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Microfossil bonebeds are concentrations of small bones mostly mm scale, with some cm scale material. They are frequently found in low-energy lacustrine deposits and as lags at the base of channel deposits. Previous studies have noted alteration of microfossil material in fluvial sites, but data concerning the origins of this alteration are limited. The purpose of this thesis was to 1) compare the amount of polish on microfossils from a channel-hosted site and lake/pond site in the Judith River Formation (JRF), and 2) attempt to replicate the polish with a rock tumbler containing bone in various stages of preservation and silts from the JRF.
For the comparative study, sediment from the fluvial site (UC-941) and lacustrine site [UC-8303) was collected and sieved in a custom oscillatory device that gently dunked sediment in water with minimal fossil breakage. The results of the comparative study show that significantly more fluvial microfossils are polished compared with lacustrine microfossils. Of the bones from the fluvial site, 78% were polished, versus 3% of bones from the lacustrine site. Additionally, only 14% of bones from UC-941 retained identifying features, as opposed to 47% of bones from UC-8303.
In the experimental study, bones ranging from 2-7 mm in size were selected from three sites. Recrystallized bone was taken from UC-8303, a fossil locality in the Judith River Formation; dry bone stripped of collagen was selected from Homestead Cave, Utah (a 13,000 year old site); and fresh bone was taken from a deer metatarsal recovered form the Upper Missouri River Breaks National Monument, Montana. The bones were run in a vibratory rock tumbler for 72 hours, and periodically removed to be photographed. SEM images were taken at the beginning and end of the experiment. The results show that collagen-free bone became polished most quickly, in an average of 3 hours, followed by fossil bone, which required an average of 6 hours. The fresh bone polished most slowly, after an average of 14 hours of tumbling.
The comparative results show polish is a feature found mainly in fluvial microfossil bonebeds, suggesting that polish is imparted by interaction with channel sediments. Contrary to previous studies that speculated that polish was acquired through predation and digestion, it appears that abrasion is also a major source of polish. The fast rate of polishing in the experiment demonstrates that a relatively short period of time is required to polish small vertebrate microfossils. Microfossils could exist in an active channel environment for as little as hours to days and become polished. This is consistent with previous studies that have suggested that fluvial microfossils are sourced by lacustrine sites and undergo minimal transport before final burial. Surface alteration appears to occur on relatively short timescales, and while bone could exist at the base of a channel for longer periods, this is not required for abrasion and polish to occur.
Ratigan, Deirdre, "Using Experimental Taphonomy to Replicate Bone Alteration in the Judith River Formation (Upper Cretaceous, Montana)" (2011). Geology Honors Projects. Paper 10.
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