Mojokerto site rediscovered?

An upcoming paper in Journal of Human Evolution by O. F. Huffman and colleagues reports on a possible location for the Mojokerto skull. A 1994 paper by Carl Swisher and colleagues dated rock from the supposed site to 1.81 million years ago.

This paper finds that the real site is a bit above that dated horizon. The 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 40Ar39Ar 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.

The paper gives a good history of attempts to find the original excavation site. An interesting heterogeneity of the matrix fill inside the skull (assessed by CT) also factors in the story.

After a long discussion of the complexities in dating the site, they conclude:

In summary, additional field and analytical results are needed to date the Mojokerto fossil more exactly than latest Pliocene or early-mid Pleistocene in age. The 0.3 Ma difference between the 40Ar/39Ar and fission-track age determinations must be resolved. For any of these radioisotopic dates to be considered other than a maximum age, better evidence must be advanced to show that the dated material was erupted shortly before deposition at Perning. Additional paleontological and magnetostratigraphic control and radioisotopic dating would seem to be required. Geochronological conclusions have to be evaluated further in terms of the potential for temporal stratigraphic breaks in the section, rates of deposition, and the regional stratigraphic (including sequence stratigraphic) context.

I don't suppose there will ever be a very good date for the specimen. But it's impressive the amount of work there has been on it in the past several years.

References:

Huffman OF, Zaim Y, Kappelman J, Ruez DR Jr, de Vos J, Rizal Y, Aziz F, Hertler C. 2006. Relocation of the 1936 Mojokerto skull discovery site near Perning, East Java. J Hum Evol (in press). DOI

Swisher CC 3rd, Curtis GH, Jacob T, Getty AG, Suprijo A, Widiasmoro. 1994. Age of the earliest known hominids in Java, Indonesia. Science 263:1118-1121. PubMed