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Chondrostei Look for this name in NCBI Wikipedia Animal Diversity Web
http://palaeo-electronica.org/content/fc-1 Benton et al. 2015
node minimum age |
Swisher et al. (1999, 2002) reported 40Ar/39Ar dates for ash beds within the lower Yixian Formation of 124.6 Ma ± 0.2 Myr, 124.6 Ma ± 0.3 Myr, 125.0 Ma ± 0.18 Myr, and 125.0 Ma ± 0.19 Myr. These internal dates for the Yixian Formation have been applied as a minimum estimate for the origin of the chondrostean crown in previous analyses (e.g., Near et al., 2012). A more conservative estimate can be obtained using the age for basalts and andesites that overlie the Jianshangou beds. 40Ar-39Ar dating yields a mean age estimate of 121 Ma ± 0.2 Myr for these volcanic units (Smith et al., 1995). We therefore set a minimum age for Protosephurus of 120.8 Ma.
node maximum age |
Chondrosteans are not represented in diverse Early Cretaceous freshwater deposits older than the Yixian Formation (e.g., Woodward, 1915-1917; Yabumoto, 1994; the interpretation of the stem acipenseriform Peipiaosteus as Late Jurassic by Near et al., 2012 and others reflects outdated age estimates). However, given the patchy record of chondrosteans generally (see Discussion), we adopt an especially generous soft maximum age for the divergence between Acipenseridae and Polyodontidae. Our proposed soft maximum age is based on the early chondrostean Chondrosteus, from the Hettangian-Sinemurian Blue Lias Formation of Dorset, England. This genus is universally recovered as a stem chondrostean in cladistic analyses (e.g., Grande et al., 2002; Hilton and Forey, 2009; Hilton et al., 2011), and represents the earliest known member of Acipenseriformes (sensu Grande and Bemis, 1996). The base of the Hettangian is dated as 201.3 Ma ± 0.2 Myr (Ogg et al., 2012a), from which we derive a soft maximum of 201.5 Ma.
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Protosephurus is resolved as the sister group of all remaining polyodontids in cladistic analyses (Grande et al., 2002; Hilton et al., 2011). Derived features supporting this arrangement include the presence of: well-developed anterior and posterior divisions of the fenestra longitudinalis, parietals extending posterior to the posttemporals, stellate bones, a serrated posterior margin of the subopercle, and ‘microctenoid’ scales (Hilton et al., 2011).
Grande, L., Fan, J., Yabumoto, Y. and Bemis, W. E. 2002. †Protopsephurus liui, a well-preserved primitive paddlefish (Acipenseriformes: Poylodontidae) from the Lower Cretaceous of China. Journal of Vertebrate Paleontology, 22:209-237.
Hilton, E.J., Grande, L. and Bemis, W.E. 2011. Skeletal anatomy of the shortnose sturgeon, Acipenser brevirostrum Lesueur, 1818, and the systematic of sturgeons (Acipenseriformes, Acipenseridae). Fieldiana Life and Earth Sciences, 3:1-168.
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