The Taphonomy of Australopithecus Sediba of the Malapa Fossil Site (Gauteng, South Africa) – By Aurore Val
Aurore Val completed a research masters in Biological Anthropology and Prehistory at the University of Bordeaux 1, between 2007 and 2009. She finished her thesis in 2013, focussing on taphonomic remains of Australopithecus found at the Malapa site in South Africa. It was a thesis co-supervised between the Universities of the Witwatersrand and Bordeaux 1, under the direction of Lucinda Backwell, Lee Berger (Wits) and Francesco d’Errico (Bordeaux 1). She is currently a postdoc with Lyn Wadley at the University of Witwatersrand and work on the remains of birds in the Middle Stone Age site of Sibudu in KwaZulu-Natal.
The discovery of the Malapa fossil site in South Africa constitutes one of the main events of the last decade in palaeoanthropology. The bone remains of two individuals belonging to a species of hominin unknown to date were found there, in a state of preservation never seen before for fossils of that age (close to two million years). A detailed study of the different geological, biological and chemical processes contributing to the fossilisation of these remains makes it possible to understand better how the two skeletons were kept in this remarkable condition. The main results of the study are presented here.
Introduction: The Cradle of Humankind
The region of the Cradle of Humankind, north of Johannesburg, listed as a Unesco World Heritage Site, is one of the richest places in the world in terms of fossils of hominins and other animal species. The word hominin refers to all the members of the human family, from the separation of our line with that of the great apes to us, Homo sapiens sapiens. Close to one third of hominin fossils collected to date come from the Cradle of Humankind. Around fifteen fossiliferous deposits have been excavated and/or are still being excavated in this region of just under 47 000 ha. The main sites of the Cradle (Sterkfontein, Swartkrans, Kromdraai and Cooper’s D) on their own have produced several hundreds of hominin fossils and several tens of thousands of fossils of other animals.
The palaeontological assemblies were formed mainly between the end of the Pliocene and the beginning of the Pleistocene, i.e. between 3 to 4 million years and 1,5 to 1 million years BP. The remains of several extinct species, including three species of Australopithecus (Australopithecus africanus, Australopithecus sediba and a third unnamed species, Australopithecus “second species”), a species related to the Australopithecus, Paranthropus robustus, and several representatives of the Homo lineage (Homo habilis and Homo erectus) were collected there (Broom, 1936, 1938, 1947; Robinson, 1953, 1961; Brain, 1981, 1993; de Ruiter et al., 2009; Berger et al., 2010). These fossils were found in association with those of many other animal species, making it possible to document in detail the geomorphological, climatic and environmental context in which our ancestors evolved. As such, various taxa (antelopes, warthogs, carnivores, rodents, reptiles and birds among others), some extinct and others still existing today in South Africa, occupied the region during the Plio-Pleistocene.
The Cradle sites are part of the vast karstic system of the dolomitic caves in Northern Gauteng, and has been known since the end of the 18 century, when limestone was exploited by several South African mining companies in the region. Limestone was one of the materials used in building Johannesburg which, at the time, was in full expansion. It also constituted a key element for the gold industry in that it was used in the purification process of the gold being mined in Gauteng. The first fossil discoveries occurred in Sterkfontein, then in Swartkrans in the 1930s and 1940s. From then on, many scientists showed great interest in the Cradle region where, to date, series of quasi-uninterrupted excavations have been led, revealing the existence of new fossils and deposits on a regular basis (Broom, 1936, 1938, 1947; Keyser et al., 1991, 2000; Berger et al. 1993; Lacruz et al., 2002; Adams et al., 2007; de Ruiter et al., 2009). The existence of Malapa, one of the most prolific sites in the region in terms of hominin fossils, has only been known since 2008. This fossil deposit was discovered by Lee Berger, a lecturer at the Evolutionary Studies Institute of the University of the Witwatersrand in Johannesburg, and Job Kibii, a researcher at the same institute, while exploring the region using Google Earth and prospecting on foot. Within the framework of this prospecting programme, close to one hundred new fossiliferous deposits were recorded, including a dozen potentially containing hominin remains (Berger, 2012).
The Malapa fossil site, in the center of the Cradle of Humankind, is 15 km North-East of Sterkfontein. Briefly exploited during the 1920s by a mining company, today it looks somewhat ordinary due to its small dimensions (around 3 meters wide by 4 meters long and slightly less than 3 meters deep).
The remains of a new hominin species, Australopithecus sediba (Berger et al., 2010), were discovered in this deposit. The holotype of this species, i.e. the skeleton that served as the basis for the morphological description of this new taxon, MH1 (which stands for Malapa Hominin 1), is an adolescent and probably a male. The second skeleton found belongs to an adult, probably a female, MH2.
Both skeletons are very well preserved and are made up of complete or almost complete bones and, in certain cases, these bones are still anatomically connected. Pieces from the skull, the feet and hands, the pelvis, the spine and the limbs were found. The anatomy of the Australopithecus sediba skeleton offers a mosaic of morphological features never seen before (see for example Berger et al., 2010; Carlson et al., 2011; Kivell et al., 2011; Zipfel et al., 2011; and Berger, 2012). This species thus combines primitive features, inherited from the great apes (a small prognathic skull and long upper limbs compared to the lower limbs) with so-called derived or modern features, similar to those of representatives of the Homo line, such as a morphologically modern pelvis, prehensile hands with an opposable thumb and small canines.
The fossil remains of Australopithecus sediba, which were found together with those of other animals (antelopes, zebras, carnivores, warthogs and rodents), have been dated at 1,977 million years BP, using three combined methods, i. e. biochronology, palaeomagnetism and direct radiometric dating (Uranium/Lead) (Dirks et al., 2010; Pickering et al., 2011). The unique morphological features as well as the age of the fossils make of Australopithecus sediba the potential ancestor of the first Homo and even Homo erectus (Berger, 2012). Another remarkable characteristic of these fossils is their excellent state of preservation, which is unique in the context of the region’s karstic sites. The presence of remains of hominins and other primates in the palaeontological deposits of the Cradle of Humankind is usually due to carnivores, leopards and hyenas in particular, accumulating their kills. This theory which is called the “carnivore-collecting hypothesis”, as proposed by researcher C. K. “Bob” Brain in the 1980s (Brain, 1981), suggests that a predator specialised in capturing primates and occupying caves or cave entrances (leopards and/or hyenas), would have contributed to the accumulation of most human and non-human primate remains found within the Plio-Pleistocene deposits of the Cradle of Humankind. Carnivores, during the capture and consumption of carcasses, leave marks on the skeleton which can be identified on the fossil remains.
Moreover, the bones of primates accumulated by felines or hyaenidae are characterised by an advanced state of fragmentation, and by the underrepresentation or even absence of certain skeletal elements, which are completely destroyed during their consumption by carnivores. Other post-deposit processes such as weathering, scavenging, rodent gnawing and sedimentary modifications contribute further to bone remains being damaged.
Consequently, the fossils of primates from Cradle sites are most often found isolated and very fragmented. Among the hundreds of hominin fossils that were collected over the decades in the caves of the region, and before the discovery of Malapa, there was no example of quasi-complete skeletons, except for “Little Foot” in Sterkfontein (Clarke, 1998, 2007). The case of Malapa, with not just one but two quasi complete skeletons, is quite unique. The possibility of bone accumulation by carnivores was, in this instance, rejected very quickly. A preliminary hypothesis explaining the unique state of preservation of the Australopithecus sediba skeletons was proposed shortly after the discovery of the fossils (Dirks et al., 2010). According to this hypothesis, the two individuals, MH1 and MH2, could have fallen into a death trap opening onto a first cavity situated in the upper section of the karstic network which, today, is completely eroded.
Where access to this cavity is difficult, neither scavengers nor rodents could have modified the bodies of MH1 and MH2, as they too would have died either from the actually fall into the death trap or from hunger and thirst, since they would not have been able to go back to the surface.
Shortly after the fall, the corpses of MH1 and MH2 would have been transported towards a deeper section of the cave by a flow of debris caused by strong summer rains. Once covered by sediment transported by subsequent flows, the Australopithecus would have been rapidly buried and started fossilising. Protected by this anaerobic environment, MH1 and MH2 would have been preserved for the next two million years.
The objective of my doctoral research, conducted in Johannesburg between 2010 and 2013, was to test the validity of this preliminary hypothesis, based mainly on geological considerations on the nature of the sediments, as well as very general observations on the state of conservation of the skeletons. My project consisted of a detailed taphonomic study of the Malapa hominins. Taphonomy (from the Greek taphos, “grave”, and nomos, “law”), as defined by Russian palaeontologist Ivan Efremov in 1940, refers to the study of all geological, biological and chemical phenomena affecting an organism when it moves from the biosphere to the lithosphere, in other words during fossilisation (Efremov, 1940; Lyman, 1994). In the karstic context of the Cradle sites, taphonomic issues are concerned with identifying how bone remains were accumulated in the caves (i.e. due to carnivores or death traps) and which processes modified fossil assemblies over time. The objective is to understand the extent to which palaeontological assemblies are representative of the past environment, within the framework of palaeoenvironmental reconstruction, past animal communities, carnivore/primate interactions and the relations between different species of primates (managing territory and competition for example).
Thanks to the taphonomic approach, one can assess the extent to which the composition and characteristics of fossil assemblies are potentially biased by their accumulation method inside the caves, and by the processes modifying them overtime.
In Malapa, the objective of my research was to identify the hominin accumulation methods inside the site, and to understand how their corpses were buried. One of the aspects of my work was to determine whether or not the corpses were transported by a flow of debris between the upper section and a deeper section of the karstic system. Finally, the idea was to propose an estimate of how long it took for the corpses of MH1 and MH2 to be buried after the two fell into the cave, and to identify which stage the corpse decomposition process was at when MH1 and MH2 were finally buried.
Several methods were selected to answer these questions. In a first phase, a classic taphonomic study of hominin fossils and associated fauna was conducted, in the form of macro- and microscopic analyses of the bones with a view to identifying whether or not bone surface modifications took place (e.g. teeth marks from carnivore and rodents or mandible marks from insects) and to measure the extent of weathering on the fossils. The degree of fragmentation for all the remains, as well as the types of fractures on the long bones (fractures on fresh bones as opposed to dry bones), were recorded. An estimate of the survival percentage of the different skeletal elements and animal mortality profiles present in the assembly were conducted. Finally, a spatial analysis of the remains of the two Australopithecus was conducted. This analysis represented an important phase as it required us to carry out a virtual reassembly onsite of the many elements of MH1 and MH2 that had been found ex situ in blocks of breached sediment, which had been moved by the miners. These virtual reassemblies have been carried out with the 3D Avizo application programme.
The results of this doctoral thesis are currently being published and, consequently, it is not possible to describe all the conclusions obtained in detail. However, general information can be shared here. Concerning the majority of the remains of MH1 and MH2 found ex situ, it was possible to carry out virtual reassemblies inside the site using Avizo. These reassemblies led to the reconstruction of the individuals’ initial position before being disturbed by the miners, in other words the position in which they were buried and preserved. The analysis of these positions and the spatial distribution of the remains inside the site, indicate a very low degree of bone dispersion and movement, contradicting the idea that the corpses were transported by a mudslide.
The hypothesis according to which the hominins fell into a death trap is confirmed, as is the absence of contribution from carnivores (predators or scavengers) and rodents. A study of the fauna associated with the remains of Australopithecus sediba reveal the presence, within the palaeontological assembly, of several other animals with a taphonomic history similar to that of the hominins.
Selecting modern methods to carry out the study (station total, CT-scanner, Synchrotron and 3D reconstruction application programme), together with the exceptional degree of preservation of the fossils in Malapa, made it possible to reach a precision level unequalled to date in describing hominin taphonomy for periods as old as these. The results show that the two Australopithecus were complete when they arrived on the site. In time, all the elements of their skeleton should be recovered, making of MH1 and MH2 the most complete hominin skeletons ever discovered. Organising a series of excavations of the breached sediment (planned for the end of 2014), leading to the discovery of fossils in situ, should help us to complement the results emanating from this research work, and to understand better the taphonomy of the bone assembly.
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