Arachana nigra was named by Pineiro et al. (2012) [Medium-sized to large temnospondyl distinguished by the following combination of characters: skull elongate with maximum width across the jugal-quadratojugal contact; interorbital longitudinal trough extending as far posteriorly as skull roof posterior margin; frontal excluded from orbital margin; orbits located slightly posterior to the skull mid-length, and set neither far apart nor close to the skull margins; ornamentation markedly pitted, lacking pustules; sensory sulci discontinuous, with infraorbital sulci lacking a step-like or Z-shaped lacrimal flexure, deeply impressed between orbits and nostrils, where they approach each other over the lacrimal; skull table posterior border sufficiently concave to allow for occipital condyles full exposure in dorsal view; skull ventral surface straight in occipital view, as occipital and quadrate condyles are aligned; lacrimals entering external nostril, thus preventing maxilla-nasal contact; supratemporal excluded from margin of moderately incised otic notches; pterygoid palatal ramus of the pterygoid contacting vomer anteriorly, excluding palatine and ectopterygoid from the interpterygoid vacuity; pterygoid posterolateral flange poorly developed or absent; parasphenoid plate bearing separate, crescentic cristae musculari; narrow cultriform process; interpterygoid vacuity widest in its anterior onethird; small basioccipital ossification, visible in palatal and occipital views, not contributing to the exoccipital condyles; shagreen on palatal bones series; smooth ornamentation on pterygoids, parasphenoid and vomers; palatine toothrow very reduced or absent posterior to palatine tusks; ectopterygoid teeth similar in size to or larger than maxillary teeth; pterygoid ascending ramus contacting squamosal dorsally (palatoquadrate fissure absent); pterygoid quadrate ramus with low longitudinal ledge; post-temporal fenestra teardrop-shaped.]. Its type specimen is FC-DPV 1369, a skull, and it is a 3D body fossil. Its type locality is Colonia Orozco, which is in a Triassic fluvial siliciclastic/conglomerate in the Buena Vista Formation of Uruguay.