PATTERNS OF MICROBIAL BIOEROSION IN BONES FROM THE CAMPANIAN JUDITH RIVER FORMATION OF MONTANA

Microbial activity in bone impacts the preservation of bone microstructure and contributes to post-mortem degradation. Common agents of bioerosion include fungus, cyanobacteria, and bacteria, and each leaves a distinctive trace: fungi and cyanobacteria impart branched tunnels, while bacteria erode complex focal chambers. These microscopic taphonomic features are not well studied in terrestrial systems, though they hold potential to provide insights into the ecology of fossil microorganisms and their impact on bone preservation. We analyzed 168 bones from bonebeds in three distinct facies of the Campanian Judith River Formation of Montana: (1) mudstones deposited in lacustrine paleoenvironments, (2) sandstones deposited in ancient fluvial channels, and (3) sandstones deposited in the marine shoreface. We classified our findings into two categories of bioerosion: Wedl-type branching tunnels and non-Wedl microscopical focal destruction (MFD). Microscopic bioerosion was evident in 84 bones. Wedl-type branching tunnels occur exclusively along the periosteal margins of bones, while non-Wedl MFD commonly occurs around vascular canals and on periosteal margins. In bones from fluvial sandstones (n=28), 79% exhibit evidence of bioerosion. Of these, 43% exhibit non-Wedl MFD, 32% exhibit Wedl-type tunnels, and 14% exhibit both types of bioerosion. In bones from terrestrial mudstones (n=98), 54% exhibit evidence of bioerosion, with 14% characterized by non-Wedl MFD, 32% characterized by Wedl-type tunnels, and ~1% exhibiting both. In bones from shoreface sandstones (n=42), 33% exhibit evidence of bioerosion. Of these, 2% exhibit non-Wedl MFD, 21% exhibit Wedl-type tunnels, and 10% exhibit both types of bioerosion. Our results indicate that Wedl-tunneling dominates in bones recovered from lacustrine mudstones and shoreface sandstones, and that both types of bioerosion occur in roughly equal proportions in bones from fluvial sandstones. The two types of bioerosion co-occur relatively infrequently in the facies we studied. Significantly, our results highlight the need for more detailed study of bioerosion in terrestrial settings, and suggest that microscopic bioerosion may be as characteristic there as in marine depositional environments, where most previous studies have been focused.