Stratigraphy, Paleoecology and Functional Morphology of Heteromorph Ammonites of the Upper Cretaceous Nanaimo Group, British Columbia and Washington

Abstract

<p>The litho- and biostratigraphy of the Upper Cretaceous Nanaimo Group of the Vancouver Island region is revised on the bases of new fossil and field data. The Extension, Ganges and Protection Formations, recently subordinated into the Extension-Protection Formation by Muller and Jeletzky (1970) are readily mappable units, and are redefined as separate formations. The Haslam Formation contains previously unrecognized turbiditic facies. A new range zone, based on the range of Baculites chicoense Trask and Submortoniceras chicoense (Trask) is superjacent to the Schmidti Zone and subjacent to the Vancouverense Zone. The Pacificum subzone of the Vancouverense Zone (Muller and Jeletzky, 1970) is given full zonal status because of the non-overlap of Metaplacenticeras cf. pacificum (Smith) and Pachydiscus suciaensis (Meek).</p> <p>The paleoecology of the Elongatum Zone is examined using multivariate techniques. Four faunal assemblages are differentiated with cluster analysis: an inocerami-ammonite assemblage, a deposit-feeding bivalve dominant assemblage, an infaunal suspension feeding bivalve assemblage, and an anomiid bivalve-ammonite assemblage. The majority of the heteromorph ammonites of the Nanaimo Group were facies independent, suggesting either a wide enviromental tolerance, or extensive post - mortal transportation.</p> <p>A detailed morphological examination of a number of Nanaimo Group heteromorph ammonites indicates near or neutral buoyancy for the complete living animal and shell.</p> <p>Volume and density estimates for the heteromorphic species Didyrnoceras (Bostrychoceras) elongaturn (Whiteaves), Glyptoxoceras subcompressum (Forbes), Ryugassella ryugasensis Wright and Matsumoto, Baculites inornatus (Meek), B. anceps pacificus Matsumoto and Pseudoxybeloceras (Cyphoceras) nanaimoense Ward and Mallory all show positive buoyancy for complete animal without cameral fluid. A new technique to simulate living positions using exact and scale models made.of microcrystalline sculpting wax of varying densities is employed. Baculitids show a near-vertical orientation in all growth stages. D. elongatum has a vertical orientation (apex upwards) until formation of the U-shaped body chamber, when a slight tilting of the entire shell takes place. G. subcompressum, with a complex sequence of orthoconic, torticonic and finally gyroconic stages, shows a variety of attitudes. The hamitid P. nanaimoense shows a progressive "tumbling" in orientation during growth.</p>