How many ways are there to measure biological shape?

New research supports the use of cladistic data for quantifying the range of skull variation in a group of legless amphibians – the caecilians.

The rich and diverse array of biological shapes around us is astounding. Shape, however, may be an elusive concept. What do we actually measure, and how?

Dr Marcello Ruta, from the School of Life Sciences, University of Lincoln, UK, was part of a team who conducted an exploratory study of morphological disparity (i.e. range of shape variation), employing both traits that are used for reconstructing evolutionary family trees and geometric morphometric approaches. For their study, they used caecilians, a group of superficially earthworm-like amphibians distributed in the tropical regions of the southern hemisphere.

Cladistics is a branch of systematics (the field of biological classification) that seeks to reconstruct evolutionary family trees using information on characters (traits) observed in organisms and grouping them together based on whether or not they share unique characteristics inherited from a common ancestor. However, those traits hold great promise in that they can also be employed to quantify morphological differences, and these differences are the basis for analyses of shape disparity.

Traditionally, morphometric analyses can be used for quantifying disparity, for instance via linear measurements or constellations of data points, or landmarks. However, cladistic data offer a bonus, in that they can be used when measurements or landmarks cannot be readily applied to vastly divergent organisms.

Dr Ruta said: “Analyses of disparity requires a scalable comparative framework. The difficulties of applying geometric morphometrics to disparity analyses of groups with vastly divergent body plans are overcome partly by the use of cladistic characters. In all instances, we found no statistically significant difference. This suggests that cladistic and geometric morphometric data appear to summarise morphological variation in comparable ways. Our results support the use of cladistic data for characterising organismal disparity.”

The paper ‘Do cladistic and morphometric data capture common patterns of morphological disparity?’ has been published in the journal Palaeontology.