Historical Constraints and the Evolution of Development 1041
represented a chronological extension of the life cycle (with juvenilization of form
then unresolvable in mode of origin), or an increase in rates of growth over an
unchanged length of life (with the paedomorphic result then attributable to the
heterochronic process of neoteny based on prolongation of rapid juvenile growth
rates and attendant retention of characteristic morphologies associated with these
rates).
I had the privilege of working with Douglas Jones, who developed the first
reliable procedures for inferring ages from growth banding (by matching isotopic
cycles, interpreted as seasonal, with morphological banding, Fig. 10- 2 —because
simple counts of banding, the standard procedure of past sclerochronological study,
had never yielded firm results). We were able to break this conceptual logjam by
determining the ages of shells throughout the trend (Jones and Gould, 1999), and
resolving the problem, thanks to unusual cooperation from nature (who rarely
provides clear answers at one extreme of a potential continuum). The larger adult
shells of later phylogenetic stages showed no increase at all in length of life, but died
at the same age as adult shells in the earliest stages in the trend (Fig. 10-3). Thus, we
could identify the correlated phenotypic trend in size and shape as a genuine case of
neoteny
10 - 1. Paedomorphosis in lower Jurassic Gryphaea. The left sequence (top to bottom) shows
ontogenetic stages of the ancestral species drawn at the same size as adults of the phylogenetic
series (the right sequence, bottom to top). From Gould, 2000e; adapted from Hallam.