Of coiled oysters and big brains: how to rescue the terminology of heterochrony, now gone astray

SUMMARY During the past decade, the terminology of heterochrony, heretofore consistent and workable, has become internally illogical and incoherent as the unfortunate result of an extension of terms, properly devised to describe shifts in developmental timing of shapes and features, to the rates and timings that cause these shifts. All the resulting, and extensive, confusion in the literature arises as a pure consequence of this error in logic and nomenclature, and not at all from disagreement about the important empirical questions described by this central concept and phenomenon in the integration of evolution and development. In particular, the claim that the same feature in human evolution (the paedomorphic shape of the human cranium) expresses either neoteny or the apparently opposite phenomenon of hypermorphosis only records the terminological error, and not any factual disagreement—for this neotenic feature has probably arisen by a prolongation of juvenile growth patterns inappropriately designated as “hypermorphosis of rate.” I show that a prominent and unchallenged case of neoteny in fossil oysters arises by exactly the same evolutionary mode. When we restore the terminology of heterochrony by the “paedomorphic” intellectual event of dropping these inadaptive terminal accretions (the illogical extension of shape categories to describe rates), then the concept of heterochrony will again make proper distinctions by designating a clearly meaningful category of evolutionary changes originating by shifts in timing for features already present in ancestors. “It's not all heterochrony”—and this particular statement of “less is more” represents heterochrony's strength as an interesting subset with definite meaning, rather than an illogical hodge‐podge apparently applicable to all phenomena, and therefore explaining nothing.