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Publication Date

2014

Document Type

Honors Project

Department

Geosciences

Keywords

Fossils-Mongolia, Paleontology-Cambrian, Paleontology-Mongolia, Taphonomy-Mongolia, Cambrian, Taphonomy, Modes of preservation, Archalocyanthans, Small shelly fossils, SSF

Abstract

Lower Cambrian archaeocyathans, the first metazoan reef builders, and other small shelly fossils (SSFs) are uniquely preserved in phosphatic lenticular horizons in the Zavkhan basin of southwestern Mongolia. Field samples were collected at the contact of the Salaa Gorge Formation and Khairkhan Formation at the Orolgiin Gorge locality (E1221). Biological samples were picked from acid insoluble residues and twenty representative fossils (ten fossils from the first sieve and ten fossils from the second sieve) were imaged using the scanning electron microscope (SEM). Petrographic analysis of complementary samples was performed to assess the nature of phosphatization. The primary goal of this work was to determine the morphological diversity of these organisms, to interpret the ancient environments in which they lived, and to understand the diversity and ecology of these enigmatic Cambrian organisms. In residue, phosphatic internal molds of archaeocyathans were observed, and not surprisingly, in thin section, internal molds of archaeocyathans were common. Phosphatic replacement of SSFs was the most common mode of preservation. Also, a glauconitic archaeocyathan was found in residue. These organisms were transported from a peri-reefal environment to a deeper marine setting. The phosphatic molds were formed when organic matter decayed and apatite (phosphate) grew around mineral grains that filled in the hollow calcified skeletons. It is possible that local anoxic and ferruginous conditions created an environment that fostered apatite nucleation. This thesis is the first in-depth analysis of phosphatized archaeocyathans and will help with understanding different modes of preservation for archaeocyathans in the lower Cambrian.

Language

English

Comments

v, 56,v pages : illustrations (some color), maps (some color). Honors Project-Smith College, 2014. Includes bibliographical references (pages 52-56)

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