Systematics
O. tamandua was originally described as Sternarchus tamandua by George Albert Boulenger in 1880; its species name is from the Tupi word tamanduá, meaning "anteater", in reference to its long snout. In 1905, Eigenmann and Ward placed this species in the genus Sternarchorhamphus, though noting that it may represent a distinct taxon. In 1913, Ellis placed this species into its own genus Orthosternarchus, from the Greek orthos ("straight"), sternon ("chest"), and archos ("rectum"), referring to the straight snout and the forward placement of the urogenital opening.
Ellis saw O. tamandua as a close relative to Sternarchorhamphus, which has been corroborated by recent morphological and molecular analyses. The relationship between these two genera and the rest of the Apteronotidae is less clear, though they are generally regarded as basal within the family. Some authors consider them to be sister taxa to Platyurosternarchus and Sternarchorhynchus, which also have elongate heads. However, in those taxa the elongation is accomplished by lengthening of the jaws, whereas in Orthosternarchus and Sternarchorhamphus it is accomplished by the lengthening of the head while the jaws remain relatively short. This suggests that elongate head shape was independently evolved between the two groups.
Read more about this topic: Orthosternarchus Tamandua