Last Interglacial fossil elephant trackways dated by OSL/AAR in coastal aeolianites, Still Bay, South Africa

1. What they did

The sea cliffs east of Still Bay, on South Africa’s southern coast, are made of old coastal dunes that have turned to rock. They hold many fossil trackways of mammals and reptiles, including the African elephant, but neither the tracks nor the rocks had been described in any detail, and their age was uncertain.

The authors:

• Described the dune rocks in two measured cliff sections: the types of layering, the ancient soils (palaeosols) and the direction of the ancient winds.
• Dated the rocks with two methods used together for the first time in South Africa: optically stimulated luminescence (OSL), which measures how long sand grains have been buried, and amino acid racemisation (AAR), which measures slow chemical change in shells and shell grains.
• Described elephant footprints at five sites along about 3 km of coast, seen in cliff faces, as casts on the underside of rock layers and as natural impressions on old ground surfaces. Where possible they measured footprint size, stride and push-up mounds, and used the length of the hind foot to estimate each elephant’s shoulder height and age from East African reference data.
• Considered how the footprints were preserved and what the elephants’ presence says about the climate, vegetation and early humans of the time.

Only the elephant tracks are described here; the other tracks are to be described elsewhere.

2. What they found

Age of the dunes and tracks

• The dunes built up during the Last Interglacial (Marine Isotope Stage 5), with OSL ages from about 140 to 91 thousand years. Dune building stopped at about 90 thousand years ago, and a mature soil formed over a long gap until new dunes arrived about 8 thousand years ago.
• The sand just below and above the main elephant trackway layer gave OSL ages of 140 and 136 thousand years. These are slightly older than the soil layer beneath them, a small age inversion the authors note; they read them as very early in the Last Interglacial.
• The dune sand contains shell grains reworked from older dunes, which explains why the whole-rock AAR results look older than the snail shells.

The landscape

• The dunes were large, partly vegetated parabolic dunes built mainly by westerly winter winds, as today.
• Elephant and other trackways are most common in the low-angle layered sand of broad, gently sloping areas between dunes, and rare on the steep dune faces.
• Near the base of the cliffs, iron-stained bands and a brackish-water slug point to a wetland, near where the freshwater lake Rietvlei lies today.

The elephant tracks, site by site

• Site 1: trampled layers stacked over about 4 m of cliff, showing a game trail used again and again over a long time. At the base, a 33 m stretch is so heavily trampled that prints overlap and single trackways cannot be followed, suggesting a sizeable herd crossed it. No accurate measurements were possible.
• Site 2: a fallen block with three prints as casts. Print A has the broadly oval shape of a hind foot, 38 by 27 cm and 11 cm deep, with a slight bulge at the front that shows the direction of travel. Print B is round, typical of a forefoot (33 cm across, 9 cm deep). The hind foot points to an elephant about 225 cm tall at the shoulder and about 13 to 14 years old: large, but not fully grown.
• Site 3A: two prints on a wind-rippled surface, about 75 cm apart. The round hind print (25 cm across, with a 3 cm wall around it) sits in a perfectly preserved wedge of displaced sand. The other print is blurred and stretched by a downslope slide and is probably the forefoot. The ripples formed before the elephant passed. The firm, cohesive displaced sand suggests rain had moistened the ground. This animal was about 160 cm at the shoulder and four to five years old.
• Site 3B: on the layer beneath, three elephant trackways (A, B and C) cross a rippled surface, with antelope trackways crossing them. Trackways A and B (6 and 5 prints, average stride 65 cm) run side by side and curve together, suggesting two very young elephants walking together and influencing each other. Trackway C splits off at about 30 degrees and is probably a third animal. Hind prints of 17 to 20 cm point to calves about 110 cm tall and less than a year old. Some short strides (about 25 cm) show changes in gait. Two toe marks, rarely seen in elephant prints, show the direction of travel. All three trackways show the narrow track width typical of elephants, whose legs swing beneath the body. Push-up mounds on the same side of every print show the elephants were walking and sliding down a slope.
• Site 4: prints seen only in cross-section, on steep dune faces (25 to 32 degrees), 35 m above sea level. These disturb the sand most deeply, up to 30 cm, perhaps because of uneven weight on the legs while negotiating a steep, unstable slope, or loose sand.

How the tracks were preserved

• Only dune sand moistened by rain or dew holds the shape of a footprint. The many well preserved tracks suggest the dunes were often wet.
• Westerly winds dry the windward side of a dune and blow sand onto the damp, sheltered side, burying prints there. Since dunes here were built mainly by winter westerlies, the prints were probably preserved mostly in winter.
• Deep footprints commonly left undertracks and overtracks in the layers below and above.
• Preservation depended on how damp and compact the sand was, the slope, and how soon the prints were buried. Burial deeper than about 2 m protected them from roots and burrowing animals, and rapid hardening of the shelly sand helped preserve them.

3. What they concluded

• These are, as far as the authors know, the only fossil footprints of the African elephant, and the only known Pleistocene elephant footprints from Africa. They are probably those of the Cape elephant, the same subspecies as the historical “Knysna elephants”.
• At about 34° 20′ S, they are the southernmost record of the African elephant so far. Sea level was lower than today, so elephants may have roamed even further south.
• Elephants seem to have crossed this dune landscape in numbers over a long period. A migration route through difficult active dunes seems unlikely; the elephants may have been drawn by water in an interdune wetland.
• Because elephants generally need plenty of browse, their presence may mean more woodland and a wetter climate than today, which fits the signs of a high water table, a wetland and frequently wetted dunes.
• Early modern humans lived in the region at the same time and almost certainly met elephants, but no elephant remains are known from Late Pleistocene cave sites along the southern coast. The authors suggest people may not yet have had the technology to hunt such formidable prey.

4. How tracking was or was not used

The study is a reading of fossil tracks. The authors identified elephant footprints by size and shape, told hind feet (oval) from forefeet (round), and worked out direction of travel from a front bulge, toe marks and push-up mounds. They measured prints, strides and track width, recognised changes in gait and downslope sliding, read the order of events from tracks crossing ripples and other tracks, and recognised undertracks and overtracks. Hind foot length was used to estimate shoulder height and age. They refer to the elephant footfall pattern and to tracking and field guide sources (Stuart and Stuart, 1994; Brown, 1999, The Science and Art of Tracking). No trailing in the present-day sense was involved.

5. Tracker or observer certification

Not reported. The footprints were interpreted by the authors, who are geologists and geographers; no tracking qualification or evaluation is described.

6. Other technologies, skills or innovations

• First joint use of optically stimulated luminescence (OSL) and amino acid racemisation (AAR) dating in South Africa.
• An age-depth model to correct the OSL cosmic dose for sand that built up in episodes over a long time.
• Hind foot length compared with East African data (Western et al., 1983) to estimate the shoulder height and age of each animal.