Rigidothrix goiseri nov. gen., nov. spec. (Rigidotrichidae nov. fam.), a new "flagship" ciliate from the Niger floodplain breaks the flexibility-dogma in the classification of stichotrichine spirotrichs (Ciliophora, Spirotrichea).

Rigidothrix goiseri nov. gen., nov. spec. was discovered in soil from the Niger floodplain near to the town of Timbuktu, Republic of Mali. Its morphology, ontogenesis, and 18S rDNA gene sequence were studied with standard methods. Rigidothrix goiseri is very conspicuous in vivo because of its average size of 230x70 microm and a distinct tail. Further main characteristics include the rigid body; the conspicuous, stylonychid frontal area; the undulating membranes in Oxytricha pattern; a mighty adoral zone of membranelles not reorganized during ontogenesis; distinct midventral rows of which those of the opisthe develop right of the parental ones; and eight dorsal kineties, of which three develop intrakinetally and five are generated dorsomarginally. Rigidothrix goiseri possesses main characteristics of oxytrichine, stylonychine, and urostyline stichotrichs, specifically, it is the first rigid stichotrich with midventral rows, and thus it breaks the flexibility dogma. Distinct similarities with the genus Uroleptus and the CEUU hypothesis suggest that R. goiseri is more closely related to the oxytrichine than urostyline stichotrichs, in spite of the conspicuous midventral rows. This is also supported by the sequence data which show that R. goiseri is almost equally similar to both Oxytricha granulifera (95.88%) and Uroleptus gallina (94.93%), but fairly different from Urostyla grandis (92.7%). The main morphological peculiarities of R. goiseri show that it represents a new genus which should be classified into a new family, the Rigidotrichidae, together with the genera Uroleptus, Territricha, and Afrophrya nov. gen., the latter comprising Rigidothrix-like stichotrichs with cyrtohymenid oral apparatus. The conspicuous size and shape make R. goiseri a biogeographic flagship likely confined to Africa.