Compaction-related deformation in Cambrian olenelloid trilobites and its implications for fossil morphometry

Morphometric analyses of silicified and nonsilicified (preserved in shale) specimens of the olenelloid trilobites Olenellus (Olenellus) gilberti Meek (in White, 1874) and Nephrolenelius geniculatus Palmer, 1998, from the Lowe r Cambrian C-Shale Member of the Pioche Formation show that even well-prese rved specimens in shales have undergone significant changes in lateral as w ell as vertical dimensions as a result of compaction. Analyses of cephalic landmarks show that in both species compaction causes posterior-directed co llapse of the anterior lobe of the glabella, adaxial deformation of the ocu lar lobes, and abaxial and anterior splaying of genal regions. These shape changes are explicable in terms of observed exoskeletal fracture patterns. Landmarks show an increase in scatter around their ontogenetic trajectories that is generally proportional to the degree of lateral shift each landmar k has undergone. Interspecific differences in compactional response may dep end on the relative convexity of the cephalon. Olenellus (Olenellus) gilber ti is a low-convexity species and shows may ked lateral shape change, parti cularly in the genal region. Nephrolenellus geniculatus is more convex and shows less severe lateral shape change. Landmarks of both species exhibit a n average trebling of the degree of scatter around their average ontogeneti c trajectories in compacted samples. Because even well-preserved specimens in shales differ in shape from their pre-compactional appearance, results o f morphometric studies utilizing metric distances between landmarks in tril obites where compaction can be detected must he interpreted with caution.