T250 Geobiology of Critical Transitions: Integrating Fossils, Proxies, and Models I

Tuesday, 3 November 2015
1:30 PM - 5:30 PM
Room 325/326

CONTENT, NATURE, AND SIGNIFICANCE OF THE RIFLE RANGE LAGERSTÄTTE (EAGER FORMATION, EARLY CAMBRIAN), CRANBROOK, BRITISH COLUMBIA

Paper number: T250-12
Program: Technical Sessions
Time: 4:05 PM – 4:20 PM

Authors

Abstract

The early Cambrian (mid-Dyeran) Rifle Range Lagerstätte at Cranbrook, British Columbia, has remained relatively poorly known despite the occurrence of Burgess Shale taxa—among the oldest from North America—and its abundant olenelloid trilobite fauna. A systematic excavation spanning three meters within a shale interval of the Eager Formation was conducted in order to better document the biota, ichnology, and depositional and taphonomic context. The low diversity assemblage is dominated by trilobites (at least four species, two of which are new), and also contains hyoliths and more rarely inarticulate brachiopods. In addition to rare Tuzoia carapaces and extremely rare Anomalocaris claws (both allochthonous), putative cyanobacteria and one sponge were recovered. The rarity of sessile benthic organisms is striking and suggests a possible taphonomic bias. The low diversity ichnofauna records the activities of a surficial to shallow tier infauna dominated by worm-like producers. An arthropod trackway was also recovered. Density of structures is generally low (ichnofabric index 1-2, bedding plane bioturbation index 1-2).

Thin (mm) interbeds and lenses of silt and sand, including sand-filled burrows, are abundant. Numerous pavements of densely-packed trilobite sclerites also occur throughout the studied interval. Most isolated trilobite cephala are preserved in a hydrodynamically stable convex-up attitude, although they show no preferred alignment. Microstratigraphic and biostratinomic data therefore indicate that the deposit accumulated in an environment that was generally calm but experienced occasional current activity that was sufficient to induce sediment winnowing or bypass. This is consistent with a distal shelf setting below storm wave base.

A wide size range of trilobite remains is preserved, and articulated specimens occur with moderate frequency. Detailed ontogenetic information for the new species of Wanneria and Olenellus offers novel insight into olenelloid phylogeny. Olenellids and biceratopsids share a derived pattern of cephalic and thoracic development relative to the plesiomorphic condition exhibited by Wanneria, Elliptocephala, and "laudoniids". Olenelloid phylogeny, revised in light of these new data, is congruent with the stratigraphic ranges of the clades.