From Fensome et al., 2019:
Saeptodinium tiandongense He Chengquan and Qian Zeshu, 1979, p.177, pl.1, figs.6–7a–b. Holotype: He Chengquan and Qian Zeshu, 1979, pl.1, fig.7. Age: Late Eocene–Oligocene.

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Translation He Chengquan and Qian Zeshu, 1979: GSC, courtesy R. Fensome

Description: He Chengquan and Qian Zeshu, 1979
A dinoflagellate cyst. The tract has an ovoidal ambital outline, with a stout apical bulge that measures up to 5 µm in length. The inner body is subcircular and has a smooth surface; if often presses closely against the outer wall, being separated from it only in the vicinity of the apical protrusion; the boundaries of the inner body are clearly defined. No transverse or longitudinal sulci can be seen. The tract is caved inward at its antapical end, in a way that forms a pair of small antapical processes whose length is approximately 5 µm. In some specimens the antapical processes are undeveloped, only protruding to a very slight extent. The wall of the tract is thin, sculptured with fine granular ornamentation, and bears folds. No archaeopyle is seen. [The tract] may contain a structure suggestive of a nucleus.
The tract of the holotype is 37.5 µm long and 33 µm wide, and the inner body has a diameter of approximately 32 µm. The ranges of variation are as follows: tract length 37.5 - 42.5 µm, tract width 33 - 37.5 µm, and inner body diameter 32-35 µm.

Affinities: He Chengquan and Qian Zeshu, 1979
This species is similar to S. tasmaniensis Harris, 1973. It is distinguished by the fact that the surface of its inner body is smooth.