comment on this calibration


 node name
Acariformes     Look for this name in NCBI   Wikipedia   Animal Diversity Web
  recommended citations
Wolfe et al. 2016a
  node minimum age
405 Ma
This fossil is known from the Early Devonian (Pragian) Rhynie Chert of Aberdeenshire, Scotland. Spore assemblages of the Windyfield and stratigraphically underlying Rhynie Chert are dated to the early but not earliest Pragian to early (earliest?) Emsian (polygonalis-emsiensis Spore Assemblage Biozone) (Parry et al., 2011). Radiometric dating of the underlying Milton of Noth Andesite at ca. 411 Ma (Parry et al.,2011, 2013) has been subject to a dispute over its temporal relationship to hot spring activity associated with the cherts (Mark et al., 2011, 2013)and predates the biostratigraphic dating of the Rhynie Chert relative to the global dating of the base of the Pragian Stage. Therefore, a minimum age constraint may be defined at 405.0 Ma for the Rhynie Chert, using the Pragian-Emsian boundary (407.6 Ma ± 2.6 Myr) as a reference.
  node maximum age
514 Ma
A soft maximum constraint comes from the oldest chelicerate W. barbarahardyae from the Emu Bay Shale on Kangaroo Island, SouthAustralia, which has been correlated based on trilobite biostratigraphy to the upper part of the P. janeae Zone in mainland South Australia (Jell in Bengtson et al., 1990; Fig. 2 in Jago et al., 2012). As this is equivalent to the Canglangpuan Stage in South China and the late Botoman inSiberia (Gehling et al., 2011, Fig. 9), the Emu Bay Shale can be dated to Cambrian Series 2, Stage 4, providing a maximum age of ~514 Ma.
 primary fossil used to date this node 
NHMUK In. 24665
Protacarus crani, Hirst, 1923
Location relative to the calibrated node: Crown

[show fossil details]
     Locality: Aberdeenshire
     Stratum: Rhynie Chert
     Geological age: Devonian, Paleozoic

More information in Fossilworks   PaleoBioDB

  phylogenetic justification
Originally described as a single species (Hirst, 1923), P. crani from the Rhynie Chert was subsequently treated as five species belonging to five different genera (Dubinin, 1962). P. crani itself, as exemplified by the holotype, was described as a member of Eupodidae, within Trombidiformes (Hirst, 1923), or potentially more basal within the Acariformes (Bernini, 1986). While the specifics of its classification may be debatable due to the lack of preserved diagnostic characters, the fan-like setae observed dorsally in P. crani support a relationship with endeostigmatids within crown group Acariformes (Bernini, 1986; Dunlop and Selden, 2009).
  phylogenetic reference(s)
Bernini, F. 1986. Current ideas on the phylogeny and the adaptive radiations of Acarida. Boll. Zool. 53, 279–313.
Dunlop, J.A., and Selden, P.A. 2009. Calibrating the chelicerate clock: a paleontological reply to Jeyaprakash and Hoy. Exp. Appl. Acarol. 48, 183–197.
 tree image (click image for full size) 
tree image
Figure 4 from Wolfe et al. (2016).