The “coprolites” that aren't: The straight poop on specimens from the Miocene of southwestern Washington State

Abstract

Coprolite‐like siderite masses from paludal deposits of the Miocene‐Pliocene(?) Wilkes Formation near Toledo, southwestern Washington, have been studied since 1939. Over the years, numerous fossil supply houses have also sold them as vertebrate coprolites. The origin of these curious objects has been attributed to formation as speleo‐thems, pseudomorphs of bryozoa, and as feces of vertebrates, including reptiles, fish, and mammals. No vertebrate fossils have been recovered from the Wilkes Formation, although plant fossils and other organic debris are common locally. Evidence cited in support of a fecal origin include the extruded shape, the variation in shape corresponding to viscosity variations, the limited length or quantity of material, and anal extrusion striations (Amstutz, 1958). A new hypothesis for the origin of these so‐called coprolites is proposed, relying on organic‐rich sediment, but inorganic processes. In this scenario, organic‐rich silt and clay was forcefully injected into hollow wood fragments and through knotholes, creating the characteristic features formerly cited as conclusive evidence for the fecal origin of these objects. Diagenetic processes then replaced the organic sediment protected within the wood fragments with siderite.

Key words:

• coprolites
• pseudofossils
• concretions
• Wilkes Formation
• Miocene
• southwestern Washington