comment on this calibration


 node name
Theria     Look for this name in NCBI   Wikipedia   Animal Diversity Web
  recommended citations Benton et al. 2015
  node minimum age
157.3 Ma
The Daxigou site of the Tiaojishan Formation, Liaoning Province, Northeastern China, source of Juramaia, has been constrained by radiometric dates to derive from deposits of just over 160 Ma in age, equivalent to the Oxfordian stage of the Late Jurassic (Luo et al., 2011). This stage has a minimum bound of 157.3 Ma ± 1.0 Myr, or 156.3 Ma (Gradstein et al., 2012), the minimum age constraint for the eutherian-metatherian split.
  node maximum age
169.6 Ma
An alternative view (Woodburne et al., 2003) places southern tribosphenic taxa (see above) on the stem to Theria, pushing the minimum age for crown Theria deeper into the Jurassic, in order to include such taxa as Ambondro (Flynn et al., 1999). Given the possibility that southern, tribosphenic mammals such as Ambondro are therian (even eutherian), we would set the soft maximum age constraint for Theria within the Jurassic (Bathonian) at 168.3 Ma ± 1.3 Myr, so 169.6 Ma.
 primary fossil used to date this node 
PM 1143
Juramaia sinensis, Luo et al., 2011
Location relative to the calibrated node: Crown

[show fossil details]
     Locality: Daxigou site
     Stratum: Tiaojishan Formation
     Geological age: Jurassic, Mesozoic

More information in Fossilworks   PaleoBioDB

  phylogenetic justification
The human-opossum branching point is synonymous with the split of marsupials and placentals, encompassing the clade Theria and containing (respectively) the metatherian and eutherian stem members of each group. Juramaia is known from a skull and front half of the skeleton that show it is therian (Luo et al., 2011). The molars are tribosphenic, so the fossil is assigned to Boreosphenida, and it shows diagnostic characters of Eutheria in the distinctive paraconule, incipient metaconule (M2 only), long preprotocrista past the paracone, and long postprotocrista past the metacone. The forelimb shows evidence for climbing in the grasping hand, a feature shared in common with other early therians.
  phylogenetic reference(s)
Luo, Z.X., Yuan, C.X., Meng, Q.J., and Ji, Q. 2011. A Jurassic eutherian mammal and divergence of marsupials and placentals. Nature, 476:442-445.
 tree image (click image for full size) 
tree image