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Newton, C.R., 1983

Norian (Late Triassic) molluscs of cordilleran allochthonous terranes: Paleoecology and paleozoogeography

Bibliographic Reference

Newton, C.R., 1983, Norian (Late Triassic) molluscs of cordilleran allochthonous terranes: Paleoecology and paleozoogeography: University of California, Santa Cruz, Ph.D. dissertation, 173 p., illust., maps.


Late Triassic molluscan faunas of the Cordilleran allochthonous terranes are, in large part, the preserved remnants of Mesozoic tropical island arc and oceanic plateau communities whose modern counterparts have the highest species richness and speciation rates in the marine realm. This report is a pilot paleoecological study of three allochthonous terrane faunas: two silicified early Norian assemblages from the Hells Canyon and southern Wrangell Mountains sections of Wrangellia, and one calcareous late Norian biota from the Peninsular terrane at Puale Bay. The southern Wrangell Mountains assemblage is an epifaunal/vagrant, detritus-feeding association that has a high molluscan species richness (75 spp. of bivalves and gastropods). The Hells Canyon fauna is overall a vagrant/epifaunal, suspension/detritus-feeding association of very high species richness (91 spp. of bivalves and gastropods). The upper Norian carbonates at Puale Bay, on the Alaska Peninsula, contain two predominant faunal associations: a lower, coral-rich fauna occurring in thick-bedded biomicrites, and an upper, depauperate fauna of Monotis subcircularis and ammonoids occuring in rhythmically-bedded, organic-rich siliceous limestones, calcareous shales, and cherts. In both the Hells Canyon and Wrangell Mountains collections, rare circular, bevelled boreholes in the umbonal regions of epibyssate pterioid bivalves suggest the presence of predaceous gastropods in early Norian shallow-water shelf communities. The Wrangellian borings, along with similar structures reported from the Alpine Cassian Formation (Fursich and Wendt, 1977), imply that the advent of gastropod carnivory occurred during the post-Paleozoic gastropod radiation, rather than during the Cretaceous 'Mesozoic molluscan revolution' of Vermeil (1978). Zoogeographic comparisons of bivalve genera at the Wrangell Mountains and Hells Canyon localities reveal that these two faunas are more similar to one another than to any other Late Triassic assemblages in the published literature (Simpson similarity = 77; Sorenson similarity = 0.65). Further, the presence of the American forms Septocardia sp. and Mysidiella cf. M. americana in both localities, and the additional occurrence of Mysidioptera cf. M. williamsi in Hells Canyon, indicate strong affinities with the East Pacific. The Puale Bay fauna shows greatest statistical similarity to the Wrangell Mountains assemblage; moreover, the association of Monotis subcircularis and Monotis salinaria suggests a tropical, East Pacific origin for the triassic rocks of the Peninsular terrane. A small collection of late Norian molluscs from the Alexander terrane contains numerous specimens of Septocardia cf. pascoensis, a form known previously only from Peru. This occurrence suggests that the Alexander terrane may have occupied a more southerly position during the Late Triassic.

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