English: Fossil bird footprint on grayish-green argillaceous lime mudstone from the Eocene of Utah, USA (rock is 5.2 cm across).

This remarkable fossil bird footprint comes from Utah's famous Soldier Summit Fossil Track Horizon. A persistent horizon of intensely bioturbated argillaceous lime mudstone occurs in the Eocene-aged Green River Formation near Soldier Summit (southern Wasatch County, north-central Utah, USA). This area was once the southwestern shore of ancient Lake Uinta. This print was made by a wading bird, probably something like a sandpiper (see Moussa, 1968).

Stratigraphy: Soldier Summit Fossil Track Horizon, Green River Formation, Middle Eocene

Locality: near Soldier Summit, Wasatch County, north-central Utah, USA

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Trace fossils are any indirect evidence of ancient life. They refer to features in rocks that do not represent parts of the body of a once-living organism. Traces include footprints, tracks, trails, burrows, borings, and bitemarks. Body fossils provide information about the morphology of ancient organisms, while trace fossils provide information about the behavior of ancient life forms. Interpreting trace fossils and determination of the identity of a trace maker can be straightforward (for example, a dinosaur footprint represents walking behavior) or not. Sediments that have trace fossils are said to be bioturbated. Burrowed textures in sedimentary rocks are referred to as bioturbation. Trace fossils have scientific names assigned to them, in the same style & manner as living organisms or body fossils.

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Reference: Moussa (1968) - Journal of Paleontology 42(6): 1433-1438.