comment on this calibration


 node name
Monotremata     Look for this name in NCBI   Wikipedia   Animal Diversity Web
  recommended citations Phillips, 2015
  node minimum age
15.97 Ma
Obdurodon dicksoni occurs in Faunal Zones B and C of the Riversleigh local faunas (northwestern Queensland). The holotype is known from the early Middle Miocene Faunal Zone C (Ringtail Site). However, slightly older Ob. dicksoni molars and a partial dentary are known from Faunal Zone B sites (Neville’s Garden and Dirk’s Towers). Early Miocene dates have consistently been attributed to Faunal Zone B sites by biocorrelation (e.g., Black, 1997; Travouillon et al., 2006). More recently Black et al. (2012) noted that U/Pb radiometric dating of speleothems now confirms this timing. However, until the new dates are published I consider the top of the Early Miocene to provide a minimum for Riversleigh Faunal Zone B and hence, for the crown Monotremata divergence.
  node maximum age
113 Ma
Potential crown monotremes are traceable at least back to the Paleocene (~61 Ma) Monotrematum sudamericanum from Argentina, which is known from several ornithorhynchid-like molars (Pascual et al., 1992) and distal femora (Forasiepi and Martinelli, 2003). Earlier (Maastrichtian) well-sampled South American faunas lack any monotremes. However, sparse Australasian fossil records provide no solid evidence for mammal faunas lacking crown monotremes until the Albian Lightning Ridge (Flannery et al., 1995) and Dinosaur Cove (Rich and Vickers-Rich, 2003) faunas. I use the base of the Albian as a soft maximum for Monotremata.
 primary fossil used to date this node 
QM F20568
Obdurodon dicksoni, Archer et al.,1992
Location relative to the calibrated node: Crown

[show fossil details]
     Locality: Riversleigh
     Stratum: Zone B
     Geological age: Miocene, Neogene, Cenozoic

More information in Fossilworks   PaleoBioDB

  phylogenetic justification
All formal and informal cladistic analyses of monotremes favour grouping Obdurodon with the modern Ornithorhynchus to the exclusion of tachyglossids (e.g., Musser, 1999; Luo et al., 2007; Rowe et al., 2008). Moreover, Phillips et al. (2009) found high statistical support for an Obdurodon - Ornithorhynchus sister-grouping, for which unambiguous synapomorphies include rostral elements (nasal, maxilla, septomaxilla) forming a board ‘bill’, a robust posterolateral maxillary process and several endocranial characters (see Macrini et al., 2006). The sister relationship between living ornithorhynchids and tachyglossids is uncontroversial in molecular and morphological studies (e.g., van Rheede et al., 2006; Luo et al., 2007; Phillips et al., 2009).
  phylogenetic reference(s)
Musser, A.M. 1999. Diversity and relationships of living and extinct monotremes. Australian Mammalogy, 21:8-9.
Luo, Z-X., Chen, P., Li, G., and Chen, M. 2007. A new eutriconodont mammal and evolutionary development in early mammals. Nature, 446:288-293.
Rowe, T., Rich, T.H., Vickers-Rich, P., Springer, M., and Woodburne, M.O. 2008. The oldest platypus and its bearing on divergence timing of the platypus and echidna clades. Proceedings of the National Academy of Sciences USA, 105:1238-1242.
 tree image (click image for full size) 
tree image
Figure 1 from Phillips (2015).