A QUANTITATIVE ANALYSIS OF FRACTURE PATTERNS DUE TO TRAMPLING ON MICROVERTEBRATES

In order to interpret the paleoecology of past ecosystems, it is necessary to understand the differential influence of taphonomic processes on element, and hence taxon preservation. One such important taphonomic process is breakage due to trampling. Variation in trample-induced modification may relate to bone shape, bone size, substrate type, and bone condition. Many studies have examined the effects of trampling on large vertebrate animals, such as cows and sheep, but few have addressed trampling effects on smaller vertebrates – those that are commonly used in paleoecological reconstruction. This study examines and compares the patterns of trample-induced bone breakage among three different taxa and size classes under controlled experimental conditions. Two mouse (Mus musculus), fish (Lepomis macrochirus), and mink (Neovison vison) skeletons were cleaned of flesh by dermestid beetles to avoid cuts or fractures made by tools. Skeletons were then disarticulated and one skeleton for each taxon was weathered in an oven at 70°C for three hours. An acting mass was applied to the weathered and unweathered bones until broken to simulate trampling in a controlled manner. Bones for each skeleton were classified into four shape classes: equant, columnar, platy or irregular. The number of bone fragments was counted for each bone shape class and then a fracture type was identified for each fragment. The nine fracture types used include sawtoothed, V-shaped, flaking, irregular perpendicular, smooth perpendicular, spiral, longitudinal, columnar, pulverized. Under controlled conditions, the mink bones broke into fewer fragments compared to the mouse or fish, suggesting that bone size plays a role in controlling trample-induced bone fragmentation. Weathered bones produced fewer fragments in all but the columnar bone class across the three taxa. Bone size also appears to influence the amount of acting mass needed to break bones; the mink needed a greater mass to induce breakage than the mouse.