Stomping Around the Sump: Miocene Pygmy Gomphothere from Esmeralda County, Nevada

摘要

Here we describe an individual gomphothere specimen recovered from Esmeralda County, Nevada. It was recovered from an area known as "The Sump," just 18 miles north of Dyer, NV, currently dominated by badland topography. Remnants of a paleo-forest are visible in the highlands of the site, while the central basin exposes mudrocks and sandstones, interfingered in places by several chalky, volcaniclastic layers. The sediments denote a past marsh environment, an ideal feeding ground for browsers of C_3 plant material, such as trees and shrubs. A slot canyon feeds the inactive channel where the discovery was made. Sedimentary units of the Esmeralda Formation in the vicinity of the north end of Fish Lake Valley have previously been interpreted as Miocene (Claredonian North American Land Mammal Age). The Gomphotheriidae were a family of proboscideans, widespread throughout the late Cenozoic era. Gomphotheres radiated throughout North America during the Miocene epoch, and diversified throughout the continent until their extinction in the late Pleistocene. Several species were participants in the Great American Biotic Interchange, but only three have been recorded from South America. Gomphotheres likely filled the niche of a dominant megafaunal browser, as their dentition and body morphology suggest that they were opportunistic feeders with a preference for C3 browse. The recovered specimen represents the maxillae, portions of the premaxillae, as well as the palatine, all in articulation. After removal of sediment with an air scribe, anterior and posterior sinus cavities were revealed. Two molars are present, and a portion of both tusks have been preserved within their cranial sockets. We attribute this specimen to Gomphotherium minor based upon the combination of small potential skull size with the extremely worn occlusal surface of the preserved molars of the individual. Modern elephants exhibit similar tooth wear towards the end of their lives, eventually succumbing to starvation. Mawby (1968) described Megabelodon minor (later synonymized with Gomphotherium by Tobien 1973) based on a tuskless mandible recovered from the same formation. Diagnostic measurements of the teeth, maxillae, and palate were taken and compared to the type specimen and other longirostrine proboscideans. We believe that the individual collected from the Sump is of the same species, but this is difficult to confirm due to our lack of a dentary present.