Oloptum is a segregate from Piptatherum. It includes only one species, O. miliaceum [≡ Oryzopsis miliaceum, ≡ Piptatherum miliaceum]. Oloptum differs morphologically from Piptatherum in having whorled panicle branches, lemmas that are not coriaceous, lemma margins that are fused at the base, and a circular disarticulation scar. (Hamasha et al. 2012; Romaschenko et al. 2012). Romaschenko et al. also demonstrateed that the lemma epidermal pattern of the two taxa differs from that of other stipoid grasses.
Hamasha et al did not include the Piptatherum thomasii, which has often been treated as a variety or subspecies of O. miliaceum, in their study. Consequently they did not provide a name for it in Oloptum. Romaschenko et al. (2012) included both taxa in their study. They found the pair formed a strongly supported clade in both their plastid and ITS trees but that the plastid tree placed the clade as sister to a clade containing Eurasian species of Achnatherum and Piptatherum virescens and P. paradoxum whereas the ITS tree showed it as having an unresolved relationship with the South Africa S. capensis and several American Stipeae.
The Oloptum clade was recognized as a distinct group within Piptatherum by Roshevits and was formally named as Piptatherum sect. Miliacea by Freitag (1975). Hamasha et al. (2012) raised it to generic rank.
References
Freitag H. 1975. The genus Piptatherum in southwest Asia. Notes from the Royal Botanic Garden, Edinburgh 33: 341-408.
Hamasha et al. 2012. Stipa (Poaceae) and allies in the Old World: molecular phylogenetics realigns genus circumscription and gives evidence on the origin of American and Australian lineages. Plant Systematics and Evolution 298: 351-367
Romaschenko et al. 2012. Systematics and evolution of the needle grasses (Poaceae: Pooideae: Stipeae) based on analysis of multiple chloroplast loci, ITS, and lemma micromorphology. Taxon 61: 18–44.