The influence of variation in larval period on adult cranial diversity in Pelobates fuscus (Anura: Pelobatidae)

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The timing and sequence of cranial ossification was studied in Pelobates fuscus tadpoles differing in the rate of their larval development. Laboratory‐reared animals, obtained from one egg mass, were examined. Some of the tadpoles metamorphosed during the first summer (normally developing tadpoles), whereas others prolonged the timing of their larval development into a second year or even later (slowly developing tadpoles). In slowly developing tadpoles the timing and sequence of cranial ossification differ in many respects from those occurring in normally developing tadpoles. Thus (a) ossification of some bones commences earlier and (b) there are some anachronisms in the sequence of ossification. As a whole, different cranial elements respond differently to prolongation of larval development. Bones which arise early in normal ontogeny (parasphenoid, frontoparietals, prootics, exoccipitals and interparietal) have a lengthened period of morphological differentiation and growth when larval development is prolonged. Consequently they show hypermorphosis in the adult with a profound effect on adult cranial morphology. Other ossifications, whose origin and growth are associated with the final metamorphic event (the transformation of the larval into the adult jaw mechanism), show no evident changes in the timing of their origin relative to somatic development.Another mechanism for altering adult morphology in animals with prolonged larval development results from the dissociation between somatic and sexual development. While somatic development in slowly developing tadpoles is very retarded and their external morphology may show no change during a large part of the larval stage, the generative system continues to develop. This situation produces tadpoles with premetamorphic somatic morphology but with almost mature generative organs. These tadpoles retain the ability to metamorphose and consequently they may transform into sexually mature animals with juvenile (early postmetamorphic) somatic morphology, i.e. into neotenic animals. In some specimens early‐originating cranial bones may be hypermorphosed whereas cranial elements normally arising in late metamorphic stages, or after metamorphosis is complete, are subject to paedomorphic reduction and under‐development.It is concluded that wide individual variation in the rate of larval development may account, at least in part, for the occurrence of intraspecific variability in anuran cranial morphology (e.g. in Pelobates syriacus).