A traditional dependence on molar traits
has obscured and mixed up fossil mammal relationships (Fig. 1) when compared to phenomic studies using skulls and skeletons of extant taxa and fossils (Fig. 2). In this way, tooth traits are shown to be like gene traits. They deliver false positives that are not validated by taxa tested by a large suite of traits from nose to tail.
Luo et al. 2011
published their cladogram nesting Juramia as the basalmost placental recovered from a cladogram of mammal interrelations based largely on tooth traits (Fig. 1).
Figure 1. From Luo et al. 2011, mammal claodgram focused on Juramaia employing many tooth traits. Compare to the LRT in figure 2 which minimizes tooth traits.
In stark contrast,
the large reptile tree (LRT, 1728+ taxa) nested Juramaia as a monotreme (Prototheria).
Figure 2. Subset of the LRT cladogram of basal Mammalia. Note the traditional clade Metatheria is a grade with new names proposed here.
Figure 3. Juramaia (Late Jurassic, 160 mya) is more completely known and nests with monotremes not placentals.
References
Luo Z-X, Yuan C-X, Men Q-J and JiQ 2011. A Jurassic eutherian mammal and divergence of marsupials and placentals. Nature 476: 442–445. doi:10.1038/nature10291.
The Pterosaur Heresies 