Morphological and Developmental Disparity in Jurassic Ammonites

Over the last two decades, the concept of morphological disparity, i.e., the morphological signal of biodiversity, has proved to be an invaluable source of information, complementing taxonomic approaches, and enriching our knowledge of large-scale clade dynamics in both paleo- and neontological contexts. Current analyses of morphological disparity, now widely applied, are focused on characterizing morphospace and its differential occupation through space and time.

Hypotheses about processes responsible for documented disparity patterns have mostly been cast in terms of external factors, while potential developmental explanations have received much less attention. This results in large part from the fact that most morphospaces and disparity studies have traditionally emphasized sampling of adult shapes. More extensive consideration of the developmental aspects underlying the emergence of these adult forms is now needed to allow a broader and more balanced array of inferences.

Here, I explore the disparity dynamics of Early Jurassic ammonites over the Pliensbachian-Toarcian interval (189.6 to 175.6 Ma). During this period, ammonites underwent two successive and drastic declines in taxonomic diversity. Investigation of the deployment of taxa in adult morphospace and developmental morphospace (allometric space) allows the appraisal of the temporal evolution of morphological and allometric disparities. Comparisons of taxonomic and disparity curves reveal marked discordances suggesting non-selective extinction patterns at both morphological and developmental levels. Another measure of allometric disparity indicates the occurrence of a peramorphocline followed by a paedomorphocline during the Toarcian. These trends are concomitant with significant changes in average adult size that compensate the heterochronic effects and explain the relative constancy of morphological disparity. The results also point out the existence of two contrasted evolutionary dynamics in Pliensbachian and Toarcian ammonites.