Silberberg Grotto Research Articles

The pectoral girdle of StW 573 (‘Little Foot’) and its implications for shoulder evolution in the Hominina

The ca. 3.67 Ma adult skeleton known as ‘Little Foot’ (StW 573), recovered from Sterkfontein Member 2 breccia in the Silberberg Grotto, is remarkable for its morphology and completeness. Preservation of clavicles and scapulae, including essentially complete right-side elements, offers opportunities to assess morphological and functional aspects of a nearly complete Australopithecus pectoral girdle. Here we describe the StW 573 pectoral girdle and offer quantitative comparisons to those of extant hominoids and selected homininans. The StW 573 pectoral girdle combines features intermediate between those of humans and other apes: a long and curved clavicle, suggesting a relatively dorsally positioned scapula; an enlarged and uniquely proportioned supraspinous fossa; a relatively cranially oriented glenoid fossa; and ape-like reinforcement of the axillary margin by a stout ventral bar. StW 573 scapulae are as follows: smaller than those of some homininans (i.e., KSD-VP-1/1 and KNM-ER 47000A), larger than others (i.e., A.L. 288-1, Sts 7, and MH2), and most similar in size to another australopith from Sterkfontein, StW 431. Moreover, StW 573 and StW 431 exhibit similar structural features along their axillary margins and inferior angles. As the StW 573 pectoral girdle (e.g., scapular configuration) has a greater affinity to that of apes—Gorilla in particular—rather than modern humans, we suggest that the StW 573 morphological pattern appears to reflect adaptations to arboreal behaviors, especially those with the hand positioned above the head, more than human-like manipulatory capabilities. When compared with less complete pectoral girdles from middle/late Miocene apes and that of the penecontemporaneous KSD-VP-1/1 (Australopithecus afarensis), and mindful of consensus views on the adaptiveness of arboreal positional behaviors soliciting abducted glenohumeral joints in early Pliocene taxa, we propose that the StW 573 pectoral girdle is a reasonable model for hypothesizing pectoral girdle configuration of the crown hominin last common ancestor.

The skull of StW 573, a 3.67 Ma Australopithecus prometheus skeleton from Sterkfontein Caves, South Africa

Here we present the first full anatomical description of the 3.67 million-year-old Australopithecus skull StW 573 that was recovered with its skeleton from the Sterkfontein Member 2 breccia in the Silberberg Grotto. Analysis demonstrates that it is most similar in multiple key morphological characters to a group of fossils from Sterkfontein Member 4 and Makapansgat that are here distinguished taxonomically as Australopithecus prometheus. This taxon contrasts with another group of fossils from those sites assigned to Australopithecus africanus. The anatomical reasons for why these groupings should not be lumped together (as is frequently done for the South African fossils) are discussed in detail. In support of this taxonomy, we also present for the first time a newly reconstructed palate of A. africanus (StW 576 from Sterkfontein Member 4), which has a uniquely complete permanent dentition. The StW 573 skull also has certain similarities with other earlier Australopithecus fossils in East Africa, assigned to Australopithecus afarensis and Australopithecus anamensis, which are discussed. One of its most interesting features is a pattern of very heavy anterior dental wear unlike that found in A. africanus but resembling that found in A. anamensis at 4.17 Ma. Because the StW 573 skull is associated with a near-complete skeleton that is also described for the first time in this special issue, we are now able to use this individual to improve our understanding of more fragmentary finds in the South African fossil record of Australopithecus.

A multiscale stratigraphic investigation of the context of StW 573 ‘Little Foot’ and Member 2, Sterkfontein Caves, South Africa

The Sterkfontein Caves is currently the world's richest Australopithecus-bearing site. Included in Sterkfontein's hominin assemblage is StW 573 (‘Little Foot’), a near-complete Australopithecus skeleton discovered in Member 2 in the Silberberg Grotto. Because of its importance to the fossil hominin record, the geological age of StW 573 has been the subject of significant debate. Three main hypotheses have been proposed regarding the formation and age of Member 2 and by association StW 573. The first proposes that Member 2 (as originally defined in the type section in the Silberberg Grotto) started to accumulate at around 2.58 Ma and that the unit is contained within the Silberberg Grotto. The second proposes that Member 2 started forming before 3.67 ± 0.16 Ma and that the deposit extends into the Milner Hall and close to the base of the cave system. The third proposes a ‘two-stage burial scenario’, in which some sediments and StW 573 represent a secondary and mixed-age accumulation reworked from a higher cave. The stratigraphic and sedimentological implications of these hypotheses are tested here through the application of a multiscale investigation of Member 2, with reference to the taphonomy of the StW 573 skeleton. The complete infilling sequence of Member 2 is described across all exposures of the deposit in the Silberberg Grotto and into the Milner Hall. Sediments are generally stratified and conformably deposited in a sequence of silty sands eroded from well-developed lateritic soils on the landscape surface. Voids, clasts and bioclasts are organized consistently across and through Member 2 conforming with the underlying deposit geometry, indicating gradual deposit accretion with no distinct collapse facies evident and only localized intra-unit postdepositional modification. The stratigraphy and sedimentology of Member 2 support a simple single-stage accumulation process of Member 2 and a primary association between the sediments of Member 2 and the StW 573 ‘Little Foot’ skeleton.

The age of fossil StW573 (‘Little Foot’): An alternative interpretation of 26Al/10Be burial data


 
 
 Following the publication (Granger DE et al., Nature 2015;522:85–88) of an 26Al/10Be burial isochron age of 3.67±0.16 Ma for the sediments encasing hominin fossil StW573 (‘Little Foot’), we consider data on chert samples presented in that publication to explore alternative age interpretations. 10Be and 26Al concentrations determined on individual chert fragments within the sediments were calculated back in time, and data from one of these fragments point to a maximum age of 2.8 Ma for the sediment package and therefore also for the fossil. An alternative hypothesis is explored, which involves re-deposition and mixing of sediment that had previously collected over time in an upper chamber, which has since been eroded. We show that it is possible for such a scenario to yield ultimately an isochron indicating an apparent age much older than the depositional age of the sediments around the fossil. A possible scenario for deposition of StW573 in Member 2 would involve the formation of an opening between the Silberberg Grotto and an upper chamber. Not only could such an opening have acted as a death trap, but it could also have disturbed the sedimentological balance in the cave, allowing unconsolidated sediment to be washed into the Silberberg Grotto. This two-staged burial model would thus allow a younger age for the fossil, consistent with the sedimentology of the deposit. This alternative age is also not in contradiction to available faunal and palaeomagnetic data.
 
 
 
 
 Significance: 
 
 
 
 Data on chert samples taken close to StW573 impose a maximum age for the fossil of 2.8 Ma – younger than the 3.67 Ma originally reported. We propose and explore a two-stage burial scenario to resolve the inconsistency and to reopen the discussion on the age of fossil StW573.
 
 
 

The stratigraphy and formation history of fossil- and artefact-bearing sediments in the Milner Hall, Sterkfontein Cave, South Africa: New interpretations and implications for palaeoanthropology and archaeology

The Sterkfontein palaeokarst deposits, the richest repository of Australopithecus fossils in the world, are currently divided into six members (M1–M6) and named the Sterkfontein Formation (Partridge and Watt, 1991). In this system, it was proposed that the passages and chambers deeper than the Silberberg Grotto formed most recently and are filled with comparatively young sediments from newer openings or reworked sediments from older, upper chambers. Here we describe the geomorphology, sedimentology, fauna and archaeological evidence from six deposits excavated near the base of the system in the Milner Hall. The use of a multidisciplinary approach enabled us to clarify the complex stratigraphic history, to track artefact-bearing deposits through multiple phases of deposition, and to identify an early deposit accumulated from the Silberberg Grotto to the base of the system. We propose that the Australopithecus-bearing Member 2 deposit originally extended down to near the base of the Milner Hall. We further propose that the vertical extent of the cave system was developed prior to the opening to the landscape surface, and that at that time the groundwater level was comparable to that seen now. We therefore argue that some of the earliest palaeoanthropological material in South Africa may have been deposited into the deepest chambers of the Sterkfontein Caves.

Stratigraphic analysis of the Sterkfontein StW 573 Australopithecus skeleton and implications for its age

StW 573, Little Foot, is the most complete Australopithecus skeleton yet discovered, with many of its bones found in their correct anatomical position. Since the discovery of the in situ skeleton in the Silberberg Grotto in 1997, several teams have attempted to date the fossil. This appeared a simple process because several flowstones are inter-bedded in the breccia above and below StW 573. Dating of these flowstones, using U–Pb (uranium-lead) isotope decay techniques, gave younger results than expected from the fauna and stratigraphic position, around 2.2 Ma (millions of years). Our recent stratigraphic, micromorphological and geochemical studies revealed that the stratigraphy is much more complicated than was previously thought, with localized post-depositional processes leading to the creation of voids within the breccia around the skeleton. These voids were then filled by multiple generations of flowstone growth. The research we present here demonstrates that the proposed dates based on the flowstone deposition can give only a minimum age for StW 573 and that the flowstone formation came after, and probably long after, the breccia deposition. If one takes account of the long evolution of these karst fillings, StW 573 appears to be significantly older than 2.2 Ma.

Palaeomagnetic analysis of the Sterkfontein palaeocave deposits: Implications for the age of the hominin fossils and stone tool industries

Palaeomagnetic analysis was conducted on speleothems from Members 1–5 at Sterkfontein Cave, South Africa. Palaeomagnetic analysis of siltstone and speleothem from the bulk of Member 4 indicate a reversed magnetic polarity that dates the deposits and its Australopithecus africanus fossils to between 2.58 and ∼2.16 Ma. Further confirmation of this age comes in the form of two short normal polarity events correlated to the Rèunion (∼2.16 Ma) and Huckleberry Ridge (∼2.05 Ma) events in speleothem capping the bulk of Member 4 and coeval with deposition of the final phase of Member 4, including A. africanus fossil Sts 5. At ∼2.16–2.05 Ma, Sts 5 is the youngest representative of A. africanus yet discovered. Palaeomagnetic analysis of the Silberberg Grotto deposits identifies a single short geomagnetic field event in flowstone overlying the StW 573 Australopithecus fossil, which is suggested to represent the Rèunion event at ∼2.16 Ma. This further supports the uranium lead age estimates of 2.3–2.2 Ma for the StW 573 fossil. Based on a reversed polarity for the deposits below the skeleton it cannot be older than 2.58 Ma. If StW 573 is considered to be a second species of Australopithecus then this indicates that two species of Australopithecus are present at Sterkfontein between 2.6 and 2.0 Ma. All of the Member 5 deposits date to less than 1.8 Ma based on a comparison of palaeomagnetic, faunal, and electron spin resonance age estimates. The StW 53 fossil bearing infill (M5A) is intermediate in age between Member 4 and the rest of Member 5 (B-C) at around 1.78–1.49 Ma. The rest of Member 5 (B-C) containing Oldowan and Acheulian stone tools and Homo and Paranthropus fossils was deposited gradually between 1.40 and 1.07 Ma, much younger than previously suggested.

Re-appraisal of the stratigraphy and determination of new U-Pb dates for the Sterkfontein hominin site, South Africa

Sterkfontein Caves is the single richest early hominin site in the world with deposits yielding one or more species of Australopithecus and possible early Homo, as well as an extensive faunal collection. The inability to date the southern African cave sites accurately or precisely has hindered attempts to integrate the hominin fossil evidence into pan-African scenarios about human evolutionary history, and especially hominin biogeography. We have used U-Pb and U-Th techniques to date sheets of calcium carbonate flowstone inter-bedded between the fossiliferous sediments. For the first time, absolute age ranges can be assigned to the fossil-bearing deposits: Member 2 is between 2.8 ± 0.28 and 2.6 ± 0.30 Ma and Member 4 between 2.65 ± 0.30 and 2.01 ± 0.05 Ma. The age of 2.01 ± 0.05 Ma for the top of Member 4 constrains the last appearance of Australopithecus africanus to 2 Ma. In the Silberberg Grotto we have reproduced the U-Pb age of ∼2.2 Ma of for the flowstones associated with StW573. We believe that these deposits, including the fossil and the flowstones, accumulated rapidly around 2.2 Ma. The stratigraphy of the site is complex as sediments are exposed both in the underground chambers and at surface. We present a new interpretation of the stratigraphy based on surface mapping, boreholes logs and U-Pb ages. Every effort was made to retain the Member system, however, only Members 2 and 4 are recognized in the boreholes. We propose that the deposits formally known as Member 3 are in fact the distal equivalents of Member 4. The sediments of Members 2 and 4 consisted of cone-like deposits and probably never filled up the cave. The U-Th ages show that there are substantial deposits younger than 400 ka in the underground cave, underlying the older deposits, highlighting again that these cave fills are not simple layer-cakes.

U-Pb Isotopic Age of the StW 573 Hominid from Sterkfontein, South Africa

Sterkfontein cave, South Africa, has yielded an australopith skeleton, StW 573, whose completeness has excited great interest in paleoanthropology. StW 573, or "Little Foot," was found 25 meters below the surface in the Silberberg Grotto. 238U-206Pb measurements on speleothems immediately above and below the fossil remains, corrected for initial 234U disequilibrium, yield ages of 2.17 +/- 0.17 million years ago (Ma) and 2.24(-0.07)(+0.09) Ma, respectively, indicating an age for StW 573 of close to 2.2 Ma. This age is in contrast to an age of approximately 3.3 Ma suggested by magnetochronology and ages of approximately 4 Ma based on 10Be and 26Al, but it is compatible with a faunal age range of 4 to 2 Ma.