comment on this calibration


 node name
Notostraca     Look for this name in NCBI   Wikipedia   Animal Diversity Web
  recommended citations
Wolfe et al. 2016
  node minimum age
121.8 Ma
This fossil was discovered in the Yixian Formation of northeastern China (Hegna and Ren, 2010). The Yixian Formation lies between the overlying Jiufotang Formation and underlying Tuchengzi Formation; together they comprise the Jehol Group (e.g. Chang et al., 2009a; Zhou, 2006). Dating of the Jehol Group has been contentious, varying from Late Jurassic to Early Cretaceous based on biostratigraphic and radiometric techniques. Recent 40Ar/39Ar dates yielded ages of 129.7 Ma±0.5 Myr for basaltic lava from the bottom of the Yixian Formation and 122.1Ma±0.3Myr for tuff layers at the bottom of the overlying Jiufotang Formation (Chang et al., 2009a). Other age estimates have fallen within this range (reviewed by Zhou, 2006). This debate underscores the point that reasonably precise radiometric dates may still be quite inaccurate. We conservatively use the younger of these estimates, so a minimum age for Yixian fossils is 121.8 Ma.
  node maximum age
521 Ma
A soft maximum age is obtained from the oldest mandibulate, Y. dianensis, which was recovered from the Yu'anshan Formation at Xiaotan section, Yongshan, Yunnan Province, attributed to the Eoredlichia–Wutingaspis Biozone (Zhang et al., 2007). Chinese Cambrian stratigraphy has been revised substantially and the Eoredlichia –Wutingaspis Biozone is no longer recognized (Peng, 2003, 2009). However, Eoredlichia is known to co-occur with Hupeidiscus, which is diagnostic of the Hupeidiscus-Sinodiscus Biozone, which is formally recognized as the second biozone of the Nangaoan Stage of the Qiandongian Series of the Cambrian of China (Peng and Babcock,2008). The Nangaoan is the proposed third stage of the Cambrian System for the International Geologic Timescale (Peng et al., 2012a).Thus, a soft maximum constraint can be established on the age of the lower boundary of the Nangaoan, which has been dated to 521 Ma (Peng et al., 2012a; Peng and Babcock, 2008).
 primary fossil used to date this node 
CNU CAL-HP-2009001
Chenops yixianensis, Hegna and Ren, 2010
Location relative to the calibrated node: Crown

[show fossil details]
     Locality: Yixian Formation
     Geological age: Cretaceous, Mesozoic

More information in Fossilworks   PaleoBioDB

  phylogenetic justification
In the morphological phylogenetic analysis of Lagebro et al. (2015), C. yixianensiswas in a polytomywith other crown Notostraca, including Lepidurus batesoni. The equal size of thoracic endopods and fourth and fifth endites may exclude C. yixianensis from crown Notostraca (Hegna and Ren, 2010). However, in previous morphological phylogenies (Hegna, 2012), C. yixianensis was sister taxon to the extant L. batesoni. This is because L. batesoni lacks elongated endites on the first thoracic appendage, suggesting a synapomorphy between C. yixianensis and the extant species, and perhaps membership in an entirely different genus (Hegna, 2012; Hegna and Ren, 2010). If indeed C. yixianensis is, based on phylogenies and endite morphology, a sister group of L. batesoni, it would remain within the crown Notostraca.
  phylogenetic reference(s)
Hegna, T.A., 2012. Phylogeny and fossil record of branchiopod crustaceans: an integrative approach (PhD) Yale University, New Haven.
Hegna, T.A., Ren, D., 2010. Two new “notostracans”, Chenops gen. nov. and Jeholops gen. nov. (Crustacea: Branchiopoda: ?Notostraca) from the Yixian Formation, Northeastern China. Acta Geol. Sin. 84, 886–894.
Lagebro, L., Gueriau, P., Hegna, T.A., Rabet, N., Butler, A.D., Budd, G.E., 2015. The oldest notostracan (Upper Devonian Strud locality, Belgium). Palaeontology 58, 497–509.
 tree image (click image for full size) 
tree image
Figure 15 from Wolfe et al. (2016).