Cifelliodon, Obdurodon, Ornithorhynchus and Tachyglossus

Ornithorhynchus anatinus (extant, Shaw 1799) is a living aquatic mammal that produces a primitive sort of milk without a typical breast and it lays eggs. Along with Akidolestes (see below), it nests basal to all other known prehistoric mammals in the large reptile tree. Juveniles have three posterior teeth in each jaw. Adults lose these and replace them with a pad. The platypus jaw is constructed differently from that of other mammals, and the jaw-opening muscle is different. The external opening of the ear still lies at the base of the jaw, not behnd the jaw joint as in other mammals. The platypus has extra bones in the shoulder girdle, including an interclavicle, which are not found in other mammals. The hind limbs sprawl in a reversion to a more reptilian configuration.

Akidolestes cifellii (Li and Luo 2006; Yixian Formation, Early Cretaceous. 124 mya; NIGPAS 139381A, B) nests as a basal toothed platypus in the large reptile tree, contra the original interpretation that considered the many similarities convergent. Akidolestes was derived from a sister to Kuehneotherium (see below) with long jaws containing many more teeth than is typical for mammals.

As a monotreme, Akidolestes was an egg-layer, yet providing nourishment for its hatchlings with an early form of milk secreting from ventral glands in its skin.

It probably was a fish and tadpole eater and had a thick, probably waterproof, furry coat.

Teeth Prior to the toothless condition found in Ornithorhynchus, phylogenetic predecessors developed more teeth. Akidolestes is one example. Keuhneotherium is yet another (see below).

Akidolestes

Obdurodon dicksoni (Archer et al. 1992; QM F20568; Oligocene to Mioncene) is a large toothed ancestor of the extant platypus.

Archer M, Jenkins FA, Hand SJ, Murray P, and Godthelp H 1992. Description of the skull and non-vestigial dentition of a Miocene platypus (Obdurodon dicksoni n. sp.) from Riversleigh, Australia, and the problem of monotreme origins. Platypus and Echidnas 15-27.

Chen M and Luo Z-X 2013. Postcranial skeleton of the Cretaceous mammal Akidolestes
cifellii and its locomotor adaptations. Journal of Mammal Evolution 20:159–189.

Li G and Luo Z-X. 2006. A Cretaceous symmetrodont therian with some monotreme-like postcranial features. Nature, 439:195–200.

Shaw G 1799. The Naturalist's Miscellany.

wiki/Ornithorhynchus
wiki/Akidolestes
wiki/Obduron

Mammalia

Ornithorhynchus

Marsupiala - Metatheria

Eomaia and kin