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

Published online by Cambridge University Press:  20 May 2016

Mark Webster  and
Nigel C. Hughes
Mark Webster
Affiliation:
Department of Earth Sciences, University of California, Riverside, California 92521
Nigel C. Hughes
Affiliation:
Department of Earth Sciences, University of California, Riverside, California 92521
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Abstract

Morphometric analyses of silicified and nonsilicified (preserved in shale) specimens of the olenelloid trilobites Olenellus (Olenellus) gilberti Meek (in White, 1874) and Nephrolenellus geniculatus Palmer, 1998, from the Lower Cambrian C-Shale Member of the Pioche Formation show that even well-preserved specimens in shales have undergone significant changes in lateral as well as vertical dimensions as a result of compaction. Analyses of cephalic landmarks show that in both species compaction causes posteriordirected collapse of the anterior lobe of the glabella, adaxial deformation of the ocular 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 landmark has undergone. Interspecific differences in compactional response may depend on the relative convexity of the cephalon. Olenellus (Olenellus) gilberti is a low-convexity species and shows marked lateral shape change, particularly in the genal region. Nephrolenellus geniculatus is more convex and shows less severe lateral shape change. Landmarks of both species exhibit an average trebling of the degree of scatter around their average ontogenetic trajectories in compacted samples. Because even well-preserved specimens in shales differ in shape from their precompactional appearance, results of morphometric studies utilizing metric distances between landmarks in trilobites where compaction can be detected must be interpreted with caution.

Type
Research Article
Information
Journal of Paleontology , Volume 73 , Issue 2: Papers from the Second International Trilobite Conference, August 1997 , March 1999 , pp. 355 - 371
Copyright
Copyright © The Paleontological Society 

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