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Competing interests: The authors have declared that no competing interests exist.
Trichomycteridae is a monophyletic group comprising seven monophyletic subfamilies (Copionodontinae, Glanapteryginae, Sarcoglanidinae, Stegophilinae, Trichogeninae, Tridentinae, and Vandelliinae) and the recognized non–monophyletic subfamily Trichomycterinae as presently conceived [1,2,3,4,5]. Among the eight genera of Trichomycterinae, Bullockia Arratia, Chang, Menu-Marque & Rojas, Eremophilus Humboldt, Hatcheria Eigenmann, and Rhizosomichthys Miles, are monotypic based on autapomorphies; Scleronema Eigenmann is a monophyletic group including three psamophyly species; Ituglanis Costa & Bockmann and Silvinichthys Arratia were proposed to allocate species previously included in Trichomycterus Valenciennes; and lastly, Trichomycterus lacks any diagnostic character being demonstrably non-monophyletic assemblage of species [2,3,4,5].
Trichomycterus is currently the largest genus within Trichomycteridae, containing over 170 valid species , which are distributed in practically all South America, Costa Rica and Panamá in Central America , and even found in an offshore island in the Pacific Ocean . Its geographical distribution ranges from lowlands of the Atlantic forest in southern Brazil, to mid-elevation streams on both sides of the Andean cordilleras, as well as the Patagonian region in Argentina . Most species inhabit fast-flowing streams . Their range of habitats is also remarkably diverse, including unusual freshwater habitats, such as sand-bottom streams, semipermanent pools, high-altitude Andean streams, and desolate creeks in Tierra del Fuego .
During recent samplings in northeastern Argentina we collected specimens of a remarkable new species of Trichomycterus from waterfalls in the Paraná River basin, which are described herein as a new species. The new taxon exhibits conspicuous characters that indicate a close relationship with T. crassicaudatus Wosiacki & de Pinna, T. igobi Wosiacki & de Pinna, and T. stawiarski (Miranda Ribeiro).
Material and methods
Specimens were collected with hand nets at Tabay stream, Jardín América, Misiones, Argentina. Appropriate actions were taken to minimize pain or discomfort of fish. Specimens were euthanized by immersion in an anesthetic solution (0.1% 2-phenoxyethanol), and then fixed in a 4% formaldehyde for one week, washed in water for one day and transferred to a 70% ethanol solution for preservation. These steps are approved by the ethical use animals of Fundación Miguel Lillo (FML) that consider animal welfare laws. Field studies did not involve endangered species. Collection permit was granted by Ministerio de Ecología of Misiones (Disp 074/15). Measurements were taken point-to-point as linear distances with digital calipers to the nearest 0.1 mm. Number of vertebrae, ribs, odontodes, unsegmented dorsal- and anal-fin rays, and caudal-fin procurrent rays were taken from 11 radiographed specimens (rd) and two cleared and stained  specimens (c&s) whenever possible. Branchiostegal rays were counted in c&s specimens. The number of post-Weberian vertebrae includes the compound caudal centrum counted as a single element. Anteriormost unbranched fin rays are represented by lower case Roman numerals and branched fin rays by Arabic numerals. The musculature of the dorsolateral head was observed in one paratype (ms). Counst of the holotype are given in parentheses. Meristic and morphometric data, nomenclature in the osteology, myology and cephalic laterosensory system follows the most recent contributions in that sense [13,14,15,16]. The new taxon plus the putative close species T. crassicaudatus, T. igobi, and T. stawiarski are treated as Trichomycterus stawiarski group in the text. Species from the Trichomycterus hasemani group were not included in the comparisons considering their well-supported relationship with non-trichomycterinae taxa [2,17]. Institutional abbreviations: CI-FML (Colección ictiológica Fundación Miguel Lillo, San Miguel de Tucumán). LGEP (Laboratorio de Genética Evolutiva-Pec