Lower Ordovician (Ibexian) trilobites from the Shallow Bay Formation, western Newfoundland, Canada.

摘要

The Shallow Bay Formation (Cow Head Group), western Newfoundland, is composed of a series of allocthonous massive limestone conglomerates and autocthonous interbedded limestones and shales. Two faunas recovered from the conglomerates exposed at the Point of Head and Back Cove sections, Cow Head Peninsula are examined herein and are the first Lower Ordovician trilobites from the Shallow Bay Formation to be comprehensively described. A single boulder recovered from unit 8 on the Point of Head contains a unique olenid-dominated trilobite fauna that is likely earliest Ordovician in age, but a latest Cambrian age cannot be entirely ruled out. The fauna is composed of seven species with the pelturine trilobite Boeckaspis geordii n. sp. dominating the assemblage. A phylogenetic analysis of Boeckaspis and other putatively closely related taxa was conducted herein. The Point of Head assemblage is unique in that it is the only known olenid-dominated fauna from Cambrian-Ordovician boundary sediments in Laurentia and it was recovered from a lithology which is atypical for olenid-dominated faunas. The second fauna consists of specimens recovered from 29 boulders collected by C.H. Kindle and H.B. Whittington from Back Cove and is likely Blackhillsian in age. A total of 73 species from 46 genera representing 20 families were identified from the Back Cove collections. Fifteen species and three genera are new and formally named herein. Phylogenetic relationships of species of Opipeuterella and Apatokephalus respectively were examined in light of newly described material from Back Cove. An attempt was made to correlate the Back Cove fauna with the Ross/Hintze trilobite-based biostratigraphic zones established in western Laurentia, but this scheme was found to be not directly applicable. The Back Cove data were included in a Q- and R-mode cluster analysis. This is the first analysis to quantitatively examine trilobite biofacies for the upper Ibexian in Laurentia. The Back Cove fauna is representative of a shelf-margin build-up biofacies. Possible earliest occurrences of the Odontopleuridae, Illaenidae, and perhaps the Proetidae or Rorringtoniidae are present in the Back Cove fauna, which lends credence to the hypothesis that the dominant post-Cambrian trilobite groups made their first appearance in shelf-margin build-up environments.