Xenurolebias pataxo differs from all species of Xenurolebias by the absence of black spots on the posterior part of the caudal peduncle in females (vs. 1-5 spots). It is also distinguished from X. myersi by its narrower head (head width 65.7-71.1% SL in males and 65.8-70.3% in females, vs. 71.5-74.6% and 74.5-81.1%, respectively), lack of round yellow spots on the distal portion of the anal fin in males (vs. presence), longer filaments on the tip of the dorsal fin in males (reaching the posterior portion of the caudal fin, vs. reaching between the base and middle of the caudal fin), and caudal fin lanceolate, with a pronounced posterior tip in larger individuals (vs. sub-lanceolate, never forming a distinctive tip posteriorly). It can be further diagnosed from X. izecksohni by having the dorsal and anal fins, in males, with short light bars on the basal portion and small round light spots of the distal portion (vs. branching bars), absence of oblique short bars on ventral portion of caudal fin in males (vs. presence), and fewer dark brown bars on the caudal fin in males (5-7 vs. 9-12) (Ref. 96072).

Description: Dorsal-fin rays 16-18 in males, 12-16 in females; anal-fin rays 21-24 in males, 19-22 in females; extremity of dorsal fin pointed and long in males, rounded to slightly pointed in females; tip of anal fin pointed in males, rounded in females; caudal fin lanceolate, forming distinctive posterior pointed extension. (Ref. 96072).