Relocation of the 1936 Mojokerto skull discovery site near Perning, East Java

Abstract

The fossil calvaria known as the Mojokerto child's skull was discovered in 1936, but uncertainties have persisted about its paleoenvironmental context and geological age because of difficulties in relocating the discovery site. Past relocation efforts were hindered by inaccuracies in old base maps, intensive post-1930s agricultural terracing, and new tree and brush growth. Fortunately geologic cross sections and site photographs from 1936-1938—not fully utilized in past relocation fieldwork—closely circumscribe site geography and geology. These documents match the conditions at just one sandstone outcrop. It is situated on the southern margin of a topographic nose at the upper end of a ∼18 m-wide gully (∼0663760 m E, 9183430 m N, UTM Zone 49M), ∼15 m southeast of the Kumai et al. (1985) relocation. The relocated discovery bed is ∼3.3 m of fossiliferous pebbly sandstone, a river-channel deposit cut into tuffaceous mudstone. The sandstone and mudstone beds correspond to original site descriptions. Pebbly sandstone is also found within the skull. The calvaria is well-preserved and taphonomically similar to large and fragile specimens found among several hundred vertebrate fossils excavated from the sandstone in 2001-2002. Since no well-preserved fossils were found intact at the surface of the sandstone, the good condition of the Mojokerto skull suggests that it was buried fully when discovered. The relocated hominin bed is the uppermost fluvial sandstone of a marine-deltaic sequence in the upper Pucangan Formation. The Mojokerto child probably died along the ancient seacoast, judging from the large extent of the deltaic facies and evidence that the calvaria experienced minimal transport. The relocated discovery bed is ∼20 m stratigraphically above the horizon from which the widely cited 1.81 ± 0.04 Ma 40Ar/ 39Ar date for the skull (Swisher et al., 1994, Science 263, 1118) was obtained. Additional field and laboratory results will be required to determine the skull's age.