family. Among the Trichomycterinae, the new taxon also lacks the diagnostic characters of Bullockia, Eremophilus, Hatcheria, Ituglanis, Rhizosomichthys, Scleronema, and Silvinichthys [3,4, 18,19,20,21,22,23]. In view of these facts, the inclusion of the new species in the genus Trichomycterus is the most plausible action for the moment.
The non-monophyletic nature of the group and the long and complicated taxonomic history has placed Trichomycterus the most formidable problem in the systematics of the Tricomycteridae . Many species of Trichomycterus have uninformative descriptions and are associated to old type material, limiting the applicability of the name only to the types . Currently, the genus contains over 170 species , however, its known diversity remains increasing along the recent years. Even with these obstacles, species groups within Trichomycterus were defined based on morphological characteristics [11,25,26,27,28,29,30,31], some of them constantly redefined and re-diagnosed, such as the Trichomycterus brasiliensis species complex [14,32,33], which, to date, its monophyly could not be consistently assessed . Besides, these characters and all species included on these groups were not placed into a comprehensive phylogenetic analysis, therefore with the possibility of representing artificial groups due to possible parallelisms.
The external and internal characters present in T. ytororo do not fit with any of the previous suggested species complexes with the exception of the group comprising T. crassicaudatus, T. igobi, and T. stawiarski. This group was proposed based on the presence of the procurrent caudal-fin rays thickly ossified and rigid with a slender distal tip; the dorsal procurrent caudal-fin rays extending along the tips of at least 10 neural spines; and 10–11 branchiostegal rays  The two first conditions are shared with in T. ytororo (Table 1, Fig 3) and contrast with the procurrent caudal–fin rays thin and flexible [11,35] extending usually for seven or eight neural spines tips, as observed in other species within the genus . In addition, the species of the Trichomycterus stawiarski group have numerous procurrent dorsal caudal-fin rays (24 or more), in comparison with other congeners (Table 1, Fig 3). Although the number of the caudal–fin procurrent rays are not available for all species, the closest value was observed in T. steindachneri DoNascimiento, Prada-Pedreros & Guerrero-Kommritz 
Besides these osteological traits, the four species show a similar color pattern consisting of two distinct layers of pigmentation: deep layer with large black blotches irregular in shape and size, overlapped by a superficial layer of small and less pigmented spots [11,35] (Figs 1 and 2). In addition, the blotches are more concentrated on the dorsum of the trunk, becoming more scattered towards ventral region. However, considering the high variation of the color pattern within the genus, only an analysis including an extensive comparative material of Trichomcyterus and other Trichomycterinae in a phylogenetic context could provide an adequate framework for interpreting these characteristics of coloration. Trichomycterus ytororo has the extensor tentaculi muscle originating on both suspensorium and neurocranium (Table 1), trait only observed in T. stawiarski and T. davisi within the genus [13,36]. At the same time, myological data for T. crassicaudatus and T. igobi and the most species on genus are not available. As consequence, the origin of this character remains uncertain considering the well-supported relationship among T. crassicaudatus, T. igobi and T. stawiarski .
The putative monophyly of the clade formed by T. crassicaudatus, + T. igobi, + T. stawiarski  would be also supported by the presence of 10–11 branchiostegal rays, contrasting with other trichomycterids which have usually 5–8 branchiostegal rays, a plesiomorphic condition . Based on this continuous character (10–11 branchiostegal rays), the recently described species, T. perkos, was proposed as possible sister taxon of this clade . Trichomycterus y