Kryptobaatar, Megaconus and Rugosodon, the multituberculates

Kryptobaata dashzevegir (Kielan-Jaworowska 1970, skull lenth 3cm; Late Cretaceous) is one of the most complete members of the Multituberculata, a clade usually placed outside most mammals, but here nests as a clade within the Rodentia, with Rattus. The mandible slid forward to engage the incisors with each other and back again to grind the molars. A flat cranium also characterizes this clade.

Image above from Digimorph.org and used with permission.

Megaconus mammaliaformis (Zhou et al. 2013, Middle Jurassic) is a member of the Haramiyida, traditionally considered pre-mammals. Here Megaconus nests with multituberculates and rodents. A deep coat of fur was preserved with this complete fossil. Zhou et al. reported the posterior mandible bones were not yet transformed into tiny ear bones. I could not confirm that and the phylogenetic nesting argues against it. The fused tibia and fibula are not seen in basal mammals, but is a synapomorphy of rodents. The calcaneum lacks an enlarged heel.

Rugosodon eurasiaticus (Yuan et al. 2013, Late Jurassic, the size of a chimpmunk) is the oldest multituberculate and it is represented by a complete skeleton. Teeth and ankle evidence indicate it was an omnivore and arboreal.

Kielan-Jaworowska Z 1970. New Upper Cretaceous multituberculate genera from Bayn Dzak, Gobi Desert. In: Kielan-Jaworowska (ed.), Results of the Polish-Mongolian Palaeontological Expeditions, pt. II. Palaeontologica Polonica 21:35-49.

Wible JR Rougier GW 2000. Cranial anatomy of Kryptobaatar dashzevegi (Mammalia, Multituberculata), and its bearing on the evolution of mammalian characters. Bulletin of the American Museum of Natural History 247: 1–120. doi:10.1206/0003-0090(2000)247<0001:CAOKDM>2.0.CO;2.

Yuan CX, Ji Q, Meng QJ, Tabrum AR, Luo ZX 2013. Earliest evolution of multituberculate mammals revealed by a new Jurassic fossil". Science 341 (6147): 779–783. doi:10.1126/science.1237970.

Zhou CF, Wu S, Martin T, Luo ZX 2013. A Jurassic mammaliaform and the earliest mammalian evolutionary adaptations. Nature 500 (7461): 163. doi:10.1038/nature12429.

wiki/Rugosodon
wiki/Kryptobaatar
wiki/Megaconus

Mammalia

Ornithorhynchus

Marsupials - Metatheria

Eomaia

Placentals - Eutheria

Carnivora

Procyon - Canis - Phoca