r fixation with exception of brighter yellow background and distal portions of fins hyaline (Fig 2).
The Tabay stream basin through 192 km from its headwaters at Campo Viera to its mouth on the Paraná River, at Jardín América (Fig 4). The stream bed is mainly composed of basaltic bedrock, in which sections with waterfalls and pools alternates all along its run. At the type locality (Tabay waterfall; Fig 5), the stream is surrounded by remnants of the Paranaense riparian forest, with its left margin degraded due to a camping site. This waterfall consists of three consecutive falls, the main one is 10m high and 20–50m wide, which drains into a narrow gorge. All specimens of Trichomycterus ytororo were captured above the waterfalls at shallow areas (about 1 meter depth or less) or in rapids witha predominantly rocky bottom and strong current. Trichomycterus davisi was the single congener recorded at the type locality, which was not collected syntopically with T. ytororo.
Stars represent the type localities. Some triangles symbols represent more than one collection locality. Numbers 1, 2, 3 indicate the Paraguay, Paraná and Iguazú Rivers, respectively.
Trichomycterus ytororo is so far known only from its type locality, the Tabay waterfalls in the Tabay stream (Fig 5), a tributary of the left bank of the Paraná River, province of Misiones, northeast of Argentina (Fig 4).
The new species proposed herein is allocated in the genus Trichomycterus (subfamily Trichomycterinae) due to the presence of all the synapomorphies for the clade that includes all the trichomycterids excepting the Copionodontinae and Trichogeninae and lacks those derived characters that support the clade comprising the subfamilies Glanapteryginae, Sarcoglanidinae, Stegophilinae, Tridentinae, and Vandelliinae . The presence of the levator internus 4 muscle originating from the neurocranial floor and the dorsal face of the posttemporo-supracleithrum, a synapomorphy for Trichomycterinae latu sensu , in the new taxon also supports its inclusion in the subfamily. 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