Microdocodon, Fossiomanus, Oligokyphus, Tritylodon and Kayentatherium

Microdocodon gracilis (Zhou et al. 2019; Middle Jurassic; 165 mya) was originally considered a docodont with hyoid bones and lacking fingers. Here Microdocodon nests at the base of the tritylodontids, lacks hyoids and has two complete sets of fingers. The medial metatarsals were longer than the others. Tiny phalanges 3.2, 4.2, 4.3 and 5.2 reappear after disappearing several nodes earlier. The limbs are long and gracile with reduced interoseal space between the crural and antebrachial elements.

Fossiomanus skull in situi and reconstructeed

Fossiomanus sinensis (Mao et al. 2021; Early Cretaceous) was correctly identified as a burrowing tritylodont. Here the phalanges are no larger than the carpals. This taxon nests between Microdocodon (above) and Riograndia + Chalminia, which has similar large dentary incisors.

Oligokyphus triserialis (Henning 1922; Late Triassic, 220 mya; 50 cm in length) was originally considered a basal mammal, but it retains a vestigial quadrate/articular vestige and it nests more primitive ly than the most primitive known mammals, like Ornithorhynchus. The increase in the incisors, the loss of canines and premolars, along with its rat-like jaw architecture indicates this was an herbivore. The teeth mashed together, but did not occlude. The cranium had a high crest, the jugal was robust. The cranium remained narrow. Epipubic bones were present.

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.

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

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 4:500–504

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

Zhou C-F, Bullar B-A S, Neander AI, Martin T and Luo Z-X 2019. New Jurassic mammaliaform sheds light on early evolution of mammal-like hyoid bones. Science 365(6450):276–279.

wiki/Kayentatherium
wiki/Oligokyphus
wiki/Tritylodon
wiki/Fossiomanus
wiki/Microdocodon