A new Harrisiothecium pollen organ from the Upper Triassic of South Central China

Extinct Mesozoic seed plants are key for understanding the evolution and relationships of seed plants, including homologies among their reproductive organs. Recent discoveries of exceptionally well-preserved fossils have greatly enhanced knowledge of the ovulate reproductive organs of Mesozoic seed plants, but research on the corresponding pollen organs is more limited. Here we describe a new species of Mesozoic pollen organ, Harrisiothecium sanduense sp. nov. based on material from the Upper Triassic Yangmeilong Formation in Hunan, South Central China. Harrisiothecium sanduense consists of a thick central axis bearing lateral secondary branches arranged mainly in a single plane and almost distichous. The lateral branches are unbranched or bifurcate, and bear one or two terminal capsules each consisting of two valves on either side of a median hinge. Each valve bears a row of six to seven elongated pollen sacs on the inner face. Harrisiothecium sanduense is similar to Harrisiothecium marsilioides from the Upper Triassic of Greenland and Hydropterangium roesleri, which we transfer to Harrisiothecium roesleri comb. nov., from the Upper Triassic of Germany, and like both species is also associated with pinnate leaves of the seed fern Ptilozamites. Harrisiothecium sanduense differs from Harrisiothecium marsilioides and Harrisiothecium roesleri in its more flattened structure with lateral branches that bear no more than one or two capsules. Harrisiothecium shares similarities to Pramelreuthia dubia, Pteroma, Pteruchus, Antevsia and Townrovia, as well as the structures that bear the pollen sacs in Bennettitales.