The skull of MCZ 2369 has a unique shape among known temnospondyls, with a long, anteriorly tapered snout, large and round orbits, a wide interorbital distance, and a very short skull table (Fig. 3D). The ratio of interorbital distance to skull length (0.28) is greater than in Dissorophus multicinctus (0.25), Conjunctio multidens (0.25), and Cacops morrisi (0.2). Even more distinctive is the size of the skull table, which reaches only 0.325 times the length of the preorbital region in Scapanops, contrasted by Cacops morrisi (0.67), Conjunctio (0.9), Dissorophus (1.0), and Broiliellus texensis (1.45). The preorbital region is tapered anteriorly. The external nares are closely spaced, and the premaxillae and nasals bound a large rounded fenestra level with the anterior parts of the narial openings. The external nares are elongate and oval, anteriorly close to the jaw margin, but posteriorly far offset from the maxilla. The outline and relative size of the external naris are most similar to those in Aspidosaurus (Berman and Lucas, 2003 and Broili, 1904). The nasal is only slightly longer than the frontal, whereas the lacrimal is greatly elongated compared to that of other dissorophoids, reaching about twice the length of the prefrontal. The prefrontal and postfrontal are separated, although the postfrontal has an anterior projection extending along the orbital margin. This feature is also found in Conjunctio and shared with more basal dissorophoids.

The skull table is not only much shorter, but also narrower than that in most other dissorophoids. The parietal is slightly narrower than the frontal, and the supratemporal is still slightly narrower than the parietal. The postparietal is relatively short but still longer than the tabular, which lacks a posterior horn. Both elements form a posterior flange. The postorbital consists of an elongate medial part and a slender lateral wing that contacts the jugal. The squamosal is extremely thin, restricted to the margin of the temporal notch. The “cheek” region is even shorter than the skull table, with the jaw joint situated well anterior to the occipital condyles. The temporal (“otic”) notch, which is wide and deep, resembles that of Dissorophus, especially in the very short distance between the notch and the posterior margin of the orbit. The squamosal makes only limited contact with the supratemporal, which otherwise forms the medial margin of the temporal notch.

Similar to other dissorophids, the skull roof bears pronounced ridges that divide its dorsal surface into smaller depressions (Fig. 1B). As in Cacops, there is a large depression on the nasals and on the anterior portion of the frontals, bounded by a transverse ridge between the orbits. An additional depression is situated on the posterior portion of the frontals and on the parietals, and the pineal foramen is located within a transversely oval pit. The dermal ornamentation is not well preserved, but was much less pronounced and regionally differentiated than in Cacops or Kamacops. As in Conjunctio, Dissorophus, and Broiliellus, it was dominated by pits and the ridges were not as high as in the advanced eucacopines. It is also knobby in places.

The large orbits are filled with numerous small elements, probably palpebral ossicles and (or) scleral plates (Fig. 1B). Scleral plates are known from Cacops morrisi (Reisz et al., 2009) and palpebral elements from Broiliellus sp. (RRS, personal examination).

The palatal surface is concealed by overlying postcranial bones, but the prepared regions expose only the internal side of the skull roof, indicating that palatal elements must have fallen off before final burial (Fig. 1C–D). The dentigerous shelves of the premaxilla and maxilla are exposed, revealing the approximate number of teeth in both elements (premaxilla: 16; maxilla: >75). The teeth are mostly subequal in size and have round bases. The teeth in the premaxilla and anterior part of maxilla are slightly larger (Carroll, 1964). The number and relative size of teeth led Carroll (1964) to refer MCZ 2369 to Conjunctio. However, similar tooth counts have also been reported for some amphibamid dissorophoids (Clack and Milner, 2010).

The ventral side of the skull is concealed by several postcranial elements, all of which are similar to the corresponding bones in other dissorophids and thus probably come from the same individual as the skull to which they are attached (Fig. 1C–D). An additional piece preserves about 14 vertebrae in articulation, five or six ribs, and the right scapula and cleithrum.

The largest limb bone is a left humerus exposed in ventral view. It is similar in its long and slender outline to that of Cacops morrisi (Reisz et al., 2009). As in C. morrisi, its distal end is weakly expanded and not fully ossified, unlike in the much larger humeri of C. aspidephorus and Dissorophus multicinctus (DeMar, 1968;Williston, 1910). Two additional elongate elements probably represent the radius and ulna, as suggested by Carroll (1964). Finally, both clavicles are also preserved, with small ventral blades typical of dissorophoids.

The few ribs, preserved in a series, are short with forked proximal portions, with spike-like uncinate processes that become shorter posteriorly. The right scapula is well ossified and with a complete, slightly expanded dorsal margin, to which the elongate cleithrum is attached as in Cacops aspidephorus. The cleithrum is more slender than in the latter taxon.

The osteoderms of Scapanops are small and square-shaped, with pitted ornamentation (Fig. 1E), and similar to those of Cacops aspidephorus and Aspidosaurus chiton (DeMar, 1966). They are slightly longer than the neural spine, and, as in Aspidosaurus, there was only a single layer of plates present. In end view, the osteoderms are shaped like an inverted V and lack a ventral projection (unlike in Conjunctio multidens; Carroll, 1964).

The description of new material of Cacops from the Early Permian fissure fillings in the Dolese Brothers limestone quarry at Fort Sill (Fröbisch and Reisz, 2012 and Reisz et al., 2009) and modern cladistic analyses of related taxa (Polley and Reisz, 2011 and Schoch, 2013) have triggered renewed interest in the interrelationships among Dissorophoidea. In the present study, we focus on: (1)

assessing the phylogenetic position of Scapanops;

Because only the skull roof is reasonably well preserved or exposed, we also include other dissorophoid taxa, notably Amphibamidae, to which Scapanops bears some resemblance in the morphology of the skull roof. Unfortunately, the palate, which differs considerably among dissorophoid clades, is not exposed in Scapanops.

The software package TNT 1.1 (Goloboff et al., 2008) was employed for the phylogenetic analysis. All characters were treated as unordered. The parsimony analysis was performed in the heuristic search mode, both under the Traditional Search and New Technology Search options. Both searches yielded the same alternative topologies.

We employed a slightly modified version of Schoch's (2012) character-taxon matrix for Dissorophidae, excluding the “wild card” taxa Brevidorsum profundum and Broiliellus texensis, both of which are in urgent need of restudy. Conjunctio multidens was coded from both the holotype and the referred specimen UCMP 40103 (“Rio Arriba taxon” of Schoch, 2012). Hence, the matrix comprised 25 taxa and 70 characters, as listed in Schoch (2012).

The analysis yielded three most parsimonious trees, each with a length of 126 steps, a Consistency Index (CI) of 0.611, and a Retention Index (RI) of 0.802. The general topology is very similar to that recovered by Schoch (2012), differing only in the more secure placement of Conjunctio among Eucacopinae. The three named species of Cacops (C. aspidephorus, C. morrisi, C. woehri) form an unresolved polytomy with the most derived eucacopines Zygosaurus and Kamacops. Fig. 4 lists the synapomorphies supporting each node.

The present findings add to our knowledge of a diverse clade of the dissorophid temnospondyls, Eucacopinae. These able terrestrial walkers (Dilkes, 2009) were more lightly built than the heavily armoured dissorophines, but had proportionately very large skulls with knobby ornament, which tended to overprint the sutures in adult specimens of Cacops aspidephorus, Kamacops, and Zygosaurus. Eucacopines are known from Texas and New Mexico, France (R. Werneburg, personal communication 2012), European Russia, and China.

The present results show that, in addition to Cacops and the derived Russian forms Kamacops and Zygosaurus, there must have been a basal eucacopine radiation represented by forms such as Conjunctio and Scapanops. These basal taxa have osteoderms similar to those of Aspidosaurus, consisting of a single row of quadrangular plates. This lends support to the hypothesis that the double rows of osteoderms must have evolved independently in eucacopines and dissorophines (Schoch, 2012).

Conjunctio shares with Cacops the general skull proportions, the large pineal foramen, and the shape of the external naris. It differs by having slightly smaller orbits and a proportionately slightly longer skull table. The heart-shaped internarial fenestra is an additional feature shared by both taxa but is also present in Scapanops. In both taxa, the postparietal has a raised posterior margin, formed of hypertrophied dermal sculpturing. They also share with Scapanops the presence of a distinct occipital flange.

Considering their substantial morphological differences, it is interesting that Conjunctio and Scapanops also share an additional apomorphy, the high number of teeth in the upper jaw. This feature was originally noted by Carroll (1964) and led to his referral of the holotype of Scapanops neglecta to Conjunctio. If our hypothesis is valid, this apomorphy was shared by basal eucacopines and then reversed to fewer and larger teeth in the least inclusive clade that includes Cacops and Kamacops. This would suggest a shift in dietary preference from small (Scapanops, Conjunctio) to larger prey (Cacops, Kamacops, Zygosaurus).

Scapanops is distinguished by its short and narrow skull table, short “cheek” region, very short distance between temporal notch and orbits, and an elongated preorbital region exceeding that of most other dissorophoids, with the exception of Kamacops and Zygosaurus. The short skull table of Cacops differs strongly from that of Scapanops in that the supratemporal and parietal have very different outlines.

Scapanops resembles amphibamids in certain features such as the small postparietal and tabular, the disproportionately large temporal notch, and its close proximity to the orbit. However, the osteoderms, cranial sutures, and the presence of the system of ridges and depressions on the skull roof are clearly dissorophid characters that are not present in any known amphibamid. Schoch (2012) has discussed in detail the distinction between amphibamids and dissorophids, which is supported by numerous characters.