Pachgenelus monus (Watson 1913) total length: 50 cm. ~195 mya early Jurassic represents a derived cynodont, between Chiniquodon and the basal mammals Megazostrodon and Ornithorhynchus. Stem mammal sisters included the herbivores, Oligokyphus and Kayentatherium (below) and Repenomamus, a large, late-surviving carnivore.
Much smaller than its predecessors, Pachygenelus was the closest sister to mammals. Living in the time of predatory dinosaurs, only the smallest cynodonts remained hidden from their view and so survived. The smaller body was more difficult to heat. Thus Pachygenelus was likely covered in dense hair, like a mammal.
Hatchlings would have been proportionately smaller and likely unable to find their own food. The parents (or just the mother) would have made a nest for them and vomitted food they could eat.
No bar of bone separated the orbit (eye socket) from the temporal fenestra (synapsid opening). The posterior teeth were wider than in predecessors. The roots of these teeth were grooved, the start of the divided root that characterizes molars in mammals. The tooth enamel had a crystalline structure like that of mammals. Pachygenelus continued to shed its teeth like a reptile, but at a smaller size, it likely had a shorter life and thus fewer tooth replacements during a lifetime.
Because of its size, Pachygenelus was likely an insect and worm eater.