I'm currently browsing with curiosity a hefty tome called Mammals From The Age Of Dinosaurs. It's about as comprehensive as one can get, and is destined to be the reference book for early mammals. Written by three of the leading experts (and most widely quoted) in the field of mammal paleontology: Zofia Kielan-Jaworowska, Richard Cifelli, and Zhe-Xi Luo. It's dated 2004; to get anything more current, you'd have to be constantly scouring the journals (which is not a bad thing, as these are times of rapid change in knowledge and understanding in this field.)
The book contains many seminal reference points, including a full survey of distribution by location and period (to just past the Mesozoic boundary), and a fully detailed survey of each major lineage of the Mesozoic.The diagrams I find particularly useful are - two (alternative) cladograms of all major mammal taxa up to eutheria (pp521 & 522); - an overview of the changed view of the evolution of the major lineages (p5); - most importantly, a diagram of the temporal distribution (through the Mesozoic) and relationships of the main lineages (p3); - a clade table (listing) of all lineages down to family level (pp 14-15).
Of major interest is the comment (p13) that only four major lineages have a significant presence after the KT boundary (end of the Mesozoic, and the dinosaurs).
Four? To the extant lineages mentioned above, the book adds multituberculates (p15). In a footnote, they elaborate the list with the multituberculate suborder Cimolodonta (lasting to the Eocene), and one dryolestoid from the Paleocene of South America. However, that note is not complete, as there are scatterings of other multituberculate taxa that are mentioned as passing through to the Paleocene. These include Ptilodontidae, and Gondwanatheria. The latter are admitted as uncertain placement (Incertae sedis) - somewhere between monotremes and (metatherians plus eutherians) - but discussed with multituberculates.
Ptilodus, a Ptilodontid
[Update 27-Mar: Dryolestes is Trechnotherian - a clade (a superset of both eutherians and metatherians) which covers all mammals that give birth to live young. I'll now exclude these from the discussion, since I'm focusing on egg-laying mammals, which it looks like the SB mammal is.]
So far, then, we have three non-therian - egg-laying - groupings surviving into the Paleocene (which ended 55 million years ago): Monotremes, multituberculates and, arguably, Gondwanatherians. To this, we now add the even more enigmatic SB mammal, surviving all the way to the Miocene, 19mya.
So what does this say about the SB mammal?
On the one hand, Worthy et al place this mammal in an unresolved trichotomy with multituberculates (which it says are more basal) and the more derived clades that include (Tinodon + the viviparous therians). In effect, pretty close to multituberculates, but no match. On the basis of the femur fragment (specifically, the greater trochanter), it's more primitive than the latter - but that's predicated on the femur and jaw fragments matching. Parsimony suggested so, but it's not a guarantee.
On the other hand, the paucity - and piecemeal nature - of the book's references to non-therian KT survivors is a good reminder that we are dealing with a matching scarcity of pertinent fossils. What has been reported so far should not be taken as a complete and reliable guide to what did survive. New Zealand has, after all, sheltered such oddities as the lizard-like Tuatara and the Leopelmatid frog, no less surprising in their uniqueness.
Next up: more on multituberculates.
Kielan-Jaworowska Z, Cifelli R L, and Luo Z-X (2004): Mammals from the age of dinosaurs : origins, evolution, and structure. New York, Columbia University Press.
Worthy T, Tennyson A, Archer M et al (2006): Miocene mammal reveals a Mesozoic ghost lineage on insular New Zealand, southwest Pacific in PNAS vol 103 no 51.