The Arco Hills Formation of east-central Idaho is a cyclically bedded mixed carbonate-siliciclastic unit which is Late Mississippian (Latest Chesterian) in age and contains abundant well-silicified brachiopods. By looking at the brachiopods of the Arco Hills Fm. in a paleoecologic aspect, it is possible to quantitatively assess sea-level changes though the Latest Chesterian.

Factors such as quantitative community analysis, functional morphology, and taphonomic features provide the basis for water depth interpretation. The brachiopods and associated other taxa are found as both in situ communities and biostratinomic accumulations. Four biofacies containing three distinct communities, interpreted to be depth controlled, have been established. Using the biota of the Arco Hills Formation as indicators of paleodepth, a detailed record of significant sea level fluctuations, deposition indicative of icehouse conditions, can be compiled for measured stratigraphic sections, especially where lithology alone fails. In comparing the depositional style and changes in faunal composition of the Arco Hills Formation to the units it overlies, it is apparent that greenhouse conditions existed prior to Arco Hills deposition.