VERTEBRATE TRACKWAYS IN LATE CENOZOIC COASTAL EOLIANITES, SOUTH AFRICA

Calcified coastal dunes rich in bioclastic carbonate, ranging in age from Middle Miocene to Holocene, occur extensively along the coast of South Africa. Primary sedimentary structures are predominantly large scale, steeply dipping, planar cross-stratification up to 5 m in thickness, interspersed three-dimensionally with smaller wedge-shaped sets and low angle interdune and toeset laminae. The eolianites represent fossilised, partially vegetated retentive dune cordons and onshore-transgressive dune plumes; trace fossils occur only in the former.

The eolianites host vertebrate (mainly mammalian) body and trace fossils, the latter including large and small herbivores, carnivores, birds and humans. At Nahoon on the southeastern coast, two occurrences ~200 ka in age each feature at least 6 trackways, preserved as casts within areas <3 m³. The ~120 ka Langebaan human prints were found on the southwestern coast. The two human traces comprise 40% of all the known pre- latest Pleistocene human prints. Combined with the (?300 ka) Terra Amata hominid footprint made in eolianite from southern France, this proportion rises to 60%. All three trackways relate to the terminal phases of interglacials (Oxygen Isotope Stages 5e, 7 and 9). Traces are preserved on bed surfaces and as casts on bed undersides. Climates characterised by rapidly oscillating damp and dry windy spells favour the genesis and preservation of traces. The quality of manually excavated of traces is inferior to those exposed by the elements, where slow etching and surficial hardening are important. The character of traces depends on substrate competency, slope angle and direction of progression of the trace makers relative to the slope.

Trace fossils in coastal eolianites have been reported from many localities around the globe. The reasons for the ubiquity of vertebrate traces in coastal eolianites are:

- the cohesiveness of moist sand, providing an effective moulding agent resistant to erosion;

- high sedimentation rates, promoting rapid burial of traces;

- rapid lithification via partial solution and reprecipitation of bioclasts;

- shoreline erosion, which effectively re-exposes trace fossil-bearing surfaces.