Updated Set 30, 2016 with new data on bat ancestors and additional taxa.
A new paper by O’Leary et al. (2013) brings new insight into the earliest radiation of placental mammals. This happened in a great radiation of clades right after the K-T extinction event, according to their results. This counters claims that placentals may have been present during the Cretaceous, but their fossils have not been found yet. Monotremes and metatherians, the ancestors of today’s egg-laying and marsupial mammals, were present during the Cretaceous. Other lineages of mammals, like Morganucodon, were present as far back as the Triassic. So that means relative stasis throughout much of the Mesozoic for mammals, followed by explosive radiation in the first third of the Paleocene.
The O’Leary et al. abstract: To discover interordinal relationships of living and fossil placental mammals and the time of origin of placentals relative to the Cretaceous-Paleogene (K-Pg) boundary, we scored 4541 phenomic characters de novo for 86 fossil and living species. Combining these data with molecular sequences, we obtained a phylogenetic tree that, when calibrated with fossils, shows that crown clade Placentalia and placental orders originated after the K-Pg boundary. Many nodes discovered using molecular data are upheld, but phenomic signals overturn molecular signals to show Sundatheria (Dermoptera + Scandentia) as the sister taxon of Primates, a close link between Proboscidea (elephants) and Sirenia (sea cows), and the monophyly of echolocating Chiroptera (bats). Our tree suggests that Placentalia first split into Xenarthra and Epitheria; extinct New World species are the oldest members of Afrotheria.
O’Leary et al. (2013) nested the tree shrews, Ptilocercus and Tupaia together.
And by gum, they do look alike (Fig. 3). And I suppose their DNA looks alike. My own, more focused studies (Fig. 2), using these extant and fossil taxa, separated these two. (Granted this set excludes several mammal clades, and that may be a problem). Ptilocercus nested between carnivorans and bats + flying lemurs, while Tupaia nested at the base of rabbits, not far from Plesiadapis and Notharctus. Several traits support these nestings and the two tree shrews are distinct in several regards (Fig.3), not least of all, their teeth. In any case these two “living fossils,” virtually unchanged for 60 million years, give some of the clearest pictures on what basal mammals looked like and what they were doing.
In the bat study (Fig 2) only one tree shrew, Ptilocercus, nests with Cynocephalus, the “flying lemur.” In the O’Leary et al. study, both tree shrews nest with Cynocephalus.
In the bat study, (Fig. 2) bats nested close to “flying lemurs” arising out of civet-like carnivores. In the O’Leary et al. study bats nested between carnivores and proto-ungulates.
In both the bat study and the O’Leary et al. study rabbits nest with primates and this clade is kin to insectivores and carnivores in that order.
So there is some agreement here.
The big problem is keeping dermopterans (“flying lemurs”) and bats together vs. separating them. This is a bigger problem when you realize that embryo bats and dermopterans both have long webbed fingers and share many other traits. But dermopterans are plant-eaters from a long way back and that may undermine what traits are now shared with insectivorous bats. Sure, megabats are fruit-eaters, but that appears to be a more recent acquisition. They still have, what appears to be, a less-derived carnivore-looking dentition, but softened and rounded in more recent times.
I wonder if tree shrews nest together because they are both living fossils? The both appear to represent very primitive (Paleocene) members of the Mammalia, from a time when there were fewer and far less diverse fauna out there. Perhaps their similar DNA reflects this.
It’s a puzzlement.
I wish I knew more about DNA in tree shrews, bats and dermopterans. If someone wants to clue me in, that would be great.
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
O’Leary, MA et al. 2013. The placental mammal ancestor and the post-K-Pg radiation of placentals. Science 339:662-667. abstract