (Blismes and Montreuillon areas) and in the Black Forest and Vosges areas. The rare amniotes found in these lacustrine layers, such as Callibrachion, the small sphenacodontid Haptodus and the taxon of uncertain affinity “Belebey” augustodinensis (both known from another site in the same formation) are allochthonous. The temnospondyl Onchiodon, a smaller relative of the genus Eryops from North America, is also present. Paleobiology The conical, thin, and slightly curved teeth of Callibrachion differ markedly from the spatulate or leaf-shaped teeth with cusps of later herbivorous caseids. They are more like those of the Eothyrididae and the basal caseid Martensius. The former were small predators, the latter was insectivorous in juvenile stage and omnivorous in adulthood. Callibrachion lacked the enlarged rib cage of derived herbivorous caseids which had a voluminous intestine necessary for digestion of high-fiber vegetation. The absence of this characteristic in Callibrachion as well as its particular dentition indicate that it was faunivorous and probably fed on insects and small vertebrates. Classification From its description in 1893 until the late 1970s, most paleontologists who studied Callibrachion believed it to be close of species now classified as Sphenacodontidae. Some authors have also considered it to be a junior synonym of the genus Haptodus. In the 1990s, Michel Laurent came to doubt the synapsid nature of Callibrachion (then classified as Haptodus gaudryi) and considered it a nomen dubium. It was not until 2015 that the animal was re-studied by Spindler and colleagues and identified as a caseasaur on the basis of its overall proportions as well as dental and osteological characteristics that exclude it from any other synapsid clades. In 2016, a phylogenetic study of caseasaurs recovered Callibrachion as a basal caseid closely related to Eocasea and Datheosaurus''. Below is the phylogenetic analysis published by Neil Brocklehurst and colleagues