Effects of taphonomic deformation on geometric morphometric analysis of fossils: a case study using the dicynodont Diictodon feliceps (Therapsida, Anomodontia)

Taphonomic deformation, the distortion of fossils as a result of
geological processes, poses problems for the use of geometric
morphometrics in addressing paleobiological questions. Signal from
biological variation, such as ontogenetic trends and sexual dimorphism,
may be lost if variation from deformation is too high. Here, we
investigate the effects of taphonomic deformation on geometric
morphometric analyses of the abundant, well known Permian
therapsid Diictodon feliceps.
Distorted Diictodon crania can be categorized into seven
typical styles of deformation: lateral compression, dorsoventral
compression, anteroposterior compression, ‘saddle-shape’ deformation
(localized collapse at cranial mid-length), anterodorsal shear,
anteroventral shear, and right/left shear. In simulated morphometric
datasets incorporating known ‘biological’ signals and subjected to uniform
shear, deformation was typically the main source of variance but accurate
‘biological’ information could be recovered in most cases. However, in
empirical datasets, not only was deformation the dominant source of
variance, but little structure associated with allometry and sexual
dimorphism was apparent, suggesting that the more varied deformation
styles suffered by actual fossils overprint biological variation. In a
principal component analysis of all anomodont therapsids,
deformed Diictodon specimens exhibit significant
dispersion around the ‘true’ position of this taxon in morphospace based
on undistorted specimens. The overall variance associated with
deformation for Anomodontia as a whole is minor, and the major axes of
variation in the study sample show a strong phylogenetic signal
instead. Although extremely problematic for studying variation in
fossil taxa at lower taxonomic levels, the cumulative effects of
deformation in this study are shown to be random, and inclusion of
deformed specimens in higher-level analyses of morphological disparity are
warranted. Mean morphologies of distorted specimens are found to
approximate the morphology of undistorted specimens, so we recommend use
of species-level means in higher-level analyses when possible.