19 La Brea Tar Pits: Old Bones and New Insights

Wednesday, 13 May 2020
8:00 AM - 12:00 PM
San Rafael

A STONE’S THROW THROUGH TIME: TAPHONOMIC VARIATION AMONG PITS AT RANCHO LA BREA

Paper number: 19-6
Program: Theme Sessions
Time: 9:40 AM – 10:00 AM

Authors

Abstract

Rancho La Brea (RLB) is an asphaltic paleontological site that preserves millions of fossils of plants and animals spanning the last 50,000 years. RLB comprises more than 130 individual fossiliferous deposits that formed through a combination of active entrapment in sticky asphalt seeps and fluvial activity. Therefore, taphonomic analysis is essential to understanding changing ecological dynamics during the late Pleistocene megafaunal extinction at RLB. Until now, the taphonomic history of RLB has only been described from one of the oldest deposits – Pit 91 (45-26 ka) – leaving a gap in our knowledge of RLB through the interglacial transition into the Holocene. We quantified taphonomic variables for mammalian megafauna in four deposits – pits 91, 13, 3, and 61/67 – ranging in age from approximately 45–11 ka. Taphonomic variables included: minimum number of individuals, ontogeny, pit wear (a taphonomic feature caused by bone-on-bone contact within deposits), weathering, and abrasion. Pit wear frequencies varied among pits with Pit 13 having substantially more pit wear, which may be the result of deposit concentration. Pits 13, 3, and 61/67 exhibited similar distributions for both weathering and abrasion stages, in which the majority of specimens had minimal weathering and abrasion. In contrast, a greater percentage of specimens from Pit 91 were weathered and abraded, which may indicate slower burial and increased fluvial action, respectively. However, discrepancies between pits 91 and 13, 3, and 61/67 could be due to methodological differences. To test for this, we resampled Pit 91 taphonomic variables. Taphonomic trends from our Pit 91 dataset closely matched trends from pits 13, 3, and 61/67 – specifically, the majority of specimens had minimal weathering and abrasion. Close analysis of specimens with disparate weathering and abrasion scores suggests that published taphonomy scores were higher in specimens with postmortem damage. In our methodology, we ignored postmortem breaks when determining weathering and abrasion stages. Further, taphonomy scores on complete specimens were similar for weathering and abrasion in both datasets. In conclusion, employment of consistent methodology and taphonomic comparison of multiple deposits are imperative to understanding the transition into the Holocene at RLB.

Geological Society of America Abstracts with Programs. Vol. 52, No. 4

doi: 10.1130/abs/2020CD-347101

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