There is an active debate about how skeletal elements are encoded along the proximodistal (PD) axis of the developing limb. Our aim here is to see whether consideration of the evolutionary morphology of the limb can contribute to our understanding of patterning mechanisms. Of special interest in this context are animals showing reiterated skeletal elements along the PD axis (e.g., dolphins and plesiosaurs with hyperphalangy). We build on previous hypotheses to propose a two-step model of PD patterning in which specification of broad domains in the early limb bud is distinct from subsequent processes that divides an initial anlage into a segmental pattern to yield individual skeletal elements. This model overcomes a major evolutionary problem with the progress zone model, which has not previously been noted: pleiotropy. Parallels with other developmental systems are briefly discussed.
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