Fig. 4.

Alpha-shapes fitted to three example bacula. a, mustelid; b, canid; c, ursid. As refinement coefficient is decreased, the volume of alpha-shapes (as a percentage of CT voxel volume) decreases. i, when this value drops below 100, the alpha-shape has ‘broken down’ and the fit passes internally of the point cloud; ii, the ‘optimal’ refinement occurs when alpha volume is exactly equal to CT volume; iii, an intermediate fit alpha-shape defined as halfway between ‘optimal’ alpha and the cnvex hull describes some coarser geometric features, such as the curvature of the mustelid baculum, but misses finer-scale detail such as the canid urethral groove; iv, the coarsest alpha-shapes are equivalent to convex hulls, fitted only to the outermost extremes of the point cloud and representing gross morphology. Due to the curved nature of the mustelid baculum, coarse alpha-volume is considerably greater than the CT voxel volume