An abundance of fossil material traditionally identified as coprolites (fossilized feces) has been found in the Miocene of Washington State for many years. Examination of these coprolites by Dolf Seilacher (the pre-eminent living paleontologist on Earth) and others has resulted in a reassessment of their identification.
“Washington coprolites” are now identified as cololites (intestinal casts). The evidence & rationale behind this identification are summarized in Seilacher et al. (2001 - Paleobiology 27(1): 7-13). These structures are composed of siderite (FeCO3 - iron carbonate), and generally have yellowish-brown coatings of limonite (FeO·OH·nH2O - hydrous iron hydroxy-oxide).
Cololite (9.6 cm across) from the Miocene Wilkes Formation of Washington State, USA. Note the longitudinal groove incised along the cololite - this is the impression of the taenia coli, a ribbon of smooth muscle that occurs along one side of the large intestine (not a feature of a coprolite!).
Ordinarily, the preservation potential of intestinal casts of vertebrates seems fairly low. Strangely, no vertebrate fossil bone or teeth remains occur in the Wilkes Formation cololite-bearing beds. Bones & teeth are normally expected to have a fairly high preservation potential.
The paradoxical situation of abundant cololites and no bones & teeth was explained by Seilacher et al. (2001): the cololites were sideritized by bacterial activity and diagenesis involving moving groundwater fronts that favored dissolution of phosphatic material (bones, teeth) & precipitation of iron carbonate.
Stratigraphy & Locality: exposures of fine-grained, tuffaceous fluvial or lacustrine sedimentary rocks of the Wilkes Formation (Upper Miocene); Salmon Creek area, southwestern Lewis County, southwestern Washington State, USA.
Notice the morphological consistency in the Wilkes Formation cololite specimens shown below.
Cololites (larger specimen is 3.4 cm tall)
Cololites (larger specimen is 4.5 cm tall)
Cololites (largest specimen is 4.7 cm tall)
Cololites (5.5 cm tall)
Cololites (larger specimen is 5.5 cm tall)
Cololites (larger specimen is 5.8 cm tall)
Cololite (4.9 cm across)