New data on the Upper Triassic bivalve Monotis in North America, and the new subgenus Pacimonotis

• Published in: Journal of Paleontology
• Main Authors: Grant-Mackie, J. A., Silberling, N. J.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Cambridge University Press (CUP) 1990
• Subjects: Alaska; Yukon
• Online Access: http://dx.doi.org/10.1017/s0022336000018400; https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0022336000018400

Re-examination of some North American collections of the late middle to early late Norian pterioid bivalve Monotis confirms the separation of M. ( M. ) alaskana Smith from the European type species M. salinaria (Schlotheim). The related M. ( M. ) haueri Kittl, as differentiated statistically, is more widely represented in southern Alaska, the Canadian Yukon, and Nevada than was formerly recognized. Species described originally from northeastern USSR and now confirmed as present in rocks from Alaska and western Canada include: M. ( Entomonotis ) pachypleura (Teller), M. ( Eomonotis ) daonellaeformis Kiparisova, and M. ( E. ) anjuensis Bytschkov and Efimova. Monotis subcircularis Gabb and related forms are transferred from the subgenus Entomonotis to the new subgenus Pacimonotis in order to give the former subgenus a more homogeneous morphologic range. Re-evaluation of the stratigraphic occurrences of these species shows that M. alaskana occurs above M. subcircularis and below M. haueri in the Wrangell Mountains, Alaska. Elsewhere in North America, however, Monotis haueri occurs at the same level as M. subcircularis and is thus a long-ranging form. These three taxa, as well as M. pachypleura , are representative of the upper Norian Cordilleranus Zone. Monotis daonellaeformis is among the oldest Monotis species of the middle Norian Columbianus Zone; M. anjuensis in North America is probably also from the Columbianus Zone but could overlap in occurrence with the upper Norian M. subcircularis.