Exceptionally preserved asphaltic coprolites expand the spatiotemporal range of a North American paleoecological proxy

www.nature.com/scientificreports OPEN Exceptionally preserved asphaltic coprolites expand the spatiotemporal range of a North American paleoecological proxy Alexis M. Mychajliw1,2,3*, Karin A. Rice1, Laura R. Tewksbury1, John R. Southon4 & Emily L. Lindsey1 As fossilized feces, coprolites represent direct evidence of animal behavior captured in the fossil record. They encapsulate past ecological interactions between a consumer and its prey and, when they contain plant material, can also guide paleoenvironmental reconstructions. Here we describe the first coprolites from the lagerstätte Rancho La Brea (RLB) in Los Angeles, California, which also represent the first confirmed coprolites from an asphaltic (“tar pit”) context globally. Combining multiple lines of evidence, including radiocarbon dating, body size reconstructions, stable isotope analysis, scanning electron microscopy, and sediment analyses, we document hundreds of rodent coprolites found in association with plant material, and tentatively assign them to the woodrat genus Neotoma. Neotoma nests (i.e., middens) and their associated coprolites inform paleoclimatic reconstructions for the arid southwestern US but are not typically preserved in coastal areas due to environmental and physiological characteristics. The serendipitous activity of an asphalt seep preserved coprolites and their original cellulosic material for 50,000 years at RLB, yielding a snapshot of coastal California during Marine Isotope Stage 3. This discovery augments the proxies available at an already critical fossil locality and highlights the potential for more comprehensive paleoenvironmental analyses at other asphaltic localities globally. Coprolites are some of the most important ichnofossils that can be recovered from a diversity of taphonomic, ecological, and geologic contexts1. As trace fossils, coprolites represent windows into the evolution of ecological interactions such as predation, herbivory, and parasitism, and can contain paleoecological proxies spanning thousands to millions of years in the past2–4. In deep time, they are permineralized or lithified, constituting a cast or mold of the original fecal matter5. Conversely, coprolites containing their original biological material have been recovered from a taxonomically diverse range of Quaternary vertebrates including, for example, ground sloths6, mammoths7, crocodiles8, and moas9. The intact digested matter present in Quaternary coprolites can provide crucial evidence for testing ecological questions that may be otherwise difficult to address in the fossil record and that can be examined with techniques suitable for bridging fossil and modern samples (e.g., identification of plant macrofossils and pollen, ancient DNA, and biomarkers10). Despite these advantages, the preservation of Quaternary coprolites is biased to cold or arid climates, permanently wet sites, caves, rock shelters, and/or high elevation areas, thereby excluding many ecosystems, deposits, and taxonomic groups from this form of study. Rancho La Brea (RLB) is a famous Quaternary lagerstätte that has yielded >5 million specimens and is located in Los Angeles, California (34°03′48″N, 118°21′20″W). Studies leveraging RLB’s abundant and well-preserved vertebrate remains have defined our understanding of late Pleistocene carnivores, through studies of morphological change11, pathologies12, and trophic interactions13,14. Far less attention, however, has historically been focused on primary producers and the lower trophic levels of food webs, thus hindering the current use of the site for paleoenvironmental proxies despite its potential (though see recent entomological progress15). While known colloquially as the “La Brea Tar Pits”, the site actually comprises a series of asphaltic deposits containing 1 La Brea Tar Pits & Museum, 5801 Wilshire Blvd, Los Angeles, California, 90036, USA. 2Institute of Low Temperature Science, Hokkaido University, Sapporo, 060-0819, Japan. 3Laboratories of Molecular Anthropology and Microbiome Research, University of Oklahoma, Norman, OK, 73019, USA. 4Department of Earth System Science, UC Irvine, Keck CCAMS Group, Irvine, California, 92697, USA. *email: Scientific Reports | (2020) 10:5069 | https://doi.org/10.1038/s41598-020-61996-y 1 www.nature.com/scientificreports/ www.nature.com/scientificreports fossil material in a matrix of gravels, sands, silts, and clays—the “pit” shapes are the result of human excavations that subsequently refilled with liquid asphalt16. RLB sits above the Salt Lake Oilfield, and throughout the Late Quaternary, Miocene-aged hydrocarbons migrated upwards from the Puente Formation reservoir (Monterey Formation equivalent in the Los Angeles Basin17), resulting in periodically surficial liquid asphalt that infiltrated sediments, entrapped organisms, and preserved biological tissues including bone collagen, insect chitin, plant cellulose, and mollusk shells18. While RLB’s climatic and elevational qualities do not align with optimal coprolite recovery conditions given its low elevation, non-arid coastal setting that favors decomposition rather than desiccation, the biological material of coprolites could theoretically be preserved by rapid impregnation with liquid asphalt. The vast majority of catalogued RLB specimens originated from >90 deposits (“pits”) that were excavated with a near-exclusive focus on the bones of large mammals and birds in the early 1900s18. Reconstructions based on megafaunal remains consider the site to have operated as a “trap” in which carnivores were attracted to struggling herbivores stuck in pooling surficial asphalt, subsequently died, and were rapidly covered19,20. Extensive disarticulation of faunal elements and lack of stratigraphic integrity in RLB deposits have been cited to invoke churning of viscous, asphalt-saturated sediments and/or trampling by panicked animals as primary processes in the formation of RLB fossil deposits19,20, both of which could preclude the preservation of intact coprolites. Other models that incorporate geologic lines of evidence instead emphasize the roles of fluviatile agents and burial by alluvium from the Santa Monica Mountains; such a high-energy depositional environment could also be unfavorable for coprolite preservation16. A new opportunity to systematically excavate RLB deposits arose when, in 2006, the construction of an adjacent underground parking garage for the Los Angeles County Museum of Art (LACMA; Fig. 1A) unearthed 16 new fossiliferous asphaltic deposits. Entire blocks were exposed and pedestalled, and custom boxes were then built around deposits, allowing them to be removed and transplanted intact from their in situ position in 23 custom-built wooden boxes (“Project 23”). Project 23’s Box 1, weighing ~123,000 lbs and measuring 4 m wide x 5 m long x 2 m deep, was the first to be excavated, and has yielded >25,000 specimens21,22. Excavation efforts in Box 1 were focused primarily o (...truncated)