Type Geological formation Other sandstone Primary Mudstone Underlies Hanna Formation | Thickness 600-2,000 m Region North America Overlies Medicine Bow Formation | |
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The Ferris Formation is a Late Cretaceous (~66 Ma) to Paleocene (66-63 Ma), fluvial-deltaic geological formation in southern Wyoming. It contains a variety of trace and body fossils, preserved in sandy fluvial channel deposits and overbank units. Dinosaur remains are fragmentary, but include Triceratops, Tyrannosaurus, dromaeosaurids, Paronychodon, Ricardoestesia, Edmontosaurus, Edmontonia, Ankylosaurus, and Pachycephalosaurus.
Some of the fluvial channels contain evidence of tidal influence and brackish water, in the form of tidal facies, shark teeth, and shrimp burrows. This demonstrates that the western shoreline of the Western Interior Sea was still within a few hundred kilometers even during the latest Cretaceous. The local K-T boundary is contained within a sandy channel deposit made up of stacked bars. Reworked Cretaceous fossils are preserved at the base of the channel complex, associated with mud rip-up clasts, and Paleocene mammal fossils are preserved in the upper portion of the bar.
The Ferris Formation is up to 2,000 m thick in the Hanna Basin and represents rapid accumulation of predominantly fine-grained sediment on a broad delta. The delta previously fed the deepwater Lewis Shale and shallow marine Fox Hills Formation. The Ferris followed behind as a system of lagoons, bays, and delta plain environments.
Vertebrate paleofauna
In addition to a variety of dinosaurs, the following taxa are known from the Ferris Formation:
Myledaphus,
Lissodus,
Cretorectolobus,
Phylodus,
Amia,
Lepisosteus,
Basilemys,
Adocus,
Leptochamops,
Brachychampsa,
and a variety of Cretaceous and Paleocene multituberculates, marsupial, and placentals.