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Abstract

The Middle Cambrian Gros Ventre Formation (north-central Wyoming, USA) contains a lagerstätte of Rusophycus (arthropod ichnofossils) preserved in convex hyporelief on the base of sandstone beds or as sand lenses within beds of silty mudstone. Rusophycus was produced by typical trilobite behavior but was preserved only under the particular confluence of conditions that were common during the deposition of the Gros Ventre Formation. These conditions include the background deposition of firm, cohesive muds bound by bacterial mats, which allowed the burrows to maintain their shape until cast by episodic sand deposition. The absence of spreiten and presence of storm event bedding within sectioned Rusophycus specimens support this scenario and contradict previous assertions that Rusophycus was formed within the substrate. Chondrites were also present, extending its range into the Middle Cambrian. Bacterial mats and microbially induced sedimentary structures were present before, during, and after the deposition of the Gros Ventre Formation. Bioturbation indices are comparable to other Middle Cambrian sites, supporting a gradual increase of bioturbation intensity during the Agronomic and Cambrian Substrate Revolutions.

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