Sterkfontein variability

In a 2002 paper on cranial remains from Sterkfontein, Lockwood and Tobias write the following in a section called "Are there multiple hominin species from Sterkfontein Member 4?":

The Group C specimens (Stw 183 and Stw 255) arguably represent a phenon, as they deviate from the A. africanus sample in the same direction. Each exhibits some derived characters of A. aethiopicus, A. robustus, and/or A. boisei. Moreover, based on dental size and morphology, Stw 252 probably belongs in this group (Clarke, 1988).
Stw 183 is the strongest craniofacial evidence for a second species of Member 4, though it is not by itself definitive (Lockwood, 1997). Stw 183 is an immature individual, and a full interpretation of this specimen relies on a comparative framework of early hominin ontogeny that is at present incomplete. Despite its youth, it possesses characters typically found in A. robustus, such as an incipient maxillary trigon and a rounded lateral portion of the inferior orbital margin (not present in any other specimen from Sterkfontein Member 4).
The temporal bones catalogued at Stw 255 (including Stw 266a) resemble A. africanus in essentially one autapomorphic character: the prominent eustachian process. Otherwise, this individual shows traits corresponding to A. boisei, especially in the relationship of the tympanic to the postglenoid and mastoid processes. On the whole, Stw 255 suggests the appearance of the temporal bone in KNM-WT 17000 of A. aethiopicus. Moreover, Spoor (1993) showed that Stw 255 has a combination of features regarding the orientation of the posterior petrosal surface that may correspond to the external anatomy of KNM-WT 17000: an unflexed cranial base combined with a petrous axis that is relatively coronally oriented in the transverse plane. Fossils catalogued as Stw 255 may also be associated with the various specimens that make up Stw 252, but this is uncertain, as a second individual (Stw 265) of similar preservation was found in close proximity to Stw 252 (Lockwood and Tobias 2002:446-447, citations in original).

They go on to say they do not think that these specimens are sufficient evidence that another species was present, and they note details of a few other fragments that are different from the sample as a whole. They differ from Clarke (e.g. 1988), who would have divided the relatively complete cranial specimens into two samples.

Some other workers have suggested that individual specimens from Sterkfontein Member 4 might represent other species besides A. africanus. Kimbel and Rak (1993) proposed that Sts 19 probably represents Homo, and that the inclusion of the specimen into A. africanus inflates the variation within that species. This proposition was tested by Ahern (1998), who found the Sterkfontein Member 4 specimens to be quite variable without Sts 19 also. In other words, this site preserves a variable sample -- especially considering nonmetric traits observed on individual specimens.

The traits of Stw 183 and Stw 255 may fall in that category of variation, and Lockwood and Tobias (2002) suggest that possibility. I think the specimens are interesting because of whose traits they share. Studying A. africanus not as a branch of a cladogram, but as a real species with possible ancestors and descendants, the occasional presence of characters of earlier and later species is to be expected. The question is whether these characters document an ancestor-descendant relation for A. africanus and the robust taxa, or whether they might be shared by collateral taxa by virtue of common ancestry alone. The half-million years preceding the origins of Homo may have been just as interesting for the study of populations as the last half-million years.

References:

Ahern JCM. 1998. Underestimating intraspecific variation: the problem with excluding Sts 19 from Australopithecus africanus. Am J Phys Anthropol 105:461-480.

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Clarke RJ. 1988. A new Australopithecus cranium from Sterkfontein and its bearing on the ancestry of Paranthropus. In (Grine FE, ed) Evolutionary history of the robust australopithecines. Aldine de Gruyter, New York. p. 285-292.

Kimbel WH and Rak Y. 1993. The importance of species taxa in paleoanthropology and an argument for the phylogenetic concept of the species category. In (Kimbel WH and Martin LB, eds) Species, species concepts, and primate evolution. Plenum Press, New York. p. 461-484.

Lockwood CA and Tobias PV. 2005. Morphology and affinities of new hominin cranial remains from Member 4 of the Sterkfontein Formation, Gauteng Province, South Africa. J Hum Evol 42:389-450.