Taxonomic assignments for the 3.4 Ma to 1.1 Ma hominin postcanine teeth from the Usno Formation and the Shungura Formation, Lower Omo Valley, Ethiopia

Sediments of the Usno Formation and the Shungura Formation date from ca. 3.75 Ma to 1.09 Ma, during which time the genus Homo appeared, Australopithecus disappeared, and the eastern African robust hominins did both. We performed an in-depth analysis of size, shape, and morphology of 108 Omo permanent postcanine dental specimens. First, we compared linear dimensions against a comparative dataset of 809 teeth from 359 specimens representing 10 hominin taxa. We then developed a linear discriminant function to predict the most-likely taxon for the Omo specimens based on the crown dimensions. We also ran cluster analyses on morphological scores and 2-dimensional cusp areas from the Omo sample to test whether these statistical clusters align with the taxa predicted by the discriminant function analysis. We found that variation in morphological scores does not align well with the taxonomic groups predicted from linear data. Cluster analyses of mandibular molar cusp areas categorize robust and non-robust groups fairly well, but maxillary molar cusp areas do not. We conclude that only Au. afarensis is found in the earliest part of the stratigraphy (3.4 Ma). By 2.95 Ma, there are two hominin lineages, Paranthropus and a more dentally gracile group in which Au. afarensis and Homo are not distinguishable until higher in the stratigraphic sequence. Paranthropus increased in frequency from 2.27 Ma until 1.9 Ma. After this time, Homo is the dominant taxon. The geologically youngest evidence of Paranthropus occurs at 1.37 Ma, after which, only teeth attributable to Homo are observed.