The genus Rapicactus was proposed in 1942 by Buxbaum and Oehme.[2] It was characterized by having thick roots with a constriction above forming a neck. However, most subsequent treatments sank the genus into a broadly circumscribed genus Turbinicarpus. The circumscription of Turbinicarpus has been described as "remarkably unstable", with species regularly transferred to other genera.[1] The broad circumscription of Turbinicarpus was recognized as polyphyletic by Hunt in 2016.[3]Aphylogenetic study published in 2019 showed that a monophyleticRapicactus was sister to Acharagma, separated from a monophyletic Turbinicarpus:[1]
^ abcVázquez-sánchez, Monserrat; Sánchez, Daniel; Terrazas, Teresa; De La Rosa-Tilapa, Alejandro & Arias, Salvador (2019), "Polyphyly of the iconic cactus genus Turbinicarpus (Cactaceae) and its generic circumscription", Botanical Journal of the Linnean Society, 190 (4): 405–420, doi:10.1093/botlinnean/boz027
