Homo erectus calvaria from Ngawi (Java) and its evolutionary implications

Ngawi 1 is an undated but well-preserved Homo erectus calvaria from Java. Previous craniometric and morphological studies have shown its similarities to late Javanese H. erectus from Ngandong as well as Sambungmacan (Sm 1 [and Sm 3]). Some researchers emphasize their morphological homogeneity, and suggest that this 'Ngandong/Sambungmacan/Ngawi group' is morphologically distinct from H. erectus from the Early Pleistocene of Sangiran and Trinil, possibly at a species-level. In this study, we reinvestigated Ngawi 1 based on the newly cleaned original specimen and using micro computed tomography with the aim of testing if such morphological discontinuity really exists within the Javanese fossil record. We metrically and non-metrically examined 33 cranial characters that are useful to distinguish earlier and later Javanese H. erectus. We also evaluated the morphology of the three Sambungmacan crania (Sm 1, 3, and 4) in the same way. The results of these and multivariate analyses support previous studies that Ngawi 1 exhibits many characteristic features of Ngandong H. erectus. However, Ngawi 1 is more or less similar to earlier Javanese H. erectus in smaller cranial size, a weak but distinct supraglabellar depression, a relatively short temporal bone, limited posterior projection of the middle part of the occipital torus, a shallower and 'roofed' mandibular fossa, and a smaller mastoid process. The three Sambungmacan crania also show general affinities to Ngandong, but are similar to earlier Javanese H. erectus in a few or more characters. Such slightly shifted character distribution is at least consistent with the hypothesis of continuous evolution of H. erectus through the Pleistocene of Java. This minor but potentially meaningful pattern of morphological variation should not be overlooked when a morphological group is defined for the specimens from Ngandong, Sambungmacan, and Ngawi. We also determined the endocranial volume of Ngawi 1 to be 959 cm(3).