GEOCHEMICAL AND SEM ANALYSES REVEAL NO CONCLUSIVE EVIDENCE THAT MORRISON FORMATION SILICIFIED MASSES ARE HERBIVORE COPROLITES

The Morrison Formation is known for its rich fossil deposits of Jurassic sauropods and other dinosaurian herbivores, as well as silicified masses that are purported to be herbivore coprolites. Confirming that a mineral mass is a coprolite requires the identification of some remnant of organic material that is consistent with ingested food. 19 specimens of putative Morrison Fm. silicified coprolites ranging in mass from 130 g to 28.1 kg were obtained from a collector in Fruitlands, Utah, including specimens with possible gastroliths embedded within the mineral mass. Numerous specimens are interpenetrated with syneresis cracks that indicate mineral replacement in an aqueous environment. Each specimen was sectioned with a rock saw to expose the interior for digital microphotography and SEM scanning in order to search for any organic textures indicative of bone or plant material. SEM imaging revealed no evidence for woody or cellular textures. An EDS and XRD analysis of the chemical composition of the specimens show them to be primarily composed of quartz with minute concentrations of trace elements. Using XRF the origin of the various colors within the sectioned mineral masses was determined by the most abundant aqueous transition metal ion within a specific color: black – reduced iron / organics, green – Ni²⁺, blue – V⁴⁺, yellow – Fe³⁺, and red – Co²⁺. Loss on ignition analysis indicated that only 1.95% specimen mass could have come from organic volatiles. A petrographic analysis identified black, possibly carbonaceous material within the matrix of all specimens, however, these regions when viewed in thin section appear to be iron mineralization along fractures, which is consistent with the low concentration of carbon detected within the specimens. Polished stones embedded within some specimens were analyzed under SEM for distinctive surface abrasion patterns indicative of confirmed gastroliths. Results from this analysis were inconclusive. Overall, the Morrison Fm. mineral masses show no conclusive evidence of organic material indicative of their being coprolites, however these analyses cannot rule out the possibility that all traces of organic material were completely replaced by silica and all traces of organic texture were destroyed by recrystallization, as sometimes occurs in the formation of petrified wood.