LOBSTER–BEARING CONCRETION FORMATION IN THE BEARPAW SHALE (LATE CRETACEOUS) OF MONTANA

Concretions collected from the Late Cretaceous Bearpaw Shale near Fort Peck, northeastern Montana, have been known to bear well-preserved specimens of the nephropid lobster Palaeonephrops browni (Whitfield, 1907) since the site was first collected by Barnum Brown in 1906. Based upon megascopic examination, Feldmann et al. (1977) described them as argillaceous limestone concretions. Six additional concretions form the basis for a more detailed taphonomic study. Five of the six concretions have not been prepared to expose the lobsters. They were destructively sampled and were studied using polished surfaces, acetate peels, thin sections, and SEM techniques. Composition of the concretions was determined by employing XRD, XRF, and EDS techniques on various parts of the concretions and on insoluble residue samples of sediment within and surrounding the lobsters. The concretions were confirmed to be calcareous, rather than phosphatic as described by Tsujita (2003) from concretions in the Bearpaw Shale in Alberta, Canada. The lobster cuticle was mostly preserved in exquisite detail by phosphatization and was surrounded in some of the specimens by an aureole of pyrite. The concretion itself was calcitic containing only about 20% insoluble material, mostly quartz and illite. Worm burrows and fecal pellets within and around the lobsters documented postmortem and postburial scavenging. It is hypothesized that the phosphatization of the cuticle was a localized phenomenon possibly in an anoxic, low pH microenvironment as the corpse was enveloped by a microbial film. Experimental and field observations on remarkable preservation by D. E. G. Briggs and others suggests that the preservation of the lobsters may have occurred within a few weeks of death. The remainder of the concretion formed in an aerated, higher pH environment. No significant chemical differentiation was observed throughout the concretions, except for some peaks in phosphorous due to fecal pellets and the lobster cuticle. Whether or not the concretions formed within a burrow system, as postulated by Tsujita (2003), could not be determined. The work was supported in part by NSF grant EF0531670 to Feldmann and Schweitzer.