We have known since Trueman's classic work of 1922 that the Lower Jurassic Gryphaea of Britain exhibit phyletic size increase and heterochronic change in shape. Since Hallam's revisionary work in the 1960s, we have recognized that pronounced and generalized juvenilization of form accompanied this increasing size. This extensive literature provides invertebrate paleontology's most famous example of a biometrically documented, continuous anagenetic trend within a discrete lineage. But Gryphaea has also provoked great frustration because a key datum, required for a full solution, had been theoretically recognized but practically unavailable. We could identify the evolution of shape as paedomorphic, but could not specify the mode of heterochrony for this paedomorphic result because we could not standardize samples by common age or developmental stage. In this paper, we provide sclerochronological data on sizes and shapes at specified ages marked by annual growth bands in two Jurassic sequences of Gryphaea: the classic Lower Jurassic series showing phyletic size increase with paedomorphosis, and an independent Middle-Upper Jurassic series illustrating neither size increase nor heterochrony. We prove that size increase in the classic series occurs entirely by faster growth (larger descendant sizes at the same ages as ancestors), and not by extended age (for descendants lived no longer than ancestors). The well-marked paedomorphosis of form probably arose as a correlated consequence of growing larger by extending and maintaining rapid juvenile growth rates-thus marking the heterochronic mode as a case of neoteny. The independent upper sequence, acting as a different replicate in a natural experiment, shows neither size increase nor heterochrony but does exhibit (in contrast with the classic sequence) evolution toward greater longevity. Hallam's flow tank experiments indicated a strong adaptive advantage in shell stability for both larger size and paedomorphic form. Neotenous development provides an evolutionary pathway to the simultaneous acquisition of both favored traits--thus showing that "constraints" due to "correlations of growth" (Darwin's own phrase for the phenomenon) may be positive in promoting joint evolutionary advantages, and not only neutral (in carrying spandrels along with primary adaptations), or negative (by imposing inadaptive "baggage" upon trends in form through developmental correlation with selected traits).