COMPARATIVE TAPHONOMY OF TWO VERTEBRATE MICROFOSSIL BONEBEDS DERIVED FROM DIFFERENT HOST FACIES IN THE CAMPANIAN JUDITH RIVER FORMATION, NORTH-CENTRAL MONTANA

Two vertebrate microfossil bonebeds (microsites) from the Campanian Judith River Formation of Montana were compared to discern how fossil preservation varies in relation to facies context. One bone assemblage accumulated via attritional mortality in a low-energy lacustrine basin. Fossil bones from this locality are preserved in a tabular bed of mudstone that also yields abundant freshwater mollusks and carbonized plant debris. The other bone assemblage is preserved at the base of a thick sandstone body characterized by inclined heterolithic stratification and trough cross-bedding. This bonebed presumably formed when the ancient channel hosting the site reworked a preexisting concentration of fossil debris. Bone-bearing matrix from both sites was washed through sieves and sorted under microscopes, and all material in the sand-sized fraction and greater was recovered for analysis. Approximately 4.0 kg of matrix was processed from the lacustrine site: by weight, 2.8% is shell debris, 0.065% is plant debris, and 0.025% is fossil bone (=694 bones/bone fragments). Approximately 2.5 kg of matrix was processed from the fluvial site: by weight, 0.35% is plant debris and 0.67% is fossil bone (=3477 bones/bone fragments). Shell debris was not recovered, although impressions of bivalve shells and gastropod steinkerns are present. Focusing on the bone fraction, one notable distinction between sites is the greater relative abundance of bone in the fluvial site, which presumably reflects the concentration of bioclastic debris upon reworking. Another distinction between sites is the overall quality of the vertebrate material. Of the 694 bones recovered from the lacustrine site, 47% retain morphological features that make identification feasible. In contrast, only 18% of the fossil bones recovered from the fluvial site exhibit potentially diagnostic morphological features. Another significant distinction is the degree of polish. Only 3% of bone in the lacustrine site exhibits polish, while 75% of the bone debris recovered from the fluvial site exhibits polish. Actualistic experiments in a vibratory rock tumbler indicate that fossilized bone pebbles, such as those presumably reworked in the Judith River record, can take on polish within a few hours of interaction with fluvial sediment.