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Resolving the systematic and phylogenetic position of isolated ovules: a case study on a new genus from the Permian of China




Isolated ovules occur in many fossil plant assemblages, where they provide important insights into seed-plant diversity and evolution. However, in many cases, the ovules cannot be attributed to individual groups of seed plants, restricting systematic and evolutionary assessments that can be made from otherwise well-characterized fossil taxa. In the present paper, we describe a new kind of ovule discovered in tuffaceous sediments from the Permian-aged Xuanwei Formation in Guizhou Province, China. This ovule has 180° rotational symmetry and an integument comprising a variably thick sarcotesta, a uniformly thick sclerotesta and a uniformly thin endotesta. The nucellus is attached to the integument at least basally and contains a collapsed seed megaspore; a nucellar apex is absent. Both the integument and nucellus are vascularized by paired bundles in the major plane of the ovule; the integumentary bundles are considerably larger than the nucellar bundles and the nucellar bundles emerge from a conical vascular pad. Generation of a three-dimensional reconstruction based on serial peels revealed the gross morphology and organization of the ovule and highlighted the presence of features consistent with cardiocarpalean-type ovules (ovule shape, histological features of the integument) and also features more typical of lagenostomalean- and trigonocarpalean-type ovules (large integumentary bundles, presence of nucellar bundles). To assess the affinity and evolutionary significance of the ovule, it has been included in a cladistic matrix of cardiocarpalean-, lagenostomalean- and trigonocarpalean-type ovules. Results place the ovule within the cardiocarpalean group of ovules known to have been produced by several plant groups, including cordaitean coniferophytes, pteridosperms and Palaeozoic conifers. The cladistic topology supports generic level distinction of the present species, requiring the establishment of Muricosperma guizhouensis Seyfullah & J.Hilton gen. & sp. nov. Lagenostomalean ovules produced by hydrasperman pteridosperms form a basal paraphyletic grade, whereas trigonocarpalean ovules produced by medullosan pteridosperms form a monophyletic group in which Stephanospermum is paraphyletic with respect to Rhynchosperma and Pachytesta. The results also place the Mississippian ovule Mitrospermum bulbosum apart from all of the Pennsylvanian species of Mitrospermum that form a strongly supported clade. Consequently, M. bulbosum is transferred to the new genus Whitaddera Seyfullah & J.Hilton as W. bulbosa (Long) Seyfullah & J.Hilton. © 2010 The Linnean Society of London, Botanical Journal of the Linnean Society, 2010, 164, 84–108.