MORPHOLOGICAL VARIABILITY AND DEVELOPMENTAL CONSTRAINTS IN CRINOID ARMS

The purpose of this study is to evaluate within-individual and between-individual variability in the morphology of crinoid arms. Some phenotypic variation in identical structures always exists within an individual organism; the source of this variation, therefore, must lie in the interactions between its development and the environment. This provides a way to look at developmental and functional constraints on growth and variation within repeated structures. For instance, fluctuating asymmetry (FA) is a method used in both paleontology and neontology to estimate the impact of developmental noise during the course of ontogeny. FA measures the deviation from bilateral symmetry in organisms (or paired structures within a single organism) from morphometric data. This is then used to explore such topics as developmental instability, resilience to environmental stressors, and selection on patterns of phenotypic integration.

However, echinoderms display several different versions of radial symmetry, their construction is modular, and in most cases the plates making up the test are variable in number and shape, which makes landmarks difficult to determine and hypotheses of homology difficult to justify. This means that methods devised for continuous and bilaterally symmetrical morphometric data with homologous landmarks are difficult to apply to many echinoderms. For this reason we propose an alternative, non-landmark-based approach to geometric morphometrics as a more suitable tool for analysis of organisms with modular growth. Here we describe a method of encoding crinoid arms as trees, then calculate the distance between the trees using an existing algorithm from computer science. This yields a quantitative measure of dissimilarity between the arms in any individual crinoid, from which we can find a radial asymmetry value for the whole individual analogous to values of fluctuating asymmetry among bilaterians. This dissimilarity measure is then used to describe changes in developmental constraint over the ontogeny of the arms of one extant species of crinoid, as well over time in a well-sampled fossil lineage.