Tenellisporites capillaris sp. nov., a new dispersed lycopsid megaspore from the Middle–Upper Triassic Badong Formation, Hunan Province, China

Dispersed megaspore fossils provide crucial insights into the diversity of heterosporous lycopsids through geological time. However, detailed systematic studies on Middle–Late Triassic dispersed lycopsid megaspores of the South China Block are lacking. Here, we report a unique dispersed lycopsid megaspore species, Tenellisporites capillaris sp. nov., from the Middle–Upper Triassic Badong Formation in Hunan Province, China. This trilete megaspore species is characterised by narrow, elevated, membraneous laesurae with ragged finger-like projections; a flattened equatorial zona; and single or rarely bifurcate spines with sharp tips on both the proximal and distal surfaces. These spines are connected basally to form a fine reticulum and gradually increase in thickness and length towards the equator. Ultrastructural analysis reveals that the megaspore wall consists of two layers: 1) a very thin and dense foot layer, 2) a thick, dense outer layer with parallel sporopollenin units. The outermost sporopollenin units transition to vertically arranged units where forming the spinous ornamentation. The morphological and ultrastructural characteristics indicate that the new megaspore species belongs to Isoetales. Our finding represents the first fossil record of Tenellisporites in Asia, enhancing our understanding of the palaeogeographic distribution and diversity of isoetalean lycopsids during the Middle–Late Triassic.