EARLY CAMBRIAN PHOSPHATIZED ARCHAEOCYATHANS AND SMALL SHELLY FOSSILS (SSFS) OF SOUTHWESTERN MONGOLIA

Archaeocyathans, an enigmatic group of calcified sponges, were among the first animals inhabiting reef environments in the Cambrian. In an exceptionally preserved fauna at the contact between the Lower Cambrian Salaagol and Khairkhan formations of southwestern Mongolia, phosphatized archaeocyathans are preserved with abundant small shelly fossils (SSFs). Insoluble residues and thin sections were examined from 8 limestone samples to understand the paleoecology of this setting and the nature of phosphatization. These samples were dissolved in 10% buffered acetic acid and 2 extractions were performed to collect 16 residue. Each residue was sieved, and the >840 µm and 420µm – 840µm fractions were examined in detail. From these residues, 160 fossil samples were examined under a light and scanning electron microscope (SEM). Phosphatization occurred in two important ways: internal molds of archaeocyathans and replacement of SSFs. In thin section, archaeocyathans, chancelloriids, hyoliths, ostracods, and brachiopods were observed. Archaeocyathans were preserved both as carbonate skeletons and as phosphatized internal molds. In the internal molds, phosphate and other mineral grains filled in the empty pore spaces of the archaeocyathan skeleton and the outer portion was calcitic. Hyoliths and chancelloriids were commonly replaced by phosphate, but calcitic examples were also present. In a few instances, phosphatic cement was observed in thin section. This assemblage was likely transported from a peri-reefal environment to a deeper water setting in the basal Khairkhan Formation. In this deep water environment, the decay of organic material within the archaeocyathans may have created the ideal redox conditions to foster the precipitation of apatite. This work corroborates previous findings that point to the critical role of organic matter and low oxygen and iron rich conditions as precursors for phosphatization and reveals aspects of the unique environment in which these organisms were preserved.