Pleistocene vertebrate tracksites on the Cape south coast of South Africa and their potential palaeoecological implications
Introduction
The Palaeo-Agulhas Plain is an ancient terrestrial ecosystem that existed off the south coast of South Africa during the Pleistocene Era at times of lowered sea levels, and is now almost completely submerged by high sea level, with only a narrow strip along the present-day coastal plain exposed above water. It has recently become the focus of research because of the likelihood that its ecology was significant for at least two major reasons: 1) the records for modern human origins in this region (e.g. Brown et al., 2009, 2012; Henshilwood et al., 2002, 2011; Marean et al., 2007), and 2) the diversity of the mega-diverse Cape Floristic Region (Bar-Matthews et al., 2010; Marean et al., 2014). Fossil faunal assemblages have been the preferred palaeoarchives in this region for building an understanding of its faunal character in the Pleistocene, and there is an excellent record of these (Klein, 1976, 1983; Marean et al., 2000; Rector and Reed, 2010). However, these faunal assemblages alone cannot paint a complete picture of past faunal communities, because the primary accumulators of these assemblages (people, carnivores, owls, etc.) have their own predation and transport biases that limit their representativeness of such communities. We have found that ichnofossils (animal trackways, tracks and other traces) provide a valuable addition to these faunal records – for example, while no giraffe skeletal material has been found in Cape south coast Pleistocene assemblages, we have identified the presence of giraffe via their tracks (Helm et al., 2018a).
On the Cape south coast ichnofossils are primarily preserved in aeolianites that are exposed on the coast adjacent to the intertidal zone. In the Pleistocene these locations were a thin zone of contact between the Palaeo-Agulhas Plain and the interior, and animals crossed this zone while moving between these areas, leaving a record of their movements and community composition independent of the preferences and behavioural habits of predators. Roberts et al. (2008) first drew attention to Late Pleistocene trackways occurring in aeolianites on the Cape south coast, describing elephant trackways east of Still Bay. We decided to tap this rich source of information by commencing a concerted ichnofossil study in 2007, and since then have developed a substantial and informative record along a 350-km stretch of the Cape south coast between Arniston in the west and Robberg in the east (Fig. 1) (Helm et al., 2017, 2018a; 2018b, 2018c; 2019a, 2019b).
The purpose of this study is to summarize our current understanding of the track record from the more than 130 tracksites from our study area, and to review the potential and limitations of ichnology to help determine the Pleistocene palaeofaunal makeup that may not be provided by skeletal material alone. In so doing we hope that our ichnological studies will help to enrich the understanding of the palaeoecology of the Palaeo-Agulhas Plain. As a result of further discoveries, articles on a number of topics of relevance to the palaeoecology of the Palaeo-Agulhas Plain are currently in preparation or under review – we allude to these briefly.
Section snippets
Pleistocene ichnofossil studies in South Africa
Late Pleistocene ichnofossils occur in aeolianites and cemented foreshore deposits along the Cape south coast of South Africa. These deposits are the consolidated remains of ancient sand dunes, inter-dune areas and beaches. Globally, aeolianite deposits are relatively common in mid-latitude regions, typically between 20° and 40° (e.g. Fairbridge and Johnson, 1978; Brooke, 2001). These fossil dune systems are sensitive palaeoenvironmental indicators as recorded in their orientation, geometry,
Methods
A ground survey to record ichnofossil sites was performed between Arniston and Robberg by the lead author (C.H.) over a twelve year period from 2007 to 2018 (Fig. 1). Repeated visits were necessary due to the ephemeral nature of many tracksites. Most trackbearing areas could be accessed on foot along beaches or by traversing along cliffs; however, some exposed cliff areas proved inaccessible. Visits were timed to yield optimal site exposures at low tide close to spring tide. Clear summer days
Taphonomic observations
Aeolianites in the ∼130 km stretch of coastline from Arniston in the west to Still Bay in the east tended to be well-cemented, and thus provided fewer suitable exposed surfaces. By contrast, aeolianites east of Still Bay tended to be less well-cemented, and to cleave readily along bedding planes; tracksites were encountered in most areas with suitable bedding plane exposures.
A variety of ichnofossil types was evident: impressions, undertracks, tracks in profile, natural casts, and vertebrate
Relation to the traditional body fossil record
Klein (1983) described palaeoenvironmental implications of changes in large mammal fauna in the fynbos region of South Africa. He used the historic faunal record as a benchmark against which to evaluate prehistoric faunas from archaeological sites and carnivore dens. Analysing the faunal remains described from such radiocarbon-dated sites within or close to our study area, Klein (1983) noted changes over time, and correlated these with global and regional Middle and Late Pleistocene
Reptile tracks
The tortoise tracksites are possibly the only known such sites in the global ichnofossil record. The only report we are aware of Roberts et al. (2008) indicated that tracks of testudinae had been found in aeolianites east of Still Bay, but no details were provided. Tortoises were the dominant taxon noted in MIS 4 faunal remains from Klipdrift Shelter (Reynard and Henshilwood, 2017), and MIS 5a faunal remains from Blombos Cave (Thompson and Henshilwood, 2014). Both sites lie within our study
Conclusions
Ichnofossils are locally common in Late Pleistocene deposits on the Cape south coast. For a relatively short stretch of coastline, the study area contains a substantial number of sites that provide insight into the Pleistocene fauna of southern Africa in a manner that complements data obtained from body fossils. Some of these recently discovered tracksites have already contributed to our knowledge of palaeoenvironments. Compared with other disciplines, ichnology has had a late start on the Cape
Data availability
Locality data including GPS co-ordinates of tracksites are reposited with the African Centre for Coastal Palaeoscience at Nelson Mandela University, South Africa (https://accp.mandela.ac.za/), and the Peace Region Palaeontology Research Centre in British Columbia, Canada (https://www.prprc.com/).
This research did not receive any specific grant from funding agencies in the public, commercial or not-for-profit sectors.
Acknowledgements
The assistance and advice of Natalie Baker, Andre Brink, Emily Brink, Gary Bronner, Lisa Buckley, Naomi Cleghorn, Kevin Cole, Richard Cowling, Mark Dixon, Hannah du Toit, Charles Edwards, Guy Gardner, Chris Gow, Rudolf Hattingh, Sinèad Hattingh, Chris Heese, Carina Helm, Daniel Helm, Linda Helm, Johan Huisamen, Thalassa Matthews, Richard McCrea, Thulani Ndlovu, Penny Noall, Keith Spencer, William Stear and Alex van den Heever are gratefully acknowledged. David Roberts was the leader in
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