Apatorhynchus opistheretmus was named by Friedman and Daeschler (2006) [Cosmine absent; surface of cranial dermal bones ornamented with very fine tubercles; sutures between bones of dermal skull roof non-interdigitating with the exception of the transverse join between the C- and E-bones; I-bones with long, distally expanded posterior processes that exceed the length of the body of the bone; B-bone separates I-bones; D-bone present and enclosed by C- and E-bones; ancillary branches of supraorbital canal in C-bones; narrow, elongated E-bones that are longer than C-bones; bone 3 straddles the level of the transverse suture between the C- and E-bones; single ossification combining the characteristics of bones K and X (bone KX); multiple canal-bearing bones flanking lateral margins of C-bones anterior to KX.]. It is not extant. Its type specimen is ANSP 21357, a partial skull (Skull roof). Its type locality is Apatorhynchus-type locality, which is in a Famennian terrestrial horizon in the Catskill Formation of Pennsylvania.