NEW CHANCELLORIIDS FROM THE EARLY CAMBRIAN SEKWI FORMATION WITH A COMMENT ON CHANCELLORIID AFFINITIES

Journal of Paleontology, Sep 2005 by Randell, Robert D, Lieberman, Bruce S, Hasiotis, Stephen T, Pope, Michael C

PRESERVATION AND TAPHONOMY

Articulated and disarticulated sclerites have largely resisted compaction and their walls and internal cavities have been replaced by secondary crystalline calcite (Fig. 4). Eleven partial or fully articulated scleritomes were collected, lying parallel to bedding surfaces. Seven scleritomes exhibit unusual relief of the body itself, especially the holotype of Archiasterella fletchergryllus n. sp. (Fig. 7.1), with a vertical thickness of 16 mm. Scleritomes are exposed on the undersurface of 10-50 mm thick pieces of calcareous siltstone, based on geopetal evidence and doming of microbial fabrics (Fig. 5). A thin, 0.2-1.5 mm layer of dark, finely granular microbialite encrusts the undersurface and associated sclerites of the holotype (Fig. 7.1). Typically, only the undersurface of a specimen is articulated, forming a low trough into which the disarticulated elements of the upper surface have collected. Disarticulated sclerites and fragments of integument appear to have acted as nuclei for multilayered microbialitic encrustations, and a consortium of microbial, cyanobacterial (possibly Girvanella Nicholson and Etheridge, 1878), and potentially algal microbiota form a coalesced, three-dimensional filamentous framework within the trough of the undersurface. No true soft-part preservation is observed, although narrow, dark cores within some sclerites appear to correspond to the central line of Butterfield and Nicholas (1996, fig. 5.5) and may record the former presence of connective tissues (see Bengtson and Hou, 2001).

Previously described examples of articulated chancelloriid scleritomes are from sites with Burgess Shale-type preservation and differ in preservational style and taphonomy from the Sekwi Formation material. For example, specimens are usually flattened,

Our interpretation of the taphonomic processes that led to the exceptional preservation of chancelloriids in the Sekwi Formation approximately corresponds to the model for preservation of Uintacrinus in the Smoky Hill Chalk (Cretaceous, Kansas), but the volume and structural development of microbialite in the Sekwi Formation is substantially greater, while the absence of cyanobacterial fabrics in the Smoky Hill Chalk may suggest it had a deeper environment of deposition.

MORPHOLOGICAL TERMINOLOGY

Terminology used below follows Sdzuy (1969) and Beresi and Rigby (1994) with a few modifications. The complete body has an attached basal end and an apical growing end. A basic chancelloriid sclerite (Fig. 3.2, 3.3) possesses a central ray perpendicular to the body wall, and lateral rays lying parallel to the body surface surround the central ray. All rays are joined in a central disc, with sutures marking the line of adpression between rays. Rays enter the central disc proximally and project distally. Individual sclerites are described by [number of lateral rays] [number of central rays]; e.g., a 5 1 sclerite has five lateral rays and one central ray. Lateral rays are classified by their orientation relative to the main body axis. Since the lateral ray oriented towards the apex is termed the vertical ray, those perpendicular to it are called the horizontal rays. The largest apically oriented ray of Archiasterella fletchergryllus n. sp. is not enclosed within the central disc by further lateral rays and is referred to as a recurved basal ray.