Most supported phylogenetic tree, according to character matrix in Table 2, of the genera Mesocrista, Platicrista, Itaquascon, Astatumen and Meplitumen (phylogenetic tree values: consistence index = 0.91; retention index = 0.83). 0 = Ancestor with ordinary AISM (i.e. not wide and flat and with caudal processes pointing postero-laterally), stylet supports present, spiral thickening present only after the stylet supports, ordinary stylet furcae and placoids. 1 = AISM became wide and flat, stylet furcae reduced slightly, placoids became elongated; Mesocrista maintained these characters. 2 = Caudal processes of the AISM pointed laterally; apices of the caudal processes of the stylet furcae became non-swollen. 3 = appearance of spiral thickening also on the anterior buccal tube; this line gave rise to Meplitumen. 4 = AISM became wider and their caudal processes reduced; Platicrista maintained these characters. 5 = Stylet furcae small and with reduced caudal processes, placoids reduced to a single, very thin, bar or absent, stylet support slendered; Itaquascon maintained these characters. 6 = Stylet supports disappearance, spiral thickening present also on the anterior buccal tube; this line gave rise to Astatumen.