LOCOMOTION IN FOSSIL CARNIVORA:
AN APPLICATION OF EIGENSURFACE ANALYSIS FOR
COMPARISON OF 3D SURFACES
We present a new geometric morphometric method called 'eigensurface analysis' for the quantitative analysis of three-dimensional (3D) surfaces. Eigensurface can be viewed as an extension of outline- and landmark-based geometric methods to deal with complete 3D surfaces of objects. We applied eigensurface analysis to the problem of functional inference based on the mammalian calcaneum bone, which is commonly preserved in isolation. Functional interpretations in vertebrate paleontology can be confidently drawn from relatively complete skeletons based on limb proportions and suites of locomotor characters. Interpretations drawn from isolated bones are often less certain, in part because the functionally important features on which the interpretations are based are the curvatures and angles of joint surfaces, which are difficult to quantify using standard linear measurements or 2D geometric morphometric approaches.
Eigensurface analysis allows the entire surface (or surface region) of a specimen to be analyzed by interpolating an evenly spaced grid of semi-landmark points from a standard 3D point-cloud, such as those generated by laser scanners. The same mathematically homologous point grid is fit into each object in a study, which allows the geometry of the grids to be analyzed in the same way one would standard landmark or outline point data. Eigensurface analysis also supports direct shape modelling within the ordination spaces formed to represent shape similarities, thus forming a critical bridge between these mathematical spaces and the qualitative assessment-comparisons of the shapes they represent.
We used the eigensurface method to characterize the calcaneum shape of modern carnivorans in respect to stance, number of digits, and locomotor style. We then quantitatively matched the calcanea of four extinct species to those categories to infer stance, digit number, and locomotor style. Four taxa—Ictitherium, an extinct hyaena, Enhydriodon, an extinct otter, Paramachairodus, an extinct sabre-toothed cat, and Cynelos, an extinct amphicyonid—whose anatomy and locomotion are known from independent evidence, were used to assess the effectiveness of eigensurface analysis of individual calcanea. Results allowed us to infer the number of toes, the stance, and the locomotor mode of all four of the taxa correctly. Cynelos was incorrectly inferred to have been digitigrade, and Enhydriodon was incorrectly inferred to have been terrestrial.
P. David Polly. Department of Geological Sciences, Indiana University, 1001 E. 10th Street
Bloomington, IN 47401 USA
Norman MacLeod, Department of Palaeontology, The Natural History Museum, Cromwell Road
London SW7 5BD United Kingdom
KEYWORDS: Carnivora, Eigensurface, Geometric Morphometrics
PE Article Number: 11.2.10A
Copyright: Palaeontological Association July 2008
Submission: 4 April 2008. Acceptance: 22 June 2008