comment on this calibration


 node name
Gnathostomata     Look for this name in NCBI   Wikipedia   Animal Diversity Web
  recommended citations Benton et al. 2015
  node minimum age
420.7 Ma
The holotype of Guiyu oneiros was recovered from the Kuanti Formation, just below the first stratigraphic occurrence of the conodont index fossil Ozarkodina crispa, at a locality near Xiaoxiang Reservoir, Qujing, Yunnan, China (Wang, 2001; Zhu et al., 2009). However, since it is not possible to discriminate whether the first occurrence of O. crispa here coincides, postdates or antedates its first occurrence in the type Ludlow of the Welsh Borderlands (it is not demonstrated that the occurrence of G. oneiros occurs within the preceding Ozarkodina snajdri biozone), the most appropriate age interpretation should be based on the geochronological age of the top of the O. crispa biozone which immediately precedes the Ludlow-Pridoli boundary in both the type Ludlow in the Welsh Borderlands of England, and at the GSSP section for the Ludlow-Pridoli in the Daleje Valley, Prague, Czech Republic where it is dated as 423 Ma ± 2.3 Myr (Melchin et al., 2012). This yields a minimum age estimate of 420.7 Ma.
  node maximum age
468.4 Ma
A maximum bound for the divergence between chondrichthyans and osteichthyans can be derived from diverse faunas of jawless, armored stem gnathostomes (‘ostracoderms’) of Middle to Late Ordovician age (Young, 1997; Erdtmann et al., 2000; Sansom et al., 1996, 2001, 2013; see calibration for crown Vertebrata-Craniata, Node 18). Isolated microremains of this age have been compared with chondrichthyans (e.g., Sansom et al., 2012), but phylogenetic interpretations of such materials are not secure, especially in the absence of articulated specimens of similar age showing unambiguous features of jawed vertebrates. We therefore propose the oldest of these Ordovician ostracoderm assemblages, from the Stairway Sandstone Formation of Mt Watt, Amadeus Basin, Northern Territory, Australia, as an estimated maximum bound for the gnathostome crown. The age of the formation is constrained by the occurrence of the conodont Lenodus sp. cf. L. variabilis in the middle part of the Stairway Sandstone, which Davies et al. (2011) interpret as evidence for an early Darriwilian age. This is based on the assumption that Lenodus sp. cf.L. variabilis falls within the taxonomic range of Lenodus variabilis and L. antivariabilis, which have a stratigraphic range limited to the upper Baltoniodus norrlandicus Zone to the Eoplacognathus variabilis and Yangtzeplacognathus crassus zones (Löfgren and Zhang, 2003). Jakobsen et al. (2014) have argued for a mid-Darriwlian age for the Stairway Sandstone on the basis of trilobites. The base of the Darriwillian is dated as 467.3 Ma ± 1.1 Myr, from which we give an estimated maximum bound of 468.4 Ma for the gnathostome crown. This value allows for the possibility that isolated scales (e.g., Karatajuté-Talimaa and Predtechenskyj, 1995) and spines (Sansom et al., 2005) of latest Ordovician-early Silurian age might represent members of the chondrichthyan total group.
 primary fossil used to date this node 
IVPP V15541
Guiyu oneiros , Zhu et al. 2009
Location relative to the calibrated node: Crown

[show fossil details]
     Locality: Xiaoxiang Reservoir, Qujing, Yunnan
     Stratum: Kuanti Formation
     Geological age: Silurian, Paleozoic

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  phylogenetic justification
Several clear features, including the presence of rhombic scales with peg-and-socket articulations, indicate that Guiyu is an osteichthyan. More specifically, Guiyu presents several synapomorphies of lobe-finned fishes (e.g., a dermal intracranial joint), with resolved phylogenetic solutions placing this genus on the sarcopterygian stem within the osteichthyan crown (Zhu et al., 2009; Friedman and Brazeau, 2010; Davis et al., 2012).
  phylogenetic reference(s)
Zhu, M., Zhao, W., Jia, L., Lu, J., Qiao, T., and Qu, Q. 2009. The oldest articulated osteichthyan reveals mosaic gnathostome characters. Nature, 458:469-474.
Friedman, M. and Brazeau, M.D. 2010. A reappraisal of the origin and basal radiation of the Osteichthyes. Journal of Vertebrate Paleontology, 30:35–56.
Davis, S.P., Finarelli, J.A., and Coates, M.I. 2012. Acanthodes and shark-like conditions in the last common ancestor of modern gnathostomes. Nature, 486:247–250.
 tree image (click image for full size) 
tree image
Figure 3 of Benton et al. (2014).