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Fossils

What are fossils?

 

Fossils are formed when organic structures are preserved through mineral replacement, or by cementing the sediments that contain the structures. There are two main kinds of fossil: (1) Body fossils represent preserved parts of the bodies of organisms. This can include the bones or vertebrate animals, or the outlines of the parts of invertebrates and plants. At the Sterkfontein Caves, we find body fossils of vertebrate animals that lived in the Cradle of Humankind millions of years ago. (2) Trace fossils (also called ichnofossils) represent the preservation of an organism’s activity or behaviour, and can include footprints, fin and tail drags, burrows and coprolites.

 

Process

 

Body fossils are normally preserved by a replacement of the structure of bones with other minerals, such as calcite (a carbonate mineral), which is composed of the chemical calcium carbonate. Calcium carbonate is abundant in the world’s rocks and sediments, and is the main constituent of limestone and dolomite.

 

Calcium carbonate can form many different minerals, but its most important attribute for fossilisation is that it is soluble. When it is dissolved in water, it can permeate bones and gradually replace the organic calcium of bones with inorganic calcium carbonate.

 

Calcium carbonate often also plays an important role in the formation of trace fossils because it can cement sediments and their structures (including footprint shapes, for example) when they are buried, and preserve them for millions of years. Sometimes, that sediment is changed to rock and the trace fossils can be preserved for hundreds of millions of years.

 

Another possible agent of fossilisation of body parts and traces are silicate minerals. These are extremely abundant in the world’s rocks and sediments and, when heated or put under pressure, can also be soluble and replace structures or cement sediments.

 

Fossilisation takes a long time, but the duration of the process depends a lot on how abundant the fossilising minerals are in the environment. In the Cradle of Humankind, the caves are made out of calcium carbonate and so carbonate minerals, like calcite, are very abundant. Bones that fall into the caves are then surrounded by calcium carbonate-rich sediments, helping the fossilisation process.

 

Fossil Discoveries at Swartkrans

 

Fossil discoveries at Swartkrans began in 1948, when Robert Broom and John Robinson moved part of their excavation team to the site with funding from the University of California’s African Expedition.

 

Broom and Robinson initially recovered fossils of Paranthropus crassidens  (later subsumed into P. robustus) from “pink breccia” later associated with Member 1 Hanging Remnant, and a single jaw bone, initially assigned to Telanthropus capensis (SK15), in a “chocolate brown” breccia of unknown stratigraphic origin. This material was accessioned under “SK”, which represents some of the most iconic fossils from the site.

 

The co-occurence of P. robustus and early Homo fossils (SK15 was later assigned to Homo erectus by Robinson in 1961) from Swartkrans led Broom and Robinson to discuss the contemporaneity of multiple hominin species on the South African landscape. These insights would eventually revise the linear model of human evolution, which held that one species was replaced by another. Instead, multiple species coexisted and some died out while others evolved into more derived forms.

 

From 1965 to 1986, Swartkrans was systematically excavated by CK Brain, who discovered an abundance of fossil hominin remains . The majority of this material was accessioned with the prefix “SKX”, which greatly increased the fossil assemblage from the site.

 

Stratigraphic divisions distinguished Members 1 through 5, although the Early Pleistocene hominin material was restricted to Member 1-3, spanning ~2.2- to >1-million years ago. During this period of research, some of the SK hominin specimens were re-evaluated, which increased the diversity of species representation. For example, Ron Clarke examined SK27 , a juvenile skull, found in the Member 1 Hanging Remnant, which he argued was Homo habilis based on features of the skull and associated teeth. Moreover, a reassessment of SK847 by Clarke, FC Howell and Brain in 1970  was published that argued the specimen represented Homo, and was later assigned to Homo ergaster .

 

In 2005, renewed excavations at Swartkrans, led by Travis Pickering and Kathleen Kuman further explored Members 1-3, and have since recovered hominin fossils primarily from the Lower Bank portion of Member 1. These are mostly fragmentary remains and isolated teeth, although this is likely a factor of the deposit itself.

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    FOSSIL ACCESS

     

    All heritage objects found in South Africa belong to the country as part of the national estate and cannot be owned, sold or destroyed by individuals . It is this heritage framework that protects the resources that tell the stories of all our pasts.

     

    In the Cradle of Humankind, every site in which heritage resources are found, e.g. fossils, stone tools, graves and stone walling, is registered and managed by the South African Heritage Resources Agency (SAHRA). It is SAHRA’s responsibility to ensure that conservation systems are in place that protect the site and any objects that are removed from that site through research.

     

    SAHRA’s role extends to issuing research permits to researchers for sampling and documentation at heritage sites, and there are excavation and curation standards that must be met in order to be issued a permit. As part of the protection of heritage objects, SAHRA appoint forma repositories that act on the behalf of the national estate to protect, curate and manage access to heritage objects and collections.

     

    In the Cradle of Humankind, two main repositories operate: Wits University, Johannesburg, and the Ditsong Museum, Pretoria. It is in these spaces that all excavated objects are kept for conservation and study. Sterkfontein and Swartkrans fossils are kept at Wits University.

     

    Access to the fossils excavated from Sterkfontein and Swartkrans is managed by the curator, in this case Dr Bernhard Zipfel. Inquiries for access can be emailed to Dr Zipfel. Access to the stone tools excavated from Sterkfontein and Swartkrans is managed by the curator of archaeology, Dr Thembi Russell.