A NEW EARLY SINEMURIAN (JURASSIC) AMMONITE SPECIES FROM THE TASEKO LAKES MAP AREA, BRITISH COLUMBIA, CANADA

Abstract

The present study is based on extensive sampling from an olistolith located near Castle Peak in the Taseko Lakes map area (British Columbia, Canada). The fauna is diverse and well preserved and similar to that illustrated from Monte Cetona in Italy by Fucini (1903). Coroniceras ( Paracoroniceras ) mutabile new species is the most abundant species of the assemblage found in the olistolith, where the ammonite association indicates an age corresponding with the Turneri Zone of the Early Sinemurian of the Northwest European zonation. C . ( P .) mutabile apparently complicates the still poorly understood systematics of the genus Coroniceras Hyatt, 1867. According to Corna and Dommergues (1995, p. 26), the genus Coroniceras and its subgenera should be considered as morphologic groups rather than natural taxonomic entities. In our new species, the complex ontogenetic variations in ornament and whorl section make subgeneric attribution difficult. The earliest stages of ontogeny recall the morphology of Eucoroniceras Spath, 1922, whereas in the last whorls C . ( P .) mutabile assumes a Caenisites -like morphology. In spite of this, we consider that this new species increases our understanding of the systematics of the Arietitidae. C . ( P .) mutabile constitutes an interesting intermediate, in terms of stratigraphic position and morphology, between the older Coroniceras of the Semicostatum Zone and the coexistent Caenisites Buckman, 1925. This suggests the existence of a phylogenetic link between these two genera, as supposed by Donovan (1987). The verification of this hypothesis by cladistic analysis constitutes an interesting theme for further research. Our new species also provides valuable new documentation of the age correlative of the Turneri Zone of the Northwest European ammonite zonation, a time interval not well represented in the Jurassic records of the Americas. The new species was found at one locality immediately …