The Arco Hills Formation contains abundant lingulate, strophomenate, and rhynchonellate brachiopods, echinoderms, corals, bryozoans, and rare vertebrate and trilobite fossil fragments. Fossils are equally abundant in carbonate and siliciclastic units; however, silicified brachiopods are limited to carbonate lithologies which occasionally have minor amounts of siliciclastic sediment. These silicified brachiopods were extracted for taxonomic, taphonomic, and paleoecologic description. Chesterian brachiopod assemblages of the Cordilleran region are poorly described and this work represents the only systematic treatment of Chesterian macrofossil assemblages in Idaho. Fifteen genera (23 species) of brachiopods are presented from Arco Hills Formation, including two new species: Anthracospirifer arcoensis and Composita idahoensis, described herein.

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