A New Assemblage of Mid-Pliocene Proboscideans from the Woranso-Mille Area, Afar Region, Ethiopia: Taxonomic, Evolutionary, and Paleoecological Considerations
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- Sanders, W.J. & Haile-Selassie, Y. J Mammal Evol (2012) 19: 105. doi:10.1007/s10914-011-9181-y
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Recent fieldwork at Woranso-Mille, Ethiopia led to the recovery of an abundant, diverse mammalian fauna that includes remains of the early hominid Australopithecus afarensis. Proboscideans are among the taxa well sampled at the site, primarily by gnathodental specimens, dated to the mid-Pliocene interval of 3.8–3.6 Ma. These fossils document traces of the last anancine gomphotheres (Anancus ultimus) in eastern Africa and several elephant taxa. Comparative study of the elephant fossils indicates the presence of cf. Mammuthus sp. “Hadar-type,” cf. Elephas ekorensis, E. recki brumpti, and cf. Loxodonta adaurora adaurora. Proboscidean evolution in the mid-Pliocene is interesting because during this time archaic elephants were completely replaced by basal members of crown elephant lineages, taxonomic diversity was high (multiple elephant species, anancine gomphotheres, stegodonts, and deinotheres), and elephants were undergoing substantial reorganization of the craniodental masticatory apparatus, presumably in response to the spread of more open habitats and greater competition for grazing resources. The Woranso-Mille sample is important because this interval is only represented elsewhere in eastern Africa by a small number of sites, and because adaptive diversification among early crown elephants requires greater clarification. Morphometric contrasts among the fossil dentition from Woranso-Mille presage the differential success of elephant lineages in eastern Africa during the Pleistocene, providing hints about the beginnings of competitive displacement. Differences between E. recki brumpti from Woranso-Mille and the slightly younger Sidi Hakoma Member of the Hadar Formation reveal the beginnings of continuous, directional morphometric change that characterized the lineage. Reconsideration of E. recki subspecies indicates that they are arbitrary lineage divisions tied to geochronological boundaries (with utility for biochronological correlation at well sampled sites) rather than real phylogenetic entities, but does not reject monophyly or anagenetic evolution of the lineage.