Abstract
Seed plants underwent first major evolutionary radiation in the Late Devonian (Famennian), as evidenced by the numerous ovules described to date. However, the early pollen organs are underrepresented, so that their structure and evolution remain poorly known. Here we report a new taxon of pollen organ Placotheca minuta from the Late Devonian. The synangium consists of many basally and more or less laterally fused microsporangia borne on the margin of a pad. The prepollen is spherical and trilete. The appearance of Famennian synangia especially in Placotheca does not support the current understanding that the earliest pollen organs closely resembled the fructifications of the ancestral progymnosperms. Placotheca indicates earlier diversification of pollen organs than previously expected and is highly derived among the early pollen organs with trilete prepollen. It is suggested that, immediately after the origination of seed plants, pollen organs had evolved at a rapid rate, whereas their prepollen remained primitively spore-like.
Highlights
Microsporangia numerous, elongate, arising from margin of pad to produce a single ring, with their tips curving toward synangial center; microsporangia basally and somewhat laterally fused and distally free
The fossils come from the Wutong Formation at the Xiangkou section, Xiangyu Town, Dongzhi County, Anhui Province, South China
Assemblages of plants, spores, fish and conchostracans indicate that the Wutong Formation is Upper Devonian (Famennian), and that the Guanshan Member is lower Famennian, the Leigutai Member upper Famennian[13,14]
Summary
All specimens have been deposited in Department of Geology, Peking University, Beijing, China. Steel needles were used to expose some pollen organs, whose photographs were made with a digital camera and microscope. A number of microsporangia and a few prepollen were observed with scanning electronic microscopy
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