Tanque Loma, a new late-Pleistocene megafaunal tar seep locality from southwest Ecuador

Fossil deposits in the petroleum-rich 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 megafauna in this region. This paper reports data from excavations at Tanque Loma, a 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. The megafauna bones are concentrated in and just above a ∼0.5 m thick asphaltic layer, but occur sparsely and with poorer preservation up to 1 m above this deposit. Several meters of presumed-Holocene sediments overlying the megafauna-bearing strata are rich in bones of microvertebrates including birds, squamates, and rodents. These are interpreted as raptor assemblages. While over 1000 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 laurillardi. Only five other megafauna taxa have been identified from this site, including Glossotherium cf. tropicorum, Holmesina occidentalis, cf. Notiomastodon platensis, Equus (Amerhippus) c.f. santaeelenae, 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 Pleistocene deposits, although terrestrial snail shells and fragmented remains of marine invertebrates are occasionally encountered. Accelerator Mass Spectrometry radiocarbon dates on Eremotherium and cf. Notiomaston bones from within and just above the asphaltic layer yielded dates of ∼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 to preserve the bones. The predominance of Eremotherium fossils at this site indicate that it may have been an area where these animals congregated, suggesting possible gregarious behavior in this taxon. The radiocarbon dates so far obtained on extinct taxa at Tanque Loma 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 pattern may be an artifact of low sampling in the region.