Intrataxonomic variability in metapodial and femur bone density related to age in guanaco (Lama guanicoe). Zooarchaeological and taphonomical implications

The aim of this paper is to present new information about bone mineral density (BMD) variability in guanaco (Lama guanicoe), a South American wild camelid; while taking into account its ontogenic development. The importance of considering intrataxonomic variability in bone density related to age for constructing zooarchaeological indices and applying them to archaeological assemblages is also discussed. The results presented here were developed from a large sample of 46 metacarpals and 45 femurs of modern guanacos, ranging in age from ca. 6 months of gestation to 12 years. Bone structural densities were measured using a biphotonic X-ray densitometer, DEXA method, Norland EXCELL and the bone mineral content (BMC) of each scanned portion or complete element was divided by its volume, which was obtained independently by water displacement. Our results indicate that the bone density of each anatomical unit is highly variable, even among individuals of the same species. According to metacarpals data, age is an important factor affecting BMD. Moreover, our study shows that the density ranges of different skeletal elements can overlap, suggesting that it is not possible to predict that a certain element is always denser than another one. Finally, we present new evidence about the potential age-related taphonomic bias in faunal assemblages, indicating that meanwhile some skeletal elements show clear differences in their density between their fused and unfused state (metacarpal) other do not (femur).     

Fluvial dispersal potential of guanaco bones (Lama guanicoe) under controlled experimental conditions: the influence of age classes to the hydrodynamic behavior

Hydrodynamic sorting is a taphonomic process able to transport and scatter bones deposited in archaeological and paleontological sites. This study presents the results of experimentation performed in an artificial flume with guanaco (Lama guanicoe) bones of different ontogenetic development, dry and saturated in water, in hydric flows velocities of 15 and 30 cm/s. The obtained results show that bone global density, the age of the individual, the dry or wet bone state, and the hydric flow velocity influence significantly bone dispersion. In this way, bones from immature individuals with unfused secondary growth centers and relatively low bulk density have better possibility of being transported than fused bones from adult individuals. Taking into account the results obtained in this experimentation and the feasibility of discriminating age categories in fossil assemblages, two bone groups with differential potential transport are presented in this paper. These transport groups constitute a methodological tool to evaluate the role hydric current may had played in the formation of a fossil assemblage.