A summary today…
featuring a long cladogram (Fig. 1), a subset from the large reptile tree (LRT, 1259 taxa) focusing on the Mammalia. This is how this LRT subset stands at present. Not much has changed other than the few node changes from the past week.
The transition from Prototheria to Theria (Metatheria)
includes long-snouted taxa, like Ukhaatherium. Nearly all Prototheria are also long-snouted (Cifelliodon is the current sole exception).
The transition from Metatheria to Eutheria (simplified)
includes small omnivorous didelphids arising from the carnivorous/herbivorous split among larger metatherians. Basal Carnivora, the most basal eutherian clade, are also omnivores. Caluromys, the extant wooly opossum, has a pouch, but nests at the base of all placental taxa (the LRT tests only skeletal traits), so it represents the size and shape of the earliest placentals (contra O’Leary et al. 2013)… basically didelphids without pouches, and fewer teeth, generally (but not always).
Basal members of most placental clades
are all Caluromys-like taxa, with a rapid radiation in the Late Triassic/Early Jurassic generating most of the major placental clades in the LRT (Fig. 1). Larger members of each of these placental clades appeared in the fossil record only after the K-T extinction event. So hardy where these basal taxa, that many still live to this day.
As shown earlier, higher eutheria are born able to able to walk or swim. They are no longer helpless with arboreal parents (tree-climbing goats the exception). Basal eutherians reproduce more like their metatherian ancestors, with helpless infants.
Figure 1. Subset of the LRT focusing on mammals. Extant taxa are colored. Thylacinus is recently extinct.
The latest competing study
(O’Leary et al. 2013, Fig. 2) recovers the highly specialized edentates, aardvarks, elephants and elephant shrews as the most primitive placentals. Carnivora + bats are quite derived in the O’Leary team cladogram, somehow giving rise to ungulates and whales. This is an untenable hypothesis. It doesn’t make sense. Evidently the O’Leary team had faith that smaller didelphid-like ancestors would fill in the enormous phylogenetic gaps in their cladogram. By contrast the LRT has all the operational taxonomic units (OTUs) it needs to produce a series of gradually accumulating derived traits between every taxon in its chart (Fig. 1). The LRT makes sense.
Figure 5. Simplified version of the O’Leary et al 2013 cladogram showing placental relations exploded after the K-T boundary.
References
O’Leary, MA et al. 2013. The placental mammal ancestor and the post-K-Pg radiation of placentals. Science 339:662-667. abstract
Wible JR, Rougier GW, Novacek MJ, Asher RJ 2007. Cretaceous eutherians and Laurasian origin for placental mammals near the K/T boundary Nature 447: 1003-1006
The Pterosaur Heresies 
For the billionth time, the tips of branches are not the ancestors of their nodes. No one suggests “edentates, aardvarks, elephants and elephant shrews [are] the most primitive placentals” or that bats+carnivores gave “rise to ungulates and whales.” Obviously that wouldn’t make sense. Elephants are as far from the placental common ancestor as we are, or mice are, or whales are. O’Leary et al. even go so far as to illustrate what their character distribution hypothesizes that common ancestor to be- an insectivory thing. Do you still not understand how to read cladograms or are you purposely trying to make their hypothesis sound ridiculous?
You didn’t read the text, Mickey. Something to the effect of: The authors HOPE or HAVE FAITH that some small insectivory thing will be found someday is somewhere in the text. The LRT provides those taxa without imagining them. And don’t you see the ridiculous-ness of the O’Leary hypothesis of relationships? And their reliance on hope, not taxa?
It’s not hope or faith to interpret cladograms correctly. It’s as if you used a tree of living mammals to claim the ancestral mammals were platypi and echidnas. Your LRT would show that too, using only living taxa. Do you think the ancestors of living mammals were platypi and echdnas?
Given those parameters, the ancestors of living mammals, including living monotremes (they count, too), would be… wait for it… salamanders and lobe-fin fish.
Salamanders and lobe-finned fish are the tips of the branches, not the nodes that are the actual ancestors. Do you really not get this? Any actual OTU you have in a cladogram is basically never the ancestor of any other OTU or clade. What about that is confusing?