Clinal variation, intraspecific heterochrony, and microevolution in the Permian bryozoan Tabulipora carbonaria

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Abstract

Clinal geographic variation across an onshore-to-offshore environmental gradient occurs in the Lower Permian trepostome bryozoan Tabulipora carbonaria collected from three widespread calcareous shales of the Wreford Megacyclothem of Kansas. Separate multivariate statistical tests dclineate significant differences between populations within each shale unit, and between onshore, intermediate, and offshore populations pooled across all three shales, suggesting that even weakly-developed gradients may initiate different intraspecific morphological responses. Growth trajectories for populations along the cline also differ significantly, indicating astogenetic (developmental) heterochrony. Populations from onshore habitats are generally paedomorphic relative to those in more offshore settings, and exhibit pre- and postdisplacement and hypermorphosis in zooecial and acanthostyle characteristics. These heterochronic processes may have increased colonial reproductive potential and the efficiency of water flow for feeding and waste disposal in colonies from onshore habitats. Variation from tightly constrained development (astogenetic plasticity) decreased monotonically in an onshore-to-offshore direction; canalized growth may characterize colonies from more stable offshore habitats, whereas greater flexibility during the growth of colonies from unstable onshore biotopes may have increased their rate of survival. Populations of colonies from stratigraphically successive calcareous shales of the Wreford display patterns of growth that are nearly identical to those found in an offshore-to-onshore direction along the cline. Both clinal and temporal patterns probably resulted from selection for more paedomorphic morphologies in onshore, perhaps unstable, habitats and represent microevolution in T. carbonaria. These local adjustments to environmental conditions may produce variation that affects the rate of macroevolutionary change. □Bryozoa, clines, heterochrony, microevolution, Permian, variability.

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