COLOLITES
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)