It is clear that this procolophonid is very similar to the derived procolophonids Leptopleuron from the Lossiemouth Sandstone Formation, Scoloparia from the Wolfville Formation, Newark Supergroup, and Hypsognathus from the Newark Supergroup. In addition certain characters are also shared with the unnamed Cromhall taxon (Fraser 1986, figure 6) and Soturnia from Brazil.
Modesto et al. (2002) generated a phylogenetic hypothesis of the Procolophonoidea based on an extensive character list derived mainly from Sues et al. (2000). Although many of these characters cannot be coded for the present specimen because it is incomplete and poorly preserved, enough of them are observable, especially in the dentition, to provide a good estimate of its phylogenetic position. Key characters that are not preserved, but would have been useful, include the contribution of the prefrontals to the skull roof and lateral walls of the snout, and characters associated with the palate and braincase.
The Abajo form possesses a low number of maxillary teeth that have bases that are labio-lingually expanded. Both of these characters are synapomorphies of the Procolophonidae (Modesto et al. 2002). Other characters observed in this specimen that are found in most procolophonids, but lacking in some basal forms, include maxillary teeth with labio-lingually expanded crowns and maxillary teeth that are inset from the lateral surface of the ‘cheek’ (Modesto et al. 2002). An additional character that is equivocal in the Abajo form is the lack of a postparietal, because the posterior margin of the skull roof is eroded. The Leptopleuroninae is a derived clade that is defined as all procolophonids more closely related to Leptopleuron than to Procolophon, and is identified by a host of characters (Modesto et al. 2002). Three of these characters can be observed in the Abajo form, making it identifiable as a leptopleuronine procolophonid. These are: two premaxillary teeth; orbit terminates well posterior to the pineal foramen; and two or more radiating quadratojugal spines (three in this specimen) (Modesto et al. 2002). One character in the analysis by Modesto et al. (2002) unites the present specimen with Leptopleuron and Hypsognathus to the exclusion of Scoloparia: the convex anteroventrally facing ventral margin of the jugal. Soturnia from Brazil also possesses this feature as well as another that defines this unnamed clade, the presence of one incisiform dentary tooth (Cisneros and Schultz 2003). If the palatine in the Abajo form does not have any teeth, and this is unconfirmed because the palate is poorly preserved and incomplete, then it would be united with Hypsognathus and Soturnia in a clade to the exclusion of all other procolophonids (Modesto et al. 2002; Cisneros and Schultz 2003). The presence of labial and lingual cusps separated by a transverse ridge in the maxillary teeth is an additional character that may unite Hypsognathus, Soturnia, the Abajo form, and the Cromhall taxon. However, the maxillary teeth of Leptopleuron have never been described in detail, so the character state is unknown in that taxon. That such a deep occlusal basin is seen in the maxillary teeth of both the Abajo form and Hypsognathus may be evidence of a close relationship between the two. The new specimen is clearly allied with derived leptopleuronine procolophonids, but the lack of any autapomorphies or unique combination of character states precludes the erection of a new taxon.
Leptopleuronine procolophonids had a cosmopolitan distribution during the Late Triassic. Although the Abajo form seems to be allied with mostly Laurasian forms, this may be an artifact of our poor knowledge of Late Triassic Gondwanan procolophonids. The new specimen, Leptopleuron, Hypsognathus, Soturnia, and the Cromhall form are evidence for a global Norian radiation of leptopleuronines, suggesting that procolophonids diversified until their extinction at the end of the Triassic.