INTRODUCTION

Modifications of vertebrate carcasses and bones have been studied in many parts of the world, from tropical Africa to northern temperate regions (Behrensmeyer 1978, 1983, Haynes 1983, Blumenschine 1986, Korth 1979, Lyman 1994, Andrews and Cook 1985, Andrews and Armour-Chelu 1998), but little is known about the effects of weathering and other modifications in mid-latitude desert environments. These are inhospitable environments for both living and dead, with high solar radiation, extreme daily temperature fluctuations, high seasonality (both daily and annual), little vegetation and little or no rain sometimes for years at a time. For survival of bones exposed on the surface, these conditions pose problems not encountered elsewhere. For example, the lack of vegetation, which could shade and protect bones from the intense solar radiation, lack of rain and the dryness of the air expose bones to extreme drying conditions; and the large daily temperature changes expose the bones to heating and cooling on a regular basis. Rainfall, when it occurs, can be extremely violent, resulting in flash floods in gully systems. It might be expected, therefore, that burial of bones would be slow, transport of bones in gullies would be rapid, and that the extremes of weathering would result in the rapid breakdown of bones exposed on the surface.

In order to test these predictions, long-term studies of bones under differing environmental conditions are needed. Only one such study has been published (Behrensmeyer 1984, Trueman et al. 2004), and the environment of exposure is a tropical African savanna with higher rainfall and greater solar radiation. A long-term monitoring experiment (20 years) in a temperate environment is in preparation (Andrews and Armour-Chelu 1998), and the results suggest that in temperate climates both the rate and the type of taphonomic modification is very different from that observed in tropical Africa. The intention of the present study was to investigate the effects of intermediate environmental conditions in the mid-latitudes.

In the 1980s, we undertook a series of fossil collections in Qatar and Abu Dhabi, organized by Peter Whybrow. Although he prematurely and unfortunately died, the present work owes much to his dedication and capacity to embrace new ideas. In 1984, we set up a series of taphonomic trials to monitor the effects of desert environments on the preservation of bones (others described in Andrews and Whybrow, in press). Most of these trials failed as a result of human interference, but the one reported here was the discovery of a recently dead camel in a broad shallow valley on Jebel Barakah, a remnant hill of Miocene deposits by the edge of the Arabian Gulf. Jebel Barakah is in the far west of Abu Dhabi (Figure 1), close to the border with Saudi Arabia. The camel was in a secluded valley well away from any likelihood of human interference so that any modifications produced during the breakdown of the skeleton are likely due to natural causes. The following analysis is based on monitoring this single individual for a period of 15 years, from 1984 to 1998, and all results must be viewed as tentative since a larger sample was not available.

Material and Methods

Five surveys were made over a period of 15 years, the periods being dictated by times of expeditions to collect Miocene fossils. The first survey was made on January 5, 1984, and the last on November 8, 1998, when we completed our programme of field work in Abu Dhabi. The specimen studied was a young adult of the Arabian camel Camelus dromedarius (Harrison and Bates 1990) in a broad shallow gully on the lower slopes of Jebel Barakah (Figure 2): latitude 24º 00' 25.7"N, longitude 52º 19' 43.1"E. The cause of death of the camel and the exact date when it died are unknown, but the bones formed a compact cluster that had not been dispersed down the gully. Many of the bones were still covered in skin or other soft tissue and, from observations of other recently dead camels in Arabia and the extent of rainfall over the preceding few years, we estimated death was about two years earlier, some time in 1982. It was a young adult individual with the postcranial epiphyses fused but some of the vertebral epiphyses either unfused or partly fused.

A base line was set up along the line of the gully in which the skeleton was first found, and fixed marker points were used to ensure that the same line was used throughout for measuring dispersal of the bones. All visible bones were recorded on each of the five surveys, but no attempt was made to locate any bones below the surface until the final collection in 1998. Measurements were in two dimensions, including length along the datum line and distance perpendicular from the line. Angles of orientation were measured, although numbers of specimens were too small for reliable results. Angles of dip were not measured since most bones were resting on the surface of the soil, and their dip was a function of the angle of slope of the ground. Some bones became buried during the course of the monitoring of the skeleton, but also some bones previously buried were uncovered by the shifting of the sands covering much of the site. Weathering was recorded following the scale published by Behrensmeyer (1978), and other damage was assessed qualitatively, particularly the presence of root marks and other aspects of soil corrosion. Presence of carnivore tooth marks was investigated.

Description of the Site

All the bones were in or close to the bottom of a broad shallow gully, which slopes slightly north of west (282º) down towards the Arabian Gulf. The gully is cut by a shallow water channel about half a metre deep (Figure 2). The substrate is unconsolidated sand overlying Miocene deposits consisting at this level of moderately calcified silts and sands. The sparse vegetation consists of widely separated clumps of grasses and low-growing thorny shrubs (Figure 2). Most of the camel bones were lying on the north side of the gully on a gently sloping platform, but some had fallen into the vertical-sided channel that cuts along the platform. There were no bones on the south side of the channel.