Abstract
An expedition to the middle Rio Purus basin uncovered a remarkable new species of the genus Spinipterus . The new species has a very distinct and conspicuous colour pattern resembling a jaguar and it is almost four times larger than Spinipterus acsi , a small specimen (32 mm L S ) from Caño Santa Rita, a right bank tributary of Río Nanay in Peru and a second specimen was reported from Rio Juruá, Amazonas State, Brazil. Although the new species is more similar in size and colour pattern to Liosomadoras , it shares the synapomorphies for Spinipterus . The new species differs from the congener by the following characters: (a) colour pattern with large black rosette‐like spots over a light yellow to brown background ( v . brown background with small dark blotches over the body); (b) adult body size reaching 104.5 mm L S ( v . maximum known size 37.1 mm L S ); (c) posterior process of cleithrum short, never reaching vertical through the dorsal‐fin origin ( v . posterior process long, surpassing vertical through the dorsal‐fin origin); (d) seven soft pectoral‐fin rays ( v . six); (e) caudal fin truncated ( v . caudal fin rounded).