Lower Bajocian (Jurassic) Ammonoid Faunas of the Western Americas

Abstract

<p>The Lower Bajocian (Jurassic) ammonoid faunas from the lower parts of the Yakoun Formation along Skidegate Inlet (Queen Charlotte Islands) belong to three distinct faunules, probably of different ages: the Zemistephanus richardsoni faunule at MacKenzie Bay (upper Sauzei Zone or lowermost Humphriesianum Zone), the Stephanoceras skidegatense - Chondroceras defontii faunule at Richardson Bay (Humphriesianum Zone) and the Stephanoceras itinsae - Chondroceras oblatum faunule on South Balch Island (upper Humphriesianum Zone).</p> <p>Taxonomic and stratigraphic comparisons with other ammonoid faunas from the western margins of both North and South American show that close affinities exist with southern Alaska, mainland British Columbia, western Alberta and eastern Oregon, and lesser ones with western interior United States, Chile and Argentina. Larger samples led to a number of taxonomic revisions by establishing the range of variation of species.</p> <p>The following dimorphic species are recognised: Stephanoceras ilinsae (McLearn) ♀ & ♂[=Stephanoceras yakounense. McLearn ♀ - Itinsaites itinsae McLearn ♂], Stephanoceras skidegatense (Whiteaves) ♀ & ♂ [microconch new], Zemistephanus richardsoni (Whiteaves) ♀ & ♂ [microconch new], Chondroceras oblatum (Whiteaves) ♀ & ♂ [microconch new] and Chondroceras defontii (McLearn) ♀ & ♂ [microconch new]. As a consequence of these dimorphic pairings the microconch genus Itinsailes McLearn, 1927 is regarded as a junior subjective synonym of Stephanoceras Waagen, 1869, and Kanastephanus McLearn, 1927 as a subjective synonym of Zemistephanus McLearn, 1927. The new species Zemistephanus alaskensis ♀ is described.</p>