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Taphonomy and paleoecology of asphaltic Pleistocene vertebrate deposits of the western Neotropics


Asphaltic deposits, or "tar pits," present a unique opportunity to investigate the paleobiology and paleoecology of Quaternary mammals due to their tendency to accumulate and preserve remains of numerous taxa, along with associated materials that can aid in paleoenvironmental and chronological analyses. This role is especially important in areas with low preservation potential or incomplete sampling, such as the Neotropics.

Fossil deposits in the asphaltic sediments of the Santa Elena Peninsula in southwestern Ecuador contain some of the largest and best-preserved assemblages of Pleistocene megafaunal remains known from the neotropics, and thus represent an opportunity to greatly expand our knowledge of Pleistocene paleoecology and the extinction of Quaternary megafuana in this region. This dissertation reports data from excavations at Tanque Loma, a new late-Pleistocene locality on the Santa Elena Peninsula that preserves a dense assemblage of megafaunal remains in hydrocarbon-saturated sediments along with microfaunal and paleobotanical material.

Chapter 1 details the results of three years of excavations and associated sedimentological, stratigraphic, systematic, taphonomic, and chronological studies at Tanque Loma. Remains of extinct Pleistocene megafauna are encountered within and up to one meter above a laterally extensive asphalt-saturated sandstone layer along with abundant plant material. Several meters of presumed-Holocene sediments overlying the megafauna-bearing strata are rich in microvertebrate remains including birds, squamates, and rodents, most likely representing raptor assemblages. While over 1,000 megafaunal bones have been identified from the Pleistocene strata at Tanque Loma, more than 85% of these remains pertain to a single species, the giant ground sloth Eremotherium laurellardi. Only five other megafauna taxa have been identified from this site, including Glossotherium tropicorum, Holmesina occidentalis, cf. Notiomastodon platensis, Equus (Amerhippus) santaelenae, and a cervid tentatively assigned to cf. Odocoileus salinae based on body size and geography. No carnivores have yet been identified from Tanque Loma, and microvertebrate remains are extremely rare in the megafauna-bearing deposits, although terrestrial snail shells and fragmented remains of marine invertebrates are occasionally encountered. Accelerator Mass Spectrometry radiocarbon dates on Eremotherium and c.f. Notiomaston bones from within and just above the asphaltic layer yielded dates of around 17,000 - 23,500 radiocarbon years BP.

Taken together, the taxonomic composition, taphonomy, geologic context, and sedimentology of Tanque Loma suggest that this site represents a bone bed assemblage in a heavily vegetated, low-energy riparian environment with secondary infiltration of asphalt that helped preserve the bones.

The large accumulation of one taxon, Eremotherium laurillardi, at Tanque Loma offers a unique opportunity to investigate the ecology and behavior of this species. Chapter 2 uses data from this and other paleontological localities as well as modern African ecosystems to investigate the formation of the E. laurillardi assemblage at Tanque Loma and the behavioral ecology and life history of this species. Multiple lines of evidence, including a monodominant taxonomic composition; a multigenerational age structure with prime adult individuals well-represented; sediments suggestive of a low-energy anoxic aquatic environment; and the presence of abundant plant material that appears to pertain to coprolites of E. laurillardi; suggest that these sloths congregated and died in a protracted mass mortality event in a marshy riparian habitat. The evidence is consistent with a mass death due to drought and/or disease in a shallow watering hole, paralleling situations observed among large wallowing herbivores in Africa today. Furthermore, several neonate and fetal individuals are present in the deposit, suggesting that this species may have had a distinct breeding season, which is also common among large herbivores in seasonally dry tropical environments.

Chapter 3 endeavors to offer context for the Tanque Loma locality by combining data from these excavations with analyses of other asphaltic vertebrate localities in the region. The most well known asphaltic paleontological locality in tropical South America is the Talara tar seeps in northwest Peru, which has yielded a great diversity of microfossils as well as extinct megafauna. In addition, two other highly productive asphaltic localities have been excavated on the Santa Elena Peninsula -- the La Carolina locality excavated by Robert Hoffstetter in the 1940's, and the Coralito locality excavated by Franz Spillmann in the 1930's and A. Gordon Edmund in the 1960's. I examined fossils from these excavations currently housed in the collections of the Museo Gustavo Orces in Quito, Ecuador, the Royal Ontario Museum in Toronto, Canada, and the Museum National d'Histoire Naturelle in Paris, France, in order to compare the depositional and environmental contexts of these different sites and to investigate the paleoecology and biogeography of the mammal taxa preserved therein. In general, the communities of megaherbivores are comparable between these geographically close sites, but Talara and La Carolina present a much more diverse assemblage of birds, micromammals, and carnivores as compared with the other two localities. Taxonomic, geomorphological, and taphonomic data indicate that these two sites were most likely "tar pit" style traps analogous to the famous Rancho La Brea locality in California, USA, while the SEP sites Coralito and Tanque Loma likely represent fossil assemblages in marshy or estuarine settings with secondary infiltration of tar. In addition, geological and taxonomic differences between the nearby localities Coralito and Tanque Loma suggest differences in local paleoenvironments and lends further support for the hypothesis of gregarious behavior in at least two species of extinct giant ground sloths.

Finally, the radiocarbon dates so far obtained on extinct taxa at Tanque Loma and the other asphaltic localities examined here are consistent with a model positing earlier extinctions of megafauna in tropical South America than of related taxa further south on the continent, although this observed pattern may be an artifact of low sampling in the region.

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