TAPHONOMY OF A MULTI-OOTAXA EGGSHELL LAG DEPOSIT, THE LATE CRETACEOUS MUSSENTUCHIT MEMBER, CEDAR MOUNTAIN FORMATION, UTAH

Taphonomic studies of eggshell deposits have focused typically on modern and ancient nesting site hence eggshells are commonly single ootaxa and not transported any significant distance. This study describes an unusual, multi-ootaxa, fragmental eggshell lag deposit in the Late Cretaceous Mussentuchit Member of the Cedar Mountain Formation located along east the San Rafael Swell, Utah and interprets the deposits with the aid of a series of neotaphonomic experiments.

The Mussentuchit egg eggshell-bearing lag deposits were excavated in the field and examined in thin section, Field Emission Scanning Electron Microscope (FESEM) and Energy Dispersive Spectroscope (EDS). Lag deposit consists of a series of three lenses composed of a graded pebbly sandstone passing vertically into mudstone, sheet flood deposit. In thin sections, shell exteriors display varying degrees of abrasion and corrosion from complete destruction to minimal on the ornamented surfaces. Also present are compaction related styolites and conjugate fractures. Near complete to partial mammillary tips are preserved. The difference in preservation from interior to exterior was replicated in tumbling experiments. FESEM analysis of the eggshells documents preserved bacteria community, corrosion of calcite grains, groove marks on exterior, the absence of cuticle and sediment-filled pores. EDS confirms the absence of cuticle. The bacterial community is not present in all fragments. Buried egg experiments documented thriving microbial communities on shell exterior indicating at least some of the transported fragments were buried in soil. Egg shells were subject to different degrees of transportation and subjected to both physical and chemical weathering processes.