A biomagnetostratigraphy for the Lower Triassic is constructed, using the ammonoid biostratigraphy from Arctic Boreal successions. Combined thermal and alternating fi eld demagnetization determines the Triassic magnetic fi eld polarity in 86% of specimens, with 36% showing linear trajectory line fi ts and the remainder showing great-circle trends toward the characteristic magnetization. Mean pole directions for the Deltadalen (λ = 50°, ϕ = 159°, dp/dm = 3.9°/5.1°), Lusitaniadalen (λ = 56°, ϕ = 163°, dp/dm = 4.4°/5.4°), and Vendomdalen (λ = 57°, ϕ = 143°, dp/dm = 4.4°/5.4°) members fall close to the European Lower Triassic apparent polar wander path. Mean directions for two of these membermeans pass the reversal test. The remanence is predominantly carried by magnetite. The polarity stratigraphy, when integrated with the ammonoid and meager conodont data, is similar to that determined from successions in the Sverdrup Basin (Canada). The Permian-Triassic boundary post-dates a pronounced palynofl oral turnover and predates a short duration reverse magnetozone (LT1n.1r). In the correlated Shangsi section (in South China), LT1n.1r occurs after the fi rst appearance datum (FAD) of H. parvus, but in the Arctic is within the Otoceras boreale Zone. The late Griesbachian to early Smithian is mostly reverse polarity, with three normal-polarity intervals, overlain by mid and late Smithian normal polarity. The Spathian contains four reverse-polarity intervals, the oldest one within the early Spathian with the remainder in the late Spathian. The transition into the Anisian is within the uppermost reverse magnetozone, a feature documented in other sections of this age. The polarity pattern is correlated to other marine sections, indicating the robustness of the biomagnetostratigraphic composite and its utility in calibrating Lower Triassic time.
The ammonoid species Czekanowskites rieberin.sp. from the Lower Anisian of Arctic Siberia (River Olenek) shows an enormous range of intraspecific variability. The 700 investigated specimens are packed in one single concretion. They are all adult with preserved living chambers and apertural structures. Morphologically they range from keeled smooth suboxycones with narrow umbilicus, through feebly ribbed platycones with a little wider umbilicus, to subcadicones with relatively wide umbilicus and straight ribs with bullae. The morphological variation is continuous; all specimmh belong to a single genetically linked population, variants of one biospecies. This phenomenon, which is quite common in boreal Triassic ammonoid faunas, has in the past led to serious taxonomic oversplitting. It presents a serious challenge to current ideas about a close correlation between modeof life and shell morphology in arnmonoids.□Amntonoidea, boreal Triassic, ecology. Siberia, variability.
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