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 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.
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.
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