ALLOMETRIC SPACE AND ALLOMETRIC DISPARITY: A NOVEL DEVELOPMENTAL PERSPECTIVE IN THE MACROEVOLUTIONARY ANALYSIS OF MORPHOLOGICAL DISPARITY

Over the last two decades, morphological disparity – a quantitative measure of the distribution of taxa in morphospace – has become a major research subject in Evolutionary Paleobiology. Taxonomic diversity and morphological disparity patterns have been documented and contrasted in numerous clades, but mostly by reference to adult phenotypes. In parallel, the ontogenetic component of morphological evolution has been early recognized of significant importance and has led to a rich and active research program (first under the heading of heterochrony, and now within the integrative field of Evolutionary Developmental Biology). Curiously, articulating both approaches has received little attention to date.

Here, I propose the use of allometric spaces – multidimensional spaces spanned by allometric coefficients – as a valuable new tool for investigating the role of development in shaping the evolution of morphological disparity. From their exploration can be derived measures of allometric disparity, complementing standard signals of morphological disparity, through a straightforward transposition of established analytical protocols.

Most evolutionary modifications of ontogenies, when recast in the context of allometry, are reducible to a few relevant types. Allometric spaces can thus be used to infer these types of developmental changes and assess their relative prevalence. These spaces offer a novel domain of investigation of phenotypic variation and should help in detecting temporal and phylogenetic trends, and studying major macroevolutionary phenomena in an explicitly developmental context.

The Ammonoidea at the Early-Middle Jurassic transition were chosen as a case study to illustrate the use of allometric spaces. I built two phenotypic spaces: a static, adult one (adult morphospace) and a dynamic, developmental one (allometric space). Disparity analyses of these spaces and their differential filling through time show that the whole clade manifests a strikingly constant occupation of both spaces, despite extensive taxonomic renewal. Further, partial disparity analyses of the main subclades reveal marked differences in their morphological and allometric disparity kinetics, and allow insights into morphological diversification intractable from the analysis of adult morphospace alone.