Chiniquodon theotonicus (von Huene 1936, formerly Probelesodon, MCZ 3781) total length: 50 cm. ~237 mya middle Triassic (Anisian/Landinian) represents a derived cynodont, between Thrinaxodon and Pachygenelus. Specimens once named Probelesodon have been referred to Chiniquodon.

The skull was larger relative to the body, but the brain remained quite small. Here the lateral temporal fenestra (synapsid openings) took up more than half the area of the skull in dorsal view. They housed very powerful jaw muscles. The braincase was ossified. The dentary exended above the cheek bone and the remaining jaw bones continued shrinking. A groove at the rear of the cheek bones may have held an ear canal leading toward the eardrum.

The torso was shorter than in prior taxa. The ribs no longer had costal plates. Three sacrals were present. The tail was shorter and more slender. Chiniquodon had a very small, slender scapula. The limbs were more slender. The pubis was tiny. The hind limbs moved parasagittally, like a mammal, rather than laterally, like a more primitive reptile. The calcaneum extended posteriorly forming a heel upon which the Achilles tendon inserted. This would have provided more spring to each step. The fibula had a sesamoid that fuses to the fibula in Ornithorhynchus.

Castorocauda lutrasimilis (Ji et al. 2006, Middle Jurassic, 42 cm long) was described a decade ago as a proximal relative to modern mammals provisioned with a broad, beaver-like tail. Tthe large reptile tree nested Castorocauda between the middle Triassic cynodont, Chiniquodon and the Early Jurassic ictidosaur (trithelodontid) cynodont Pachygenelus.

Castorocauda did not have a mammalian jaw joint. The angular, articular, quadrate and surangular were large and behind the dentary, as in other non-mammalian cynodonts. The skull had a postorbital bar. The pelvis was typically cynodont, not mammalian. The femur is short.

Castorocauda is one of the last known non-mammalian cynodonts in the Middle Jurassic. The reconstructed skull is shown below. Tritylodontids were also late-surviving cynodonts.

Probainognathus jensi (Romer 1970; Carnian, Late Triassic, 230 mya; skull 10cm length) had such large lateral temporal fenestrae that they extended to half the skull and rotated the orbits to face anteriorly. The dentary squamosal joint was incipient. Note the long contact of the postorbital with the small, narrow frontal. About 36 specimens are known. Juvenile online here.

Dvinia prima (Amalitskii 1922; Late Permian, 254mya; 7 cm skull) nests between Chiniquodon and Pachygenelus in the TST. The lower canine fit into a maxillary opening. The molars had a circle of cusps around a single large cusp. The postorbital is very tiny, the vestige that would be lost in derived taxa. The lateral temporal fenestrae were huge, divided by a narrow crest in which a smal brain was tucked.

Amalitskii VP 1922. Diagnoses of the new forms of vertebrates and plants from the upper Permian of North Dvia: Bulletin de l’Académie des Sciences de l'URSS, Math and Natural Sciences, 1922, p. 329-340. and in Izv. Ross. Akad. Nauk, Ser. 6 25 (1), 1–12.

Huene F von 1936. Die Fossilien Reptilien des südamerikanischen Gondwanalandes an der Zeitenwende (Denwa-Molteno-Unterkeuper = Ober-Karnisch). Ergebnisse der Sauriergrabungen in Südbrasilien 1928/29 part II.) 93-159.

Ji Q, Luo Z-X, Yuan C-X, Tabrum AR 2006. A swimming mammaliaform from the Middle Jurassic and ecomorphological diversification of early mammals. Science. 311: 1123–1127.

Martinez RN and Forster CA 1996. The skull of Probelesodon sanjuanensis, sp. nov., from the Late Triassic Ischigualasto Formation of Argentina. Journal of Vertebrate Paleontology 16:2:285-291.

Romer AS 1969. The Brazilian Triassic cynodont reptiles Belesodon and Chiniquodon. Breviora 332: 1-16 (1969)

Romer AS 1969. The Chanares (Argentina) Triassic reptile fauna. V. A new chiniquodontid cynodont, Probelesodon lewisi - cynodont ancestry. Breviora 333:1-24.

Romer AS 1970. The Chanares (Argentina) Triassic reptile fauna. VI. A cynodont with an incipient squamosal-dentary articulation". Breviora 344: 1-18.

Romer AS 1973. The Chanares (Argentina Triassic reptile fauna. XIX. Postcranial materials of the cynodonts Probelesodon and Probainognathus. Breviora 407:1-26.

Martin T 2005. Postcranial anatomy of Haldanodon exspectatus (Mammalia, Docodonta) from the Late Jurassic (Kimmeridgian) of Portugal and Its bearing for mammalian evolution, Zoological Journal of the Linnean Society, 145, p. 219-248

wiki/Chiniquodon
wiki/Probainognathus
wiki/Castorocauda
wiki/Dvinia