Ontogenetic changes of some ferns and pteridosperms from the Kasimovian and the Gzhelian of the Donets Basin have been identified on the basis of an analysis of the pinnules and pinnae morphological changes. An increase of the axis thickness and degree of segmentation of the leaf plate of the last order pinnae are identified in the process of development of the pinnate frond of ferns and pteridosperms, that are accompanied by a reducing of compaction in location of pinnules on an axis, e.g. a change of pinnules from adnate and touching to distant, and also by an increase of the pinnules size and a change of a form of the pinnules lateral margins. Ontogenetic changes of some ferns are also approved in an increasing of dichotomy amount of the pinnules lateral veins. Ontogenetic changes of the pteridosperms with callipterid foliage are also approved in an increasing of segmentation of the pinnules leaf plate.

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