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Chronology and spatial distribution of large mammal bones in PIT 91, Rancho La Brea


The Rancho La Brea tar pits represent a collection of Pleistocene fossils from an unusual sedimentary environment. A taphonomic analysis of a single tar seep, Pit 91, reveals a complex history of deposition and diagenesis for specimens found there. Radiometric dating of 46 bones from Pit 91 documents at least two episodes of deposition, one from 45,000 to 35,000 yr and another, shorter interval from 26,500 to 23,000 yr. Interestingly, the law of superposition was not upheld consistently in this case study, as some younger bones were found at a greater depth than older bones, implying that taphonomic time averaging took place. Bones are distributed as disarticulated elements in two large concentrations that span both depositional episodes. In general, long bones are oriented horizontally, with little or no preference for cardinal orientation. Degree of weathering or abrasion is not correlated with depth. Bone-on-bone contact (pit wear), however, increases with depth, suggesting possible compaction of bones through time. These results, combined with the disarticulation common to nearly all recovered specimens, suggest a postentrapment journey for the bones unique to asphalt deposits.

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