McDonald, Shauna , Tomescu, Alexandru MF .
Morphology of Late Ordovician (Ashgillian) compression macrofossils from Cononcocheague Mountain (Pennsylvania): a window into early continental biotas.
ABUNDANT compression macrofossils occur at multiple levels in Ashgillian (Upper Ordovician) rocks of the Reedsville Formation – Oswego Sandstone transition at Conococheague Mountain, Pennsylvania. The lithology of fossiliferous intervals varies from shale to siltstone, and sedimentary structures suggest depositional environments corresponding to a transition from shoreline settings (Reedsville), to deltaic and low energy fluvial plain (Oswego). C/S ratios of fossiliferous layers are consistent with freshwater deposits and corroborate sedimentological data supporting a continental origin for the fossils. Combined with the Late Ordovician age, these distinguish the Conococheague Mountain fossils as one of the oldest megascopic continental biotas. Characterization of fossils from several layers has focused on documenting qualitative and quantitative aspects of morphology: size, shape, margin completeness and geometry, type of preservation, and surface features. This data will allow for assessment of disparity within the fossil assemblages, as well as for exploration of relationships between morphology and lithology, stratigraphic position, and chemistry of fossils. Study of the compressions has revealed thalloid, primarily isodiametric morphologies, with sizes up to 30 mm, and sometimes exhibiting variations in thickness. Most fossils fall within one of two types of preservation, carbonaceous or oxidized, but some specimens exhibit both types. Numerous specimens have entire, rounded margins, and some display protrusions. Salient surface features, found in many specimens, include circular gaps in the carbonaceous material with a definite pattern of distribution, shallow pitting, and minute bumps (the latter occurring mostly around margins). These results reveal unforeseen variety in the apparently uniform fossil assemblages at Conococheague Mountain. They also show that detailed characterization of morphology is an important first step in investigations that combine chemical analyses with studies of internal structure, and which have the potential to open broad perspectives on the thalloid biotas that characterize early stages of the colonization of land.
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1 - Humboldt State University, Department of Biological Sciences, Arcata, California, 95521, USA
Presentation Type: Poster:Posters for Sections
Location: Auditorium/Bell Memorial Union
Date: Tuesday, August 1st, 2006
Time: 12:30 PM