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.

Kayentatherium wellesi (Kermack 1982; Early Jurassic; 1m in ln length, largest skull 10 cm in length) had relatively longer limnbs and a shorter torso than Oligokyphus. The incisors were longer and the jugals were deeper. The coracoid was retained. Note the anterior spine on the scapula.

Tritylodon longaevus (Owen 1884; Early Jurassic, 190 mya; 12 cm skull length) was a tritylodont cynodont with a three-cusped tooth. A sister to Kayentatherium and very similar to it, Tritylodon was a pre-mamalian rodent-mimic. Traditional trees nest tritylodontids as a separate order from Pachygenelus, but here they nest together.

Chaliminia musteloides (Bonaparte et al. 2001, Martinelli and Rougier 2007; Late Triassic, 215 mya; PVL 3857) is a close relative of Pachygenelus. Note the large dentary incisors and short premaxillary incisors.

Riograndia guaibensis (Bonaparte et al. 2001, Soares et al. 2011; Late Triassic, 215 mya) is a trithelodontid close to Pachygenelus, basal to mammaliaformes.

From the Soares et al. abstact: "Riograndia shows a suite of important anatomical features quite derived among the non-mammaliaform eucynodonts, such as the partial closure of the medial orbital wall and braincase, extensive secondary osseous palate, wide primary palate, basicranium with jugular foramen separated from the periphery of fenestra rotunda, narrow zygomatic arch and much reduced postdentary bones."

Bonaparte JF, Ferigolo J and Ribeiro AM 2001. A primitive Late Triassic 'ictidosaur' from Rio Grande Do Sul, Brazil. Palaeontology 44(4):623-635.

Hennig E 1922. Die Säugerzähne des württembergischen Rhät-Lias-Bonebeds. Neues Jahrbuch für Mineralogie, Geologie und Paläontologie 46:181-267.

Martineli AG and Rougier GW 2007. On Chaliminia musteloides (Eucynodontia: Tritheledontidae) from the Late Triassic of Argentina, and a phylogeny of Ictidosauria. Journal of Vertebrate Paleontology 27( 2): 442-460

Owen R 1844. Description of certain fossil crania, discovered by A. G. Bain, Esq., in sandstone rocks at the south-eastern extremity of Africa, referable to different species of an extinct genus of Reptilia (Dicynodon), and indicative of a new tribe or suborder of Sauria. Proc. Geol. Soc. London 4:500–4

Rubidge BS and Sidor CA 2001. Evolutionary patterns among Permo-Triassic Therapsids. Annual Review Ecological Systems 32:449-480. online here

Kermack DM 1982. A new tritylodontid from the Kayenta formation of Arizona. Zoological Journal of the Linnean Society 76:1-17.

Watson DMS 1913. On a new cynodont from the Stormberg. Geological Magazine, new series, decade 5 10(4):145-148

wiki/Kayentatherium
wiki/Oligokyphus
wiki/Pachygenelus
/wiki/Tritylodon
/wiki/Riograndia