''Rapicactus'' is a genus of

cactus A cactus (, or less commonly, cactus) is a member of the plant family Cactaceae, a family comprising about 127 genera with some 1750 known species of the order Caryophyllales. The word ''cactus'' derives, through Latin, from the Ancient Greek ...

in the tribe

Cacteae Cacteae is a tribe of plants of the family Cactaceae found mainly in North America especially Mexico. , the internal classification of the family Cactaceae remained uncertain and subject to change. A classification incorporating many of the insi ...

, subfamily

Cactoideae The Cactoideae are the largest subfamily of the cactus family, Cactaceae. Around 80% of cactus species belong to this subfamily. , the internal classification of the family Cactaceae remained uncertain and subject to change. A classification inc ...

. It has been synonymized with ''

Turbinicarpus ''Turbinicarpus'' is a genus of very small to medium-sized cacti, which inhabit the north-eastern regions of Mexico, in particular the states of San Luis Potosí, Guanajuato, Nuevo León, Querétaro, Hidalgo, Coahuila, Tamaulipas and Zacateca ...

'' but

molecular phylogenetic Molecular phylogenetics () is the branch of phylogeny that analyzes genetic, hereditary molecular differences, predominantly in DNA sequences, to gain information on an organism's evolutionary relationships. From these analyses, it is possible to ...

studies have supported its

monophyly In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic grou ...

and separation from that genus.

Taxonomy

The genus ''Rapicactus'' was proposed in 1942 by Buxbaum and Oehme. 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 In geometry, the circumscribed circle or circumcircle of a polygon is a circle that passes through all the vertices of the polygon. The center of this circle is called the circumcenter and its radius is called the circumradius. Not every polyg ...

genus ''Turbinicarpus''. The circumscription of ''Turbinicarpus'' has been described as "remarkably unstable", with species regularly transferred to other genera. The broad circumscription of ''Turbinicarpus'' was recognized as

polyphyletic A polyphyletic group is an assemblage of organisms or other evolving elements that is of mixed evolutionary origin. The term is often applied to groups that share similar features known as homoplasies, which are explained as a result of converg ...

by Hunt in 2016. A

phylogenetic In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...

study published in 2019 showed that a

monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gro ...

''Rapicactus'' was sister to ''

Acharagma ''Acharagma'' is a genus of two cactus species from northern Mexico. Description These cacti are usually solitary but sometimes occur in small clusters. The globose stems tend to be about 3–7 cm in diameter. The ribs have tubercles, wit ...

'', separated from a monophyletic ''Turbinicarpus'':

Species

Plants of the World Online Plants of the World Online (POWO) is an online database published by the Royal Botanic Gardens, Kew. It was launched in March 2017 with the ultimate aim being "to enable users to access information on all the world's known seed-bearing plants by ...

accepted four species:

References

Cactoideae Cactoideae genera {{Cactus-stub