Coprolite

Coprolites are fossilized fecal matter expelled by an organism and preserved in the rock record. The term is commonly applied to vertebrate excretions, whereas 'fecal pellet' is often used for smaller invertebrate excretions (but see Knaust, 2020). Coprolites of various shapes and sizes are common to abundant in multiple layers of the Green River Formation (Grande, 1984; 2013).

This specimen exhibits a characteristic ropy morphology, interpreted as the feces of teleost fish (Edwards, 1976). The ropy coprolites lack bony or scaly remains and microfossils, and they consist mostly of the phosphatic mineral apatite (Edwards, 1976). Wells et al. (1993) found a strong positive correlation between the abundance of coprolites and the quality of fossil fish preservation.

Several authors have noted the soft nature and proneness to disaggregation of fish excretions (Edwards, 1976; Grande, 2013; Woolley, 2001), variously inferring rapid mineralization, lack of environmental disturbance, or rapid rates of burial as key factor in their unusual preservation. A distinct feature of these coprolites is their tridimensional nature, in contrast to associated fossil fish which are flattened and fully enclosed between two sub-millimetric laminae. As seen in this specimen, the coprolite is instead enveloped by multiple laminae, suggesting sediment accumulation while part of the coprolite remained emergent. This feature has potential implications for the rate of single-lamina deposition.

References

Grande, L., 1984. Paleontology of the Green River Formation, with a review of the fish fauna. Geological Survey of Wyoming, Bulletin 63, p. 333. (1st ed. 1980.)

Grande, L., 2013. The lost world of Fossil Lake: Snapshots from deep time. University of Chicago Press. p. 425.

Knaust, D., 2020. Invertebrate coprolites and cololites revised. Papers in Palaeontology, 6(3), pp. 385-423. doi:10.1002/spp2.1297.

Wells, N.A., Ferber, C.T. and Ohman, J.C., 1993. Discriminant analysis of fish-bearing deposits in the Eocene Green River Formation of Utah and Wyoming. Palaios, pp. 81-100.

Woolly, D.A., 2001. Fish preservation, fish coprolites and the Green River Formation. J. Creat., 15(1), pp. 105-111.