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http://palaeo-electronica.org/content/fc-1 Benton et al. 2015
node minimum age |
The minimum constraint on the divergence date for birds and crocodiles falls in the late Olenekian, at the upper range of estimated ages for the holotype of Ctenosauriscus koeneni (GZG.V.4191), which comes from the Solling Formation. This unit is part of the upper Middle Buntsandstein, and is placed below the Olenekian/ Anisian boundary on lithostratigraphic evidence. The Olenekian/ Anisian boundary is dated at 247.1 Ma, based on radioisotopic evidence (Mundil et al., 2010; Gradstein et al., 2012, p. 706), and Kozur and Bachmann (2005, 2008) used counts of the number of short-eccentricity Milankovitch cycles within the Solling Formation to infer a date of ~247.5 Ma for the base of the Solling Formation. Butler et al. (2011) used this information to estimate an age of approximately 247.5–247.2 Ma (latest Olenekian) for the holotype of Ctenosauriscus koeneni (Geowissenschaftliches Zentrum der Universität Göttingen, Göttingen, Germany, GZG.V.4191). Similar in age is a further ctenosauriscid, Bystrowisuchus flerovi Sennikov, 2012, from the Lipovskaya Svita of the Donskaya Luka locality in southern European Russia (Sennikov, 2012), dated as Yarenskian Gorizont, and hence also late Olenekian (Benton et al., 2004). These dates are older than the 239 Ma estimated by Benton et al. (2009) as the minimum age constraint on this node, and an increase of 4 Myr over the date given by Benton and Donoghue (2007) which was based on less constrained dating of the Anisian Arizonasaurus.
node maximum age |
The soft maximum constraint may be assessed from the age distribution of immediate outgroups to Avesuchia, the Proterochampsidae, Euparkeriidae, Erythrosuchidae, and Proterosuchidae (Gauthier, 1986; Sereno, 1991; Benton, 1999; Brusatte et al., 2010; Nesbitt, 2011). Numerous fossil sites from around the world in the early Olenekian and the Induan, the stage below, have produced representatives of these outgroups, but not of avesuchians. For safety, the maximum bound is extended to the oldest archosaur sensu lato, Archosaurus from the Vyazniki beds of European Russia, dated as latest Wuchiapingian (Newell et al., 2010). The age range of the Wuchiapingian is 259.8 ± 0.4 Myr - 254.2 Ma ± 0.3 Myr (Gradstein et al., 2012, p. 675), and so the soft maximum age constraint is determined as 260.2 Ma.
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Ctenosauriscus koeneni is a crown archosaur and a crurotarsan, based on extensive morphological phylogenetic analyses, summarised by Butler et al. (2011).
Butler, R.J., Brusatte, S.L., Reich, M., Nesbitt, S.J., Schoch, R.R. and Hornung, J.J. 2011. The sail-backed reptile Ctenosauriscus from the latest Early Triassic of Germany and the timing and biogeography of the early archosaur radiation. PLoS ONE, 6 (10):e25693.
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