Structure and affinities of Athrotaxites yumenensis sp. nov. (Cupressaceae) from the Lower Cretaceous of northwestern China
Dong et al
Compressions of leafy twigs, pollen cones and seed cones of Athrotaxites yumenensis C. Dong et B.N. Sun sp. nov. (Cupressaceae) are abundant in certain layers of the Lower Cretaceous Zhonggou Formation in western Gansu Province, northwestern China. The leafy branches are stiff and ascending. The leaves are scale-like, spirally arranged and closely appressed to the stem. The seed cones are located terminally, globular, with 8–11 helically arranged bract-scale complexes. The bracts are intimately fused with, and more massive than, the seed-bearing portion, and each scale has two seeds. The seeds are oval, 0.9–1.1 mm long and 1 mm wide with two wings derived from the seed coat. A single pollen cone occurs at the ends of the ordinary branches with 10–16 microsporophylls. The microsporophylls are wedge-shaped and spirally arranged. Terminal microsporophylls fall away from the cone axis after pollen dispersal. The pollen sacs are ovate and produced more than 20 pollen grains per sac. The pollen grains are spherical, lack air bladders and have small germinal papilla and pores. Compared to the extant Tasmanian genus Athrotaxis in morphological, structural and cuticular features of leafy shoots, as well as seed cones and pollen cones, the study fossil consistently shares many characteristics with the extant genus, especially the extant species Athrotaxis cupressoides, but differs from it in other aspects; thus, it has been placed in a new genus. The similar characteristics, however, suggest that the fossil has the closest affinity with the extant Athrotaxis cupressoides. A detailed comparison of leaf morphology and the macro- and microstructure of the pollen and seed cones with related fossil species of Athrotaxis and Athrotaxis-like species reveals that the present fossil is a new species. Additionally, the occurrence of the fossil in the Zhonggou Formation and its similar characteristics with another fossil species, Athrotaxites berryi, suggests that Athrotaxites yumenensis sp. nov. might be an intermediate between Athrotaxites berryi and the extant Athrotaxis and that the new species is more closely aligned with extant Athrotaxis than with Athrotaxites berryi.