How heating and cooling and wetting and drying can destroy dense faunal elements and lead to differential preservation

Abstract

Recent taphonomic research has demonstrated that bulk bone density significantly determines survivability in many archaeological and paleontological contexts. Researchers often assume that higher density skeletal elements invariably preserve better than lower density skeletal elements. This assumption, however, is not universally valid. Here we present data on the taphonomy and preservation of assemblages from the Geelbek Dunes of the Western Cape Province, South Africa. These data indicate that skeletal preservation of fossil assemblages from deflation hollows can reflect an unexpectedly high proportion of porous, low density skeletal parts. High density skeletal parts including teeth and the shafts of long bones often shatter as a result of the repeated thermal shock and wetting and drying of the kind observed at Geelbek. This effect is greatest for highly mineralized bones from large mammals. The data from Geelbek demonstrate that survivability is affected by the size of the animal. Small mammals are represented by high proportions of non-mineralized, dense, skeletal parts, especially long bones.

Introduction

Context is everything in archaeology. There are few universally valid explanations that can be applied independent of the contextual analysis of the materials under study. This paper addresses the way in which bone density, animal size and mineralization affect survivability. The results from the Geelbek Dunes of the Western Cape Province, South Africa lead us to question the assumption that high density faunal elements invariably survive better than lower density elements. The realization that simple rules do not always apply in taphonomic studies, while at times frustrating, opens new lines of evidence for determining how assemblages formed.

Paleontologists and zooarchaeologists have long recognized that high density faunal elements often survive better than low density skeletal parts. Many researchers (e.g., Zapfe, 1939, Guthrie, 1967, Hill, 1980, Brain, 1981, Lyman, 1984) have demonstrated this by using a wide variety of archaeological and paleontological data. Lyman in particular has addressed this issue in a number of publications (Lyman, 1982, Lyman, 1994, Lyman et al., 1992), but unlike researchers who use density models uncritically, Lyman and his co-authors often stress that bone density is not the only factor that determines survivability:

…uncritical use of the structural density of bones may be shown to lead to potentially incorrect conclusions…The quick-fix of bone density, while a necessary part of our conclusion, is not sufficient when used alone…(Lyman et al., 1992, pp. 571–572).

Discussions of density-mediated preservation have increasingly taken on the aura of universal applicability. Current discourse on density-mediated survivability is less a question of whether the model works, but rather about who can devise the best method for quantifying skeletal density. Lam and Pearson, 2004, Lam and Pearson, 2005 have rightly stressed caution in the use of density values in archaeological analysis. This caution is due primarily to “problems involved in the derivation of bone density values” (Lam and Pearson, 2004, p. 100). The underlying message in much of the recent work by Lam, Marean, Pickering, Symmons and others is that high density skeletal elements outlast low density elements (e.g., Lam et al., 1998, Lam et al., 1999, Lam et al., 2003, Pickering et al., 2003, Symmons, 2004). Despite Lyman's call for a critical use of the notion that high bone density increases survivability, many zooarchaeologists assume that this is the case in all settings and for all species, as long as density is properly measured. Our questioning this assumption in no way implies that we reject the mainstream ideas about bone density and survivability. Clearly, in many settings high density elements do survive better than low density elements, taking into account common taphonomic influences such as transport, carnivore ravaging and dispersal. Here we demonstrate the existence of a taphonomic setting where low density bones survive better than high density bones.

This paper is not about measuring bone density. We follow Lam et al.'s (1999) position that patterns of bone density are generally robust and cross-cut species. We are also content to leave the issues related to optimizing methods of determining bone density to other scholars. Instead, our aim is to demonstrate that high density skeletal elements do not always survive better than porous, lower density elements. We hope that the data and ideas presented here will open new lines of analysis for studying faunal assemblages by demonstrating that bones from animals of dissimilar sizes respond differently in varying taphonomic settings and that higher density bones do not always survive better than lower density bones.

The Geelbek Dunes are located in the West Coast National Park in the sandveld 5 km east of the Atlantic Coast of South Africa and 2 km east of the southern end of Langebaan Lagoon (Fig. 1). This near-coastal environment receives annual rainfall of about 300 mm. The arid climate supports fynbos vegetation. While daily fluctuations in air temperature can vary by up to 30 °C, the ground surface experiences even greater swings in temperature. Most mornings the dunes are blanketed in dense fog that deposits moisture on the surfaces of plants, fossils, and artifacts. As the day heats up, this moisture evaporates. These conditions create a daily environment of heating and cooling and wetting and drying for the exposed fossils and artifacts.

For at least 40 years, informal visits to the Geelbek Dunes have resulted in the discovery of archaeological finds and abundant faunal remains. Between 1998 and 2006 a multidisciplinary team of archaeologists and Quaternary scientists led by researchers from the University of Tübingen's Department of Early Prehistory and Quaternary Ecology conducted a systematic study of the dunes. The team documented 23 open-air, archaeological and paleontological localities among 114 deflation hollows and selected both archaeological and paleontological sites for studying the paleoenvironments and human adaptations of the near-coastal environment (Conard et al., 1999, Conard, 2002, Kandel et al., 2003, Kandel, 2004, Dietl et al., 2005, Conard and Kandel, 2006). These localities included Middle Stone Age (MSA), Later Stone Age (LSA) and historic period sites of varying composition and find density.

Although these localities are presently within a highly mobile dune field, these modern dunes migrated into this region only recently (Franceschini, 2003). Previously the sandveld in this area was stabilized by low, scrubby fynbos vegetation, as evinced by calcified rhizoliths (fossilized roots) that are visible in the loosely consolidated geological horizon designated Ancient Dune II by Felix-Henningsen et al. (2003). The deflation hollows within the mobile dunes serve as windows into the earlier landscape. As the dunes migrate northward, deflation continually exposes faunal remains, lithic artifacts and other classes of finds. We take advantage of these open deflation surfaces to study vast areas. In total we sampled more than eight hectares of deflated surfaces. This large sample can be used to model the mobility and settlement dynamics of the Stone Age inhabitants of the Western Cape. The systematic surface collections at Geelbek resulted in over 30,000 piece-plotted artifacts and faunal remains. The field crew also excavated 1887 excavation units of 1 m² and 140 units of 1/4 m². The crew screened the sediments from these shallow excavations through 10 and 1 mm mesh to gain samples of small materials that were not readily visible during surface collection.