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

Abstract

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.

Three-dimensional geometric morphometric analysis of the skull of Protoceratops andrewsi supports a socio-sexual signalling role for the ceratopsian frill

“…In common with previous studies, size was found to be strongly correlated with whole-skull shape in P. andrewsi [ 13 , 32 , 33 ], accounting for around 52% of total shape variation in this dataset and reflecting drastic changes in skull shape during ontogeny [ 28 ]. Although all modules showed some degree of change, the parietal-squamosal frill consistently displayed the highest rates of size and shape change through ontogeny, as well as the highest morphological variance independent of size, expanding significantly during ontogeny to form a large, conspicuous structure ( figure 3 a,b ).…”

“…PC4 (4.9% of total shape variation) shows obvious signs of taphonomic deformation, i.e. flattening of the skull and deformation of the orbit [ 32 ] (electronic supplementary material, figure S6). Although arguably not as clear in PCs 2 and 3, it is likely that effects of taphonomic deformation are distributed more subtly among these and subsequent PC axes.…”

“…All fossil specimens are potentially subject to taphonomic deformation [ 32 ]. The dilation component of taphonomic deformation describes symmetrical plastic deformation, resulting in dorsoventral or lateral compression of specimens.…”

“…This is found in socio-sexual traits in a wide range of extant taxa from the dung beetle genus Onthophagus to bovids such as Connochaetes [ 19 ]. When dealing with fossil datasets, it is also important to consider the effects of taphonomic deformation, which may increase morphological variance [ 32 ].…”

A revision ofProlimulus woodwardiFritsch, 1899 with comparison to other highly paedomorphic belinurids

“…Here, we demonstrate this condition by examining exclusively belinurid species: the constructed morphospace is polarized by species with genal spines (e.g., Euproops and Belinurus ) and those lacking the morphology (e.g., Prolimulus , Alanops ). Although morphospace is impacted by taphonomic modification of fossils ( Kammerer et al, 2020 ), this is not apparent within the first two PCs ( Bicknell et al, 2019b ). Furthermore, as the cuticular xiphosurid exoskeleton requires exceptional preservation conditions, these fossils are seldom subject to the tectonic strain observed in trilobites ( Cooper, 1990 ; Hughes & Jell, 1992 ).…”