TRACE FOSSIL EVIDENCE FOR THE PRESENCE OF SOCIAL INSECTS IN THE LATE JURASSIC MORRISON FORMATION OF EASTERN UTAH

The body fossil record of insects and other arthropods in the Morrison Formation is poorly known, thus the studies of trace fossils are vital for faunal reconstruction. A newly recognized social insect trace has been recovered in the upper Morrision Formation. The depositional environment of this trace fossil assemblage is interpreted as a distal floodplain. A series of mounds eroded in topographic relief in interbedded fine-grained sandstone mudstone facies were sampled to determine burrow morphology and mineralogy. QEMSCAN analysis revealed an abundance of silicified, hollow spheroids in the facies, amongst a mixture of fined-grained quartz and clay. Seven mounds eroding in relief were observed to contain these spheroids. The facies in which these spheroids are found contain extensively bioturbated purple and white silt. Silica replacement of organic material during diagenesis is the likely origin of these sphereoids. The evidence provided supports the interpretation this trace fossil assemblage was produced by social insects. Any hypothesis regarding specific identity and ecology of the tracemaker is extremely interpretive. Possible rhizolith structures within the trace fossils, suggest this trace fossil assemblage is likely an ichnologic fabric representing a subterranean biota, including herbaceous plants. The extensive silicification of the mounds and their regular, complex network of chambers and galleries, indicate social insects were present within this biota.