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The Paleoecology of Tentaculitids from the Silurian Arisaig Group, Nova Scotia, Canada

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The Paleoecology of Tentaculitids from the Silurian Arisaig Group, Nova Scotia, Canada Wittmer, J. 2007. 20th Annual Keck Symposium; http://keck.wooster.edu/publications THE PALEOECOLOGY OF TENTACULITIDS FROM THE SILURIAN ARISAIG GROUP, NOVA SCOTIA, CANADA JACALYN WITTMER Beloit College Research Advisors: Hilary Lackey and Carl Mendelson INTRODUCTION Ten fossiliferous beds from the Arisaig Group (see Lackey et al., Table 1, this volume) contain The Silurian rocks of Nova Scotia record a abundant tentaculitids lying parallel to bedding diverse sedimentologic and paleontologic planes; vertically oriented shells occur rarely. history. The aim of this research is to These beds are composed predominantly of understand the paleoecology of portions shales and siltstones, and contain fossiliferous of the Arisaig Group by studying certain grainstone lenses that are interpreted to be paleoecological indicators associated with tempestites (Boucot et al., 1974). tentaculitids. Tentaculitid orientation can yield clues about the local depositional METHODS environment. This information may be augmented by taxonomic and petrographic Fieldwork was done in Arisaig, Nova Scotia, studies of tentaculitid-bearing beds, allowing between July 16 and August 8, 2006. Ten beds a better understanding of the Silurian marine that contained abundant tentaculitids (>15) and environment. were exposed along the shoreline were selected for detailed study from the following formations Tentaculitids are annulated, conical, shelled in the Silurian section at Arisaig: Ross Brook organisms of uncertain biologic affinities that Formation (4), Doctors Brook Formation (3), range from the Ordovician to the end of the Moydart Formation (3) (Fig. 1). Each bed was Devonian (Cornell et al., 2003). The bilateral measured and described using a Jacobs staff shells are typically compartmentalized by (outfitted with a protractor to account for dip), a transverse septa with conical embryonic meter stick, and a Brunton compass. The strata chambers. They are thought to be related to one meter above and below the selected beds cephalopods because they have a tapered cone- were described and rudimentary stratigraphic shaped shell made of calcite; however, they lack columns were constructed. Detailed drawings a siphuncle. Some have suggested that these of the ten fossil beds were made, concentrating organisms are remotely related to pteropods on the orientation of tentaculitids in the field. (pelagic gastropods), but most pteropods have a Digital photographs were taken of the fossil very thin asymmetrical shell and did not emerge beds, the sedimentary beds above and below the until the Tertiary, long after tentaculitids went fossil beds, and the adjacent stratigraphy. extinct (Orlov et al., 1976). Because of these uncertainties, tentaculitids have been placed into a separate (probably molluscan) class, the Cricoconarida (Fisher, 1966). 283 Wittmer, J. 2007. 20th Annual Keck Symposium; http://keck.wooster.edu/publications 284 Wittmer, J. 2007. 20th Annual Keck Symposium; http://keck.wooster.edu/publications Photographs of the beds were imported into RESULTS Adobe Illustrator 10 to trace and measure the orientation of tentaculitids (Fig. 2). Data were Ross Brook Formation then transferred to Stereonet for construction of The orientation of the tentaculitid shells from rose diagrams using 10° intervals on equal-area the upper Ross Brook Formation (beds A-D) plots. Because the beds typically dipped, true display bimodal and polymodal distributions orientations of tentaculitids were determined via (Fig. 1). Beds A, C, and D are bimodal, with stereonet analysis. modes at 80° and 280° (A), 150° and 320° (C), and 180° and 350° (D). Bed B has a polymodal distribution with modes at 70°, 150°, and 270°. Bed C has a relatively weaker orientation at 30° and 210°, yielding a quadrimodal distribution. 285 Wittmer, J. 2007. 20th Annual Keck Symposium; http://keck.wooster.edu/publications Doctors Brook Formation Order Tentaculitida Lyashenko, 1955 Bed E exhibits a nearly trimodal pattern with Family Tentaculitidae Walcott, 1886 modes of 50°, 170°, and 320° (Fig. 1). Beds F Genus Tentaculites von Schlotheim, 1820 and G are bimodal, with modes at 30° and 220° Tentaculites canadensis Ami, 1894 (F) and 170° and 350° (G). Description: Length 6-23 mm, width 0.5-4.5 Moydart Formation mm. The embryonic chamber is narrow with In the green member of the formation beds H a blunt point and the internal wall undulates and J show a dominant bimodal distribution slightly. Ornamentation consists of evenly with modes at 90° and 250° (H) and 50° and spaced primary rings that are lipped, giving a 280° (J) (Fig. 1). Bed I exhibits a polymodal stacked cup appearance (Fig. 3). distribution, with modes at 90°, 190°, 270°, and 320°. Occurrence: Upper Ross Brook Formation (Hurst and Pickerill, 1986). TENTACULITID SYSTEMATICS Tentaculites sp. Description: Length 3-9 mm, width 0.5-1 According to McLearn (1924) and Flower mm (Doctors Brook Formation). Length 9-12 (1943), tentaculitid fossils in the Arisaig Group mm, width <1-3 mm (Moydart Formation). belong to Tentaculites canadensis Ami, 1894. Embryonic chamber is narrow with a blunt On the basis of more recent and detailed studies point. The inner wall undulates slightly and of Fisher (1966) and Larsson (1979), I believe the external ornamentation consists of evenly that the diversity of Arisaig tentaculitids is much spaced and lipped primary rings. This species greater. Preliminary taxonomic analysis yields overlaps in size with T. canadensis, but is four genera and at least five species in the study typically significantly smaller. area. Taxonomic characters of tentaculitids include the size of the shell, spacing and shape Occurrence: Doctors Brook Formation of the rings, shape of the embryonic chamber, (phosphatized), Moydart Formation (pyritized and cross-sectional morphology of the inner and phosphatized). wall (Fig. 2A). Below are brief descriptions of the tentaculitids examined in the Arisaig Group. Family Rossiitidae Lyashenko, 1969 Genus Dicricoconus Fisher, 1962 Class Cricoconarida Lyashenko, 1957 Dicricoconus sp. 286 Wittmer, J. 2007. 20th Annual Keck Symposium; http://keck.wooster.edu/publications Description: Length 8-12 mm, width 0.5-1 mm. downcurrent in relatively rapid flow, and Embryonic chamber has a narrow blunt point upcurrent in relatively slow flow (Nagle, 1967; and the internal wall is smooth. This genus has Allen, 1984). During waning stages of storms, uniformly spaced primary rings separated by wave action may produce oscillatory flow, secondary rings that are lipped. causing high-spired shells (such as tentaculitids) to orient parallel to the wave crests (see Myrow Occurrence: Doctors Brook Formation. and Southard, 1996). Around half of the apices point in one direction and the other half point Family Volynitidae Lyashenko, 1969 in the opposite direction, yielding bimodal Genus Odessites Lyashenko, 1969 paleocurrent roses. Differential filling of a shell Odessites sp. may cause the center of gravity to migrate; in this case, unidirectional flow might result in a Description: Length 2.5-9 mm, width 0.5-0.8 bimodal pattern. Furthermore, the biology of mm. The shape of the embryonic chamber tentaculitids is unknown, and may play a part has a blunt point with an irregular inner wall. in shell alignment. The interpretation of rose Ornamentation consists of annulations that diagrams based on tentaculitid orientation is alternate with widely and equally spaced rings. thus complex. Occurrence: Moydart Formation. Ross Brook Formation Tentaculitid orientations within beds A- Genus Podolites Lyashenko, 1969 D display bimodal distributions, but the Podolites sp. predominant modes vary from 80° and 280° to 180° and 350°. According to Beck Description: Length 2.5-9 mm, width 0.5-0.8 and Strother (2001), the upper Ross Brook mm. The embryonic chamber has a blunt point. Formation demonstrates a middle to inner shelf Shells are straight with primary and secondary environment where storm waves commonly rings that contain curved annulets. The inner occur; the bimodal rose diagrams support this wall is irregular and lacks correspondence with interpretation (Boggs, 2006). Within the shaly the primary rings, secondary rings, and annulets siltstones (above and below the tempestites), on the outer wall of the shell. local evidence of tidal and other nearshore processes occur, including flame structures, Occurrence: Moydart Formation. flaser bedding, ripple marks, and bioturbated beds. Some of the tentaculitids that show DISCUSSION preferred alignment within this formation were found perpendicular to ripple marks, further Orientation Analysis supporting an inner shelf environment. The Tentaculitid orientation is dependent upon at polymodal distribution in bed B could be due least three variables: nature of the flow (e.g., to bioturbation, which would alter the original unidirectional, bidirectional/oscillatory), shell orientation of the shells. hydrodynamics, and position of the center of gravity of the shell (Lindholm, 1987). Tentaculites canadensis specimens are Unidirectional flow typically gives rise to preserved as complete shells or external molds; a strong alignment of tentaculitids along broken shells are rare, suggesting only modest their long axes (Flower, 1943; Fisher, 1966; transport. The associated fauna includes the Cornell et al., 2003). But the shell apex points brachiopod Camarotoechia rossonia (Hurst and Pickerill, 1986) and trilobite fragments. 287 Wittmer, J. 2007. 20th Annual Keck Symposium; http://keck.wooster.edu/publications Doctors
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