File:Vertebrate cololites (lower sigmoid colon & rectum) (Wilkes Formation, Upper Miocene; Salmon Creek area, Lewis County, southwestern Washington State, USA) 1 (15440175116).jpg

Vertebratea cololites (larger specimen is 3.4 cm tall) from the Miocene of Washington State, USA.

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 the famous paleontologist Dolf Seilacher 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).

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.

Many Wilkes Formation cololite specimens are morphologically consistent, especially those that represent the lower sigmoid colon and rectum (see above photo). Some specimens have a longitudinal groove incised along the cololite, which is an 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!).

Stratigraphy: fine-grained, tuffaceous fluvial or lacustrine sedimentary rocks of the Wilkes Formation, Upper Miocene