CHARACTERIZATION OF “SQUARE OBJECTS” FROM THE BEAR GULCH LIMESTONE (UPPER MISSISSIPPIAN, MONTANA)

The Bear Gulch Limestone is a fine-grained, non-bioturbated, laminated, and homogeneous unit known primarily for its incredibly well-preserved fossils. This Upper Mississippian-age plattenkalk is believed to have been deposited within a highly productive and complex embayment that lay on the boundary between tropical and arid environments. It is hypothesized that rapid deposition of sediments during summers resulted in the death and preservation of the specimens found throughout the beds. The fossils of the Bear Gulch include sharks and other vertebrates, a variety of invertebrates including sponges and worms, and a number of enigmatic fossils labeled as “square objects” or “blobs”.

“Square objects” come in a range of colors and sizes; they are preserved both as films, impressions, and possibly casts. Many of the square objects have a mottled appearance. Previous research on some square objects found them to be composed of an amorphous carbon film (Thomas 2004). The present study focused on describing and characterizing the range of square objects identified from the Bear Gulch limestone using a variety of tools including basic morphometric measurements and comparisons, photographic techniques (including RTI) and analysis (e.g. Image J, Meshlab), optical light microscopy and geochemical elemental analysis and mapping (Raman and micro-XRF). “Square objects” examined for this study came from the collections at the Carnegie Museum and/ or were collected by the authors and represent both isolated specimens and slabs filled with many organisms.

The data collected clearly demonstrate that the “square objects” of the Bear Gulch actually represent a variety of different organisms in differing states of preservation. They are distinguished from each other by shape, size, textural features, color variations and patterns, visible impressions of structures and slight elemental compositional differences. The data support a variety of likely biological affinities for these objects including Ctenophora, Cnidaria, Porifera, and Tunicate as well as provide information regarding their environment at time of preservation (e.g. mass stranding or life position).