The effect of weathering on bird bone survivorship in modern and fossil saline-alkaline lake environments

Abstract

A modern Lesser Flamingo (Phoeniconaias minor) assemblage was collected along the shoreline of Lake Emakat, a saline-alkaline lake in northern Tanzania. Taphonomic analysis found the assemblage to be heavily weathered. This is likely due to the bone's heightened exposure to solar radiation and corrosive soil and water chemistries, as is expected to occur in such depositional environments.Analysis found that deep, wide, longitudinal cracks penetrate the medullar cavities of both weathered and unweathered long bones. The cause and taphonomic consequence of these cracks are addressed here, using data from Lake Emakat and from controlled studies. Results support repeated (episodic) submersion, followed by drying, as the causal mechanism behind thesewet-drycracks. Mineral salt uptake by bone may explain the early appearance and prevalence of these cracks in saline-alkaline lake settings, as compared to other depositional settings.The rate of weathering and incidence of wet-dry cracking varies significantly across limb elements. This difference correlates to element specific resistance properties to external loading forces. Heavy weathering weakens the structural integrity of bone and can accelerate its fragmentation. This can lead to bird bone loss in nearshore and ephemeral wetland settings, which may then affect resulting skeletal part, diversity, and richness profiles. Heavy weathering can therefore obscure important taphonomic and paleoecological information.The weathering data collected here are then applied to a fossil bird assemblage from the FLK Complex, (late Pliocene), Olduvai Gorge, in Tanzania. Results provide evidence for the effect of weathering on paleoecological and behavioral interpretations. Weathering should be considered when analyzing fossil bird assemblages.

The paleoecology of Pleistocene birds from Middle Bed II, at Olduvai Gorge, Tanzania, and the environmental context of the Oldowan-Acheulean transition

“…Prassack (2011) further shows that bone shaft thickness, pneumaticity, shape, and collagen fiber orientation may affect a bone's susceptibility to weathering, and especially weather-related cracking, and that this can skew skeletal part profiles of avifaunal assemblages by disproportionately destroying distal limb elements. Bone density also has a high likelihood of affecting taxon-specific differential survivorship (Livingston, 1989;Prassack, 2011), but quantitative data on density-mediated differential survivorship across taxa is limited to non-African birds (Dirrigl, 2001;Cruz and Elkin, 2003;Dumont, 2010;Ksepka et al, 2015). Body size, locomotion type, and other variables may also have an effect on resulting skeletal part profiles.…”

“…The taphonomic histories of avifaunal remains provide information that, in conjunction with the community profile, can be used to make stronger inferences about a site's environmental context (Emslie et al, 1996;Behrensmeyer et al, 2003;Cruz, 2007;Prassack, 2010Prassack, , 2011Prassack, , 2014. The taphonomic agents most likely to alter the community profile through differential destruction and/or transport of a birds bones are weathering, carnivoran feeding, and fluvial transport.…”

“…Wing (humerus, ulna, and carpometacarpus) over leg (femur, tibiotarsus, and tarsometatarsus) abundance was used to infer skeletal part bias resulting from some taphonomic phenomenon (otherwise wings/wings+legs=0.5; Ericson, 1987). Wing abundance is often attributed to anthropogenic causes (Gotfredsen, 1997;Bovy, 2002), but carnivores can produce a similar profile (Livingston, 1989;Oliver and Graham, 1994;Prassack, 2011). Prassack (2011) further shows that bone shaft thickness, pneumaticity, shape, and collagen fiber orientation may affect a bone's susceptibility to weathering, and especially weather-related cracking, and that this can skew skeletal part profiles of avifaunal assemblages by disproportionately destroying distal limb elements.…”

Patterns of Aquatic Decay and Disarticulation in Juvenile Indo-Pacific Crocodiles (Crocodylus Porosus), and Implications for the Taphonomic Interpretation of Fossil Crocodyliform Material

“…The benefit of actualistic taphonomy is that the time taken to progress through the five stages of vertebrate decay --(1) fresh, (2) bloated, (3) active decay, (4) advanced decay, and (5) remains (modified from Payne, 1965;Anderson and Hobischak, 2004;Cambra-Moo et al, 2008) --along with the level of skeletal articulation during these five stages, can be observed in real time, providing an "…empirical database of cause (taphonomic process) and effect (preservational bias)" (Allison et al, 1991, p. 78). A combination of relational analogies and actualistic taphonomy have been successfully used to interpret the taphonomic histories of fossil fish (Elder and Smith, 1984;Elder, 1985;Elder et al, 1988;Hellawell and Orr, 2012;Iniesto et al, 2013), reptiles (Brand et al, 2003a,b;Beardmore et al, 2012a,b;Meyer, 2012;Richter and Wuttke, 2012;Smith and Wuttke, 2012), mammals (Weigelt, 1989;Brand et al, 2003b;Noto, 2009;Behrensmeyer and Miller, 2012;Schwermann et al, 2012), and avian dinosaurs (Davis and Briggs, 1998;Brand et al, 2003b;Cruz, 2007;Faux and Padian, 2007;Prassack, 2011). Even with the potential pitfalls of assuming taphonomic uniformitarianism (see discussion in Gifford, 1981;Elder et al, 1988;Brasier et al, 2011), actualistic taphonomy can provide a more powerful analytical tool than analogical reasoning alone (Young, 1989;Denys, 2002;Noto, 2009).…”

Reconstruction of the burial position of two hominin skeletons (Australopithecus sediba) from the early Pleistocene Malapa cave site, South Africa

“…The flow took place after the skeletons had reached the dry stage of decay, and had likely undergone partial natural mummification. The occurrence of superficial or slightly deeper cracks on some of the bone surfaces is consistent with weathering in a moist (Andrews & Armour‐Chelu, ; Ross & Cunningham, ; Stone, Dickel, & Doran, ; Tappen, ) and dark cave, and indicative of a period of subaerial exposure before final deposition within the debris flow (Behrensmeyer, ; Crow, ; Janjua & Rogers, ; Junod & Pokines, ; Lyman & Fox, ; Prassak, ). This indicates that both hominins had reached the stage of decomposition where only desiccated tissue and bones remain, a condition supported by the preservation of both unstable and permanent articulations.…”