INVERTEBRATES OF THE UPPER MISSISSIPPIAN BEAR GULCH LIMESTONE – A TALE OF JELLIES AND TUNICATES ?

The Bear Gulch Limestone is a Mississippian age plattenkalk deposit best known for the numerous exceptionally preserved fossil fish and sharks contained within it. Well-preserved invertebrates have also been identified within the Bear Gulch including several species of shrimp, worms, sponges, and limulids. Additionally, numerous enigmatic fossils, generally referred to as “square objects” or “blobs”, have been found within this limestone. These fossils – many of which are preserved only as impressions or carbon films – contain textures and patterns that have, for the most part, eluded ready assignment to any particular phylum.

For this study, “square objects” obtained from the collections of the Carnegie Museum and/or collected by the authors, were examined using a variety of photographic (including RTI), geochemical, and microscopy techniques in order to identify distinguishing features that might allow for the assignment of some of these square objects to known taxonomic groups. Based on the results of these analyses, two of the square objects have been assigned possible taxonomic classifications and a third appears to have possible affinities to two possible groups.

The first and smallest of these is believed to belong to the cnidaria and has features that appear similar to modern Rhizostome jellyfish. This fossil, unlike the others, is preserved in relief and not simply as a flat thin film. The second square object, which is ~ 2.3 cm in length, is preserved primarily as a thin film with distinct coloration. Some of the coloration corresponds to specific chemical signatures, as determined by micro-XRF. These chemical signatures combined with features revealed through optical microscopy, photography, and RTI support assigning this square object to the pelagic tunicates. The largest of the square objects (5.5 cm x 8.5 cm) contains distinctive mineralized hard parts scattered throughout the body of the organism. These, combined with geochemical data obtained with Raman and micro-XRF, support tentatively assigning this specimen to the porifera, although there is some evidence that supports a possible tunicate affiliation.