Confronting shell morphodynamics and sclerochronology: leaving behind phenomenology to move toward integrative laws of growth

Sclerochronological methods are described as a means to reconstruct, from increments recorded on a shell transect (i.e. "sclerochonological growth"), the fluctuations in past environments. This was proposed by an erroneous analogy with tree-ring dendrochronology. However, shells do not grow like trees. Almost all molluscan shells grow by adding increments at the shell edge, while preserving their shape. This is called "terminal growth". The advantage of this property is that there is a generic mathematical model that can quantify the shell expansion (morphological growth). 

Nonetheless, this generic model is not compatible with observed shell incrementation and accretion processes. This is because increment widths should increase geometrically in the mathematical model along a transect. We remarked that despite studying the same object, morphodynamics and sclerochronology have followed divergent paths in past decades, without incorporating advances of the other speciality. 

We have now addressed this problem head-on by developing a new mathematical framework to combine incremental shell growth within shell morphodynamics. This model is designed to be able to confront a theoretical prediction of shell incrementation with a measured one. The method combines morphodynamic modelled shapes with 3D shell scans and increments measured from shell transects. 

Our work in this area highlights several heretofore unrecognized fundamental problems in morphodynamics and sclerochronology which concern the way individual variability is accounted for in both areas. Regarding the reconstruction of environmental trends, we find that the arbitrary choice of one particular "best" shell transect and averaging over groups of individuals can be a source of significant bias. It is time to revise shell science methods to consider the entire incremental geometry (or growth 'ring') so that unbiased estimates of environmental changes can be provided using sclerochonological data.