Large-diameter (2–10 cm) crustacean burrows preserved in the latest Cretaceous and early Paleocene estuarine barforms of southern Wyoming's Ferris Formation exhibit simple, nonbranching architecture, elliptical cross sections, J- and U-shaped shaft morphologies, and walking-leg impressions along the burrow walls. The burrows are classified as Psilonichnus isp. based on their morphology and environmental occurrence. In the Ferris Formation, Psilonichnus is found within multistory channel deposits and is associated with lithofacies that indicate strong tidal influence. Thalassinoides is more abundant in single-story channels with shallower flow depths, and Camborygma is found in alluvial deposits of the overlying Paleocene Hanna Formation. The presence of Psilonichnus in estuarine barforms of the Ferris Formation supports reconstructions of the Western Interior Sea showing a much wider distribution during the Cretaceous–Paleogene transition than has normally been accepted.

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