Orientation patterns of wildebeest bones on the lake Masek floodplain (Serengeti, Tanzania) and their relevance to interpret anisotropy in the Olduvai lacustrine floodplain Lucía Cobo-Sánchez a , Julia Aramendi a , Manuel Domínguez-Rodrigo a, b, * a Department of Prehistory, Complutense University, Prof. Aranguren s/n, 28040 Madrid, Spain b IDEA (Instituto de Evolución en África), Museo de los Orígenes, Plaza de San Andrés 2, 28005 Madrid, Spain article info Article history: Available online xxx abstract A study of bone orientation patterns at a wildebeest bonebed deposited in the floodplain of Lake Masek (Tanzania) shows that portions of the assemblage exhibit anisotropy probably caused by the combined action of lake transgression and regression, and the watersheet formed during the rainy season, which is also responsible for a large part of the microtopographical features of the surface where the bonebed was formed. Some taphonomic indicators (absence of polished or abraded bone, similar representation of Voorhies’ anatomical groups I and III) show that the anisotropic trend was formed under very low- energy conditions. Anisotropy can be locally produced in autochthonous assemblages. Bone orienta- tion patterns need to be combined with other taphonomic techniques to determine the depositional nature of any given assemblage. Ó 2013 Elsevier Ltd and INQUA. All rights reserved. 1. Introduction Mass drownings of wildebeest at Lakes Masek and Ndutu have long been known in ecological studies of the Serengeti (Tanzania) (Sinclair, 1979). Located in the north-west side of the Ngorongoro Conservation Area, Lake Masek is surrounded by the vast plains of Serengeti National Park. These drownings have been documented during the crossing of the lake in the wet season. The recurrent nature of these drownings is linked to the migratory path that wildebeests undertake by crossing Lake Masek in their annual movement towards the northern grassland plains. In 1973, more than 3000 calves were documented to have died during the lake crossing (Sinclair, 1979). This and subsequent death events have enabled the formation of an authentic bonebed, which is charac- terized by being autochthonous, by having an extremely dense concentration of bones (see Fig. 1) and by displaying a biased skeletal part profile due to the combined action of the lake water, the watersheets formed during the rains and by carnivores consuming and transporting away determined elements. From a paleontological point of view, this modern assemblage is an opportunity to create an analogical framework, which can contribute to the interpretation of prehistoric bonebeds. A pre- liminary taphonomic study was carried out on this area by Capaldo and Peters (1995) who quantified the number of individuals and skeletal parts. Despite the large concentration of subadult in- dividuals previously documented (Sinclair, 1979), Capaldo and Peters (1995) found that adults were better represented skele- tally. This is accounted for by the accumulation being a palimpsest of multiple depositional events, which respond to mass deaths of hundreds of individuals and sequential deaths of single individuals or small groups. Capaldo and Peters (1995) also documented that most skeletal elements were complete and that axial and long bones were predominant. Previous taphonomic research on a similar bone concentration of wildebeest was carried out by Dechant-Boaz (1982) on the Mara River (Kenya), where even excavation at selected spots was conducted. Rapid sedimentation in this alluvial setting enabled the preservation of large numbers of axial elements as well as fairly complete skeletons. The extensive accumulation of carcasses in Lake Masek occurs on the lacustrine floodplain, which during the rains becomes a mudflat. Capaldo and Peters (1995) report that most of the accu- mulation occurred along w400 m on a narrow band of w20 m. They also documented that the accumulation was highly variable and that the floodplain was affected by several erosion features, such as shallow channels (i.e., rills) and sedimentary fractures/steps as can be observed today. In their study, Capaldo and Peters carried out a selective survey along this dense strip of the bonebed, and an * Corresponding author. Department of Prehistory, Complutense University, Prof. Aranguren s/n, 28040 Madrid, Spain. E-mail address: m.dominguez.rodrigo@gmail.com (M. Domínguez-Rodrigo). Contents lists available at ScienceDirect Quaternary International journal homepage: www.elsevier.com/locate/quaint 1040-6182/$ e see front matter Ó 2013 Elsevier Ltd and INQUA. All rights reserved. http://dx.doi.org/10.1016/j.quaint.2013.07.130 Quaternary International xxx (2013) 1e8 Please cite this article in press as: Cobo-Sánchez, L., et al., Orientation patterns of wildebeest bones on the lake Masek floodplain (Serengeti, Tanzania) and their relevance to interpret anisotropy in the Olduvai lacustrine floodplain, Quaternary International (2013), http://dx.doi.org/ 10.1016/j.quaint.2013.07.130