THREE-DIMENSIONAL RE-EVALUATION OF THE DEFORMATION REMOVAL TECHNIQUE BASED ON "JIGSAW PUZZLING"
Retrodeformation is the process of removing distortions in fossils caused by tectonic or overburden stresses. These methods are very important for paleontologists because they can be used to estimate true fossil shapes necessary for studies in systematics, phylogenetics, and functional morphology. Deformation may be brittle, breaking the specimen apart but leaving the original shape of the broken pieces unaltered, or plastic, altering the original shape without breakage. The effects of plastic deformation are often overlooked in fossils with extensive brittle deformation. Anthropological studies dealing with brittlely-deformed bones tend to rely on simple "jigsaw puzzle" reconstruction, in which fractured pieces are manually or digitally placed back together. These methods assume that most of the deformation is brittle, and that the original fossil shape can be restored without plastic retrodeformation. We tested the validity of jigsaw puzzle reconstruction by using three-dimensional (3-D) computational techniques. More specifically, we examined if plastically deformed skull pieces can be arranged to form a 'symmetrical skull' that morphologically differs from the true skull under a digitally controlled environment. A cranium of a Woolly Monkey (Lagothrix lagotricha) was digitized using a 3-D laser scanner. We virtually fragmented and then plastically deformed this skull to create skull pieces with known plastic deformation. We found that these pieces fit well onto the true skull and an incorrect skull shape equally well, suggesting that "jigsaw puzzle" methods can lead to inaccurate specimen reconstruction that superficially appears 'correct'. We conclude the plastic deformation must be removed before "jigsaw puzzle" fossil reconstruction.
Alec A. Boyd. Department of Geology University of California Davis, One Shields Avenue, Physics/Geology Davis California, 95616, United States
Ryosuke Motani. Department of Geology University of California Davis, One Shields Avenue, Physics/Geology Davis California, 95616, United States
KEY WORDS: deformation, removal, reconstruction, hominids
PE Article Number: 11.2.7A
Copyright: Palaeontological Association July 2008
Submission: 8 January 2008. Acceptance: 20 May 2008