Ammonoid Succession of Setorym River (Verkhoyansk Area) and Problem of Permian- Triassic Boundary in Boreal Realm

摘要

The presence of a single Otoceras species (O. boreale) , morphologically very variable, at the base of the Nekuchan Formation in Verkhoyansk, we believe, is to be obvious. Some morphological evidence leaves no doubt that two described morphs of O. boreale are a strictly corresponding sexual dimorphic pair. It is very likely that Kummefs idea that Canadian O. concavum Tozer is an invalid species is truthful, considering the range of variability seen in larger Siberian and Himalayan Otoceras fauna. Just above the upper Tatarian Imtachan Formation, the six stages of ammonoid succession can be recognized within the lower part of the Nekuchan Formation in the Setorym River Section: (a) Otoceras bore-ale; (b) Otoceras boreale―Tompophiceras pascoei; (c) Otoceras boreale― Tompophiceras pascoei―Aldanoceras; (d) Tompophiceras pascoei―Otoceras boreale―Aldanoceras; (e) Tompophiceras mor-pheous ―T. pascoei―Aldanoceras; (f) Tompophiceras morpheous―T. pascoei―Wordieoceras do-mokhotovi―Ophiceras transitorium ; (g) Tompophiceras morpheous―T. pascoei, corresponding to the Otoceras boreale and Tompophiceras morpheous zones. In spite of the domination of Otocerataceae or Xenodiscaceae in both of these zones and the presence of some Permian type conodonts in the lower part of the Otoceras boreale Zone, they seem to be early Induan in age on the basis of the following arguments: (1) in contrast to the underlying regressive type sediments of the Upper Tatarian Imtachan Formation, both the Otoceras boreale and the Tompophiceras morpheous zones of the lowermost part of the Nekuchan Formation correspond to the single transgressive cycle; (2) typical early Induan ani-monoids (Ophiceras and Wordieoceras) have been recognized in the Tompophiceras morpheous zone; (3) all described ammonoid succession stages (a―g) are characterized by very gradual changes and therefore correspond to the different parts of the single zone or to the different zones of the same stage, but not to the different systems (Permian and Triassic): (4) elsewhere in the Boreal realm (Arctic Canada) , the conodont index species for the base of the Triassic, Hindeodus parvus, has been reported from the Otoceras boreale Zone. A new scheme of the phytogeny for the Otocerataceae and its Induan-Olenekian offspring (Araxceratidae―Otoceratidae―Vavilovitidae n. fam.― Proptychitidae―Arctocer-atidae) and Xenodiscaceae is offered.