 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 ytororo has nine branchiostegal rays, a condition not present T. igobi, T. crassicaudatus and T. stawiarski also unusual among trichomycterids 
Even so, the aforementioned presumably derived character states shared by the species herein proposed as belonging to the Trichomycterus stawiarski group (Table 1) suggest their close relationship and the reduced number of branchiostegal rays in T. ytororo could be interpreted as a homoplastic feature. However, a comprehensive phylogenetic analysis of the genus is still needed to assess if those character states are more parsimoniously attributed to common ancestry or parallelisms and to test the monophyly of this putative clade.
Trichomycterus crassicaudatus, T. igobi, and T stawiarski are endemic to the Iguazú (or Iguaçu, in Brazil) River basin, a left bank tributary of the Paraná River, while T. ytororo inhabits a downstream section of the same basin (Fig 5). Curiously, all these species are known from few collecting sites or only from their type localities (as T. ytororo), possibly indicating some ecological constraints in their distributions.
We thank Pablo Pereyra (FML) for figure 3. We thank Celeste Kummer for assistance with the radiograph of the holotype and paratypes. Mercedes Azpelicueta (CONICET) for constant support. This work was possible due to the invaluable help of Diego Baldo and Juan Martín Ferro. We thank Pedro Sanchez for logistical assistance in the field. Ministerio de Ecología, Misiones for collecting permits. We thank Carlos DoNascimiento (IAvH) and an anonymous reviewer for careful reviews in the manuscript. We are also thankful to Fundación Miguel Lillo, CONICET (PIP–11420110100301 to JMM) and FONCyT (PICT–2011–0992 to JMM); CNPq (Proc. 152354/2016-6 to JF).
- Conceptualization: GA JMM GET MB FA JF.
- Data curation: GET JF.
- Formal analysis: JF GET GA JMM.
- Funding acquisition: JMM JF.
- Investigation: GET JF.
- Resources: GET GA JMM.
- Visualization: FA GET JMM.
- Writing – original draft: GET JMM JF.
- Writing – review & editing: GA MB FA.
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