Updated August 20, 2016 with an updated cladogram with more taxa.
The oddest of all mammals,
the platypus (Ornithorhynchus, Shaw 1799; Fig. 1), is also the most primitive of all living mammals, other than its egg-laying sisters, the echindas (Tachyglossus and Zaglossus).
The only issue
raised by the large reptile tree (subset Fig. 2) is the nesting of Ornithohynchus more primitive than Megazostrodon + Hadrocodium rather than more derived, as indicated by Li and Luo 2006). That would make it the most primitive of all known mammals, given the present dataset. (That will become expanded tomorrow with greater insight).
Akidolestes cifellii (Li and Luo 2006) is a spalacotheroid symmetrodont, described as a “relative of modern therians (marsupials + placentals) shares several traits with Ornithorhynchus, but has premaxillary teeth and a narrow snout.” According to Li and Luo, it was a monotreme mimic. That may be challenged tomorrow with more details to come.
Li and Luo 2006. A Cretaceous symmetrodont therian with some monotreme-like postcranial features. Nature 439|12 January 2006|doi:10.1038/nature04168.
Shaw G 1799. The Naturalist’s Miscellany.