We use probability plots to compare metric data for a growth series of the Permian nectridean Diplocaulus magnicornis Cope to a normal (Gaussian) probability distribution function and demonstrate that the ontogeny of Diplocaulus is not a single continuous process. The plots are bimodal, revealing a population of small, ontogenetically young animals whose growth is governed by a distinct process. This population, which comprises ∼20% of the growth series, is not evident in allometric studies. These young animals have a mean mid-line skull length of ∼21 mm, and their orbits are relatively larger, and rounder, than the orbits of the older animals, whose mean skull length is ∼94 mm. Although there is no evidence of lissamphibian-type metamorphosis in the Nectridea, the probability plots clearly demonstrate a two-stage ontogeny, best described as an ontogenetic morphological saltation. This saltation may have been an alternative strategy to the metamorphic heterochrony found in lissamphibians and their relatives.
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Vol. 21 • No. 4