From Williams et al., 2017:

[Ammonidium Lister, 1970, p.48–49. Emendations: Le Hérissé, 1989, p.80–81, as a revised diagnosis; Sarjeant and Vavrdova, 1997, p.15.
Acritarch genus

Type species: originally as Baltisphaeridium microcladum, Downie, 1963 (pl.92, fig.6)] ; Ammonidium microcladum, Lister, 1970