CRYPTIC, OVAL MICROBIALITES (ENDOLITES) FROM THE MIDDLE CAMBRIAN LEDGER FM., SOUTH-CENTRAL PENNSYLVANIA, USA

Modern reefs in high energy settings adapt by building robust coral frameworks that can withstand normal current activity and wave action. In the Middle Cambrian, coral framebuilders were absent so to exploit high energy ecological niches, microbial organosedimentary constructers, primarily cyanobacteria, (+/- algae and bacteria), developed similarly robust morphologies. As seen in the Cambrian Ledger Fm., reef microbialite consists of weakly bedded sheets composed of shrubs and stubby strands of calcified Epiphyton- and Angulocellularia-like elements, described previously by de Wet and co-authors. Centimeter-scale stromatolites, thrombolites, oncolites, and oval, multiple-layered, organic, cryptic structures, informally termed endolites, form distinctive structures. Endolites are elliptical, 3-15 cm diameter forms that occur in clusters inside of reef cavities. They consist of a fenestral-rich microbialite core consisting of clotted fabrics and stubby strands, coated with fine-scale parallel laminations, overlain by microbial microshrubs, often overlain by further parallel laminations and another generation of microshrubs, which may be interspersed with microbial pillars and cuspate wispy laminations. We suggest that the elliptical form represents a biological strategy to maximize access to nutrients and enhance growth even within the low light levels inside of submarine cavities. By increasing surface area, the microbial consortium could be more effective at competing for nutrients and/or light. The endolites are entombed in multiple generations of fibrous submarine cement or oolitic grainstone and cement.

The Ledger microbial assemblage closely resembles living cryptic, mat and domal cyanobacterial forms reported from Tikehau Atoll, French Polynesia, and Lizard Island, Australia. Enigmatic Middle Cambrian fossil microbial morphologies may be better understood by analogy with these modern cyanobacterial communities.