5.1. Skull

The skull of P. nasutus is elongate, 1.3 times longer than wide, and the snout has a parabolic outline (Figs. 2–5). The skull tapers gently towards the nares, contrasting the more triangular-shaped skulls of the other four Parotosuchus species (Schroeder 1913; Konzhukova 1955; Damiani 2002; Sulej & Niedźwiedzki 2013). In contrast to these, the quadrate condyles are substantially more posterior than the occipital ones, a feature that is more consistent with basal capitosaurs and rhinesuchids (Eltink et al. 2016a, b).

The oval orbits are relatively small (ratio orbit length: skull length is 0.12), especially compared with Eryosuchus (0.17) or Mastodonsaurus (0.2). The preorbital skull is more than three times longer than the postorbital skull table (3.2), a feature to which the species name nasutus referred. The distance between the orbits is only slightly wider than that between the nares, giving a rather wide tip of the snout (Fig. 5C, D). The nares are nearly as long as the orbits, but only half as broad.

The 3D structure of the skull, which is well preserved in most specimens, is intermediate in height between Late Palaeozoic eryopiform temnospondyls (e.g., Eryops: Sawin 1941; Sclerocephalus: Schoch & Witzmann 2009) and other capitosauroids, which have much more flattened skulls (e.g., Cyclotosaurus, see Sulej & Majer 2009; Mastodonsaurus, see Schoch 1999). The orbits form the most exposed region of the skull roof, facing dorsolaterally. Their rim is not as elevated as in the more heavily ossified P. orenburgensis, but the general surface structure of the interorbital region is very similar in P. nasutus. In contrast to the taxon from Russia, the pineal foramen was not located on a raised area.

The sutures of the dermal skull bones are consistent with those of many capitosauroids, among which the separation of prefrontal and postfrontal is the most significant. In contrast to some specimens of Cyclotosaurus, the separation of the two elements is always established in P. nasutus, although the extent to which the frontal contributes to the orbit margin varies within and between individuals. Characteristic features of P. nasutus are the prefrobtals and frontais having pointed anterior ends at same level, supratemporals being as wide as postorbitals and substantially wider than parietais, and the parietals are much longer than the postparietals. Contributing to the different skull outline, the cheek is slenderer than in other Parotosuchus species, with a narrower squamosal. The postorbital is slightly more pointed than in P. orenburgensis, and laterally deeply wedged into the jugal. The postorbital portion lateral to the orbit is as wide as that posterior to the eye opening.

Fig. 5.

Skull material referred to Parotosuchus nasutus, skull root in dorsal view, B. small skull (16 cm long, GPS.MLU.2016.40), C, D large skull (34.7 cm long, GPS.MLU.2018.02).

Fig. 6.

Skull material referred to Parotosuchus nasutus, skull root in dorsal view. A, left portion of small skull (GPS.MLU.2018.06), B, anterior part of skull with associated mandibles (GPS.MLU.2016.01), C, left-central portion of skull (GPS.MLU.2016.23), D, fairly complete skull, only internal bone surface preserved (GPS.MLU.2018.07). E, skull roof (MNVD 11166).

Fig. 7.

Skull material referred to Parotosuchus nasutus, palate and occiput. A, C, E, GPS.MLU.2018.01. A, good palate, B, anterior palate (restored), C, close-up of basicranial region, D, occiput, E, oblique posterodorsal view on occiput (SMNS 5957), F, symphyseal region (SMNS 12799), G, mandible (GPS.MLU.2016.07).

Fig. 8.

Skull material referred to Parotosuchus nasutus, palate and occiput. A, good palate (GPS.MLU.2018.01), B, medium-sized palate (GPS.MLU.2018.36), C, palate and skull roof (GPS.MLU.2018.05), D, anterior palate (MSB G403), E, large anterior palate (GPS.MLU.2018.03), F, occiput (GPS.MLU.2018.01).

Unlike in “higher” capitosauroids, which have more laterally directed tabular horns, the tabular is always posterolaterally aligned and has a well-rounded end. Typically, the tabular is larger by area than the postparietal, a feature consistent throughout the genus Parotosuchus. In contrast to other Parotosuchus species, especially P. haughtoni, the squamosal embayment is proportionally larger and more open posterolaterally. Similar to other Parotosuchus species, the posterior margin of the skull table is semilunar.

5.2. Palate

The palate bones are thinner than in closely related species, notably P. orenburgensis and P. helgolandicus. This results from the proportionally much larger interpterygoid vacuities, as well as the more slender and thin pterygoids, palatines, and ectopterygoids (Figs. 7, 8). The tooth patches known from many Palaeozoic temnospondyls are entirely absent, so that the dentition is confined to the two tooth rows on the maxilla and the vomer-palatine-ectopterygoid arcade. Only two pairs of fangs are present: one large on the vomer, a smaller one on the palatine, whereas the ectopterygoid bears only a row of uniform, small teeth.

The vomer is flat with a smooth surface, except for the narrow countersunk region in the sagittal midline, where the cultriform process is exposed. This region (fodina vomeralis) is narrower than in P. orenburgensis and anteriorly not expanded. There is a continuous row of small teeth medial to the choana and posterior to the premaxilla, which is aligned medial to the vomerine fang pair. The crowns of these teeth are aligned posteromedially. The fangs themselves also have posteromedially oriented crowns.

The premaxilla and vomer are sutured anterior to the vomerine fangs, leaving a wide unpaired opening that occupies more than two-thirds of the width of the snout, as measured at the transverse premaxillo-vomerine suture. The anterior palatal opening is crescent-shaped, with a gently rounded posterior margin, markedly rounded lateral margins, and a large anteromedial constriction by a rounded posteromedial process of the premaxillae. The suture between the premaxillae is strongly serrated within this process. The premaxilla, which is more rounded than in P. orenburgensis and P. helgolandicus, houses 10 to 12 medium-sized teeth, which are longer than the vomerine and palatine teeth, but markedly smaller than the anterior maxilla teeth. Like the vomerine teeth, but less strongly so, their crowns are curved inwards, towards the buccal cavity.

The choana is characteristically elongate but very narrow, more slender than in most other capitosaurs. It is still wider and proportionally larger than in the other three Parotosuchus species, but in adults of P. nasutus it is clearly narrower than in juveniles.

The pterygoid is elongated and the slender palatine ramus is about twice as long as the stout quadrate ramus. The basipterygoid ramus is very short and sutured with the basal plate of the parasphenoid. The length of this suture measures only half the width of the basal plate, in contrast to mastodonsaurids and cyclotosaurids, where it is posteriorly much extended. The entire basipterygoid and palatine rami are covered by polygonal ridges, as well as the anteriormost portion of the quadrate ramus. The ornament is more extensive than in P. orenburgensis and P. helgolandicus.

The parasphenoid is slender with a differentiated cultriform process measuring four times the length of the rectangular basal plate. There is a medial ventral ridge that rises on the anteriormost portion of the basal plate, close to the merger with the cultriform process. This ridge extends only along the posterior half of the process, which is anteriorly flattened. The basal plate is markedly wider posteriorly, and its anteromedial portion forms a bowl-shaped depression. It is edentulous but covered by numerous radially arranged grooves converging in a point close to the centre of the basal plate, and additional tiny tubercles on top of the ridge and along its flanks. The posterior margin of the parasphenoid houses paired muscular pockets, partially subdivided by a medial process. Only the anterior margin of the pockets is formed by the parasphenoid, the bulk of these paired depressions extends broadly onto the ventral surface of the exoccipitals. Lateral to the pockets, the exoccipitals and parasphenoid form a ventrally bulging suture that probably housed an opening of the eustachian tube.

5.3. Braincase and occiput

The occiput is exposed in a range of skulls, while in SMNS 5957, the exocciptals have been dorsally freed from their dermal bone cover (Fig. 7E). The occipital condyles are smaller and more widely separated than in the other three Parotosuchus species of which complete skulls are known, giving a substantially larger foramen magnum than in these. As the thickness of dermal bones is not as excessive as in the other Parotosuchus species, the postparietals and tabulars form only a thin dorsal margin of the occiput. There is a well offset lamina ascendens of the pterygoid, well preserved in SMNS 5957, forming an almost vertically aligned lamella with a sigmoidal curvature in dorsal view (Fig. 8B). The paraquadrate foramen is rounded and located close to the ventrodistal margin of the quadratojugal, at the margin of a large oval depression that covers most of the occipital area of the quadratojugal. The crista obliqua of the pterygoid is present, but not as pronounced as in trematosauroids (Bystrow & Efremov 1940). The exoccipitals are massive, with transversely oval and paired occipital condyles well separated by a wide gap left by the unossified basioccipital. The vertical columns are robust but only narrowly sutured to the tabulars, with broad dorsomedial portions bearing two deep grooves and framed by three marked crests on their occipital surface (Fig. 8B). The exoccipitals are sutured to the parasphenoid but fail to meet the pterygoids. Laterally, the exoccipital and parasphenoid leave a slit-like opening bordered by a bulge, probably the opening of the eustachian tube (Schoch 2000a, 2002a). Ventrally, the exoccipitals have a markedly roughened surface probably as attachment area for hypaxial musculature. The otic and epipterygoid regions are not exposed in any specimen. In P. orenburgensis, the epipterygoid is well-preserved, forming a flat, curved sheet of bone similar to the central portion of the palatoquadrate in Mastodonsaurus giganteus (Schoch 2002b), albeit less extended and lacking the additional processes.

5.4.Mandible

Only few diagnostic remains of mandibles are preserved (Fig. 7F, G). In P. nasutus, the lower jaw is markedly deeper and has a more convex ventral margin than in co-occurring Trematosaurus. It is consistent with the mandible of P. orenburgensis in general proportions, although the Meckelian fenestra is substantially longer and lower. The mandible is also less deep than in the Russian taxon, more similar to other medium-sized capitosauroids, such as Kupferzellia (Schoch 1997). The post-glenoid region (sensu Jupp & Warren 1985) is short and rounded, with the hamate process present but much lower than in mastodonsaurids or cyclotosaurids, consistent with that of P. orenburgensis (Konzhukova 1965).

The symphysis (SMNS 12799; Fig. 7F) bears: a continuous row of marginal teeth, similar in size to those further posterior but with narrower bases; a pair of tusks (of which in the studied specimen the medial one is much larger); and a posterior row of five teeth, which are slightly smaller than the marginal teeth and whose bases are rounded. The anterior coronoid and presplenial are well preserved and their sutures are consistent with those in Mastodonsaurus (Schoch 1999).

5.5.Visceral skeleton

The only two preserved visceral elements are the stapes and basibranchial, which form the usual ossification of this part of the skeleton. The stapes is elongate and anteroposteriorly compressed. It is preserved in articulation on both sides of GPS.MLU.2018.01, which permits only its posteroventral side to be examined. The distal end points into the squamosal embayment, close to the suture between tabular and squamosal. The proximal portion is bifurcate, with a ventral process apparently articulating with the parasphenoid. There is a large stapedial foramen in the proximal half of the element, and a second foramen at about mid-level of the stapes. The basibranchial is only preserved in one specimen, where it is exposed due to an oblique break between skull roof and palate. It forms a slender and small, poorly differentiated element, much less pronounced than in stereospondylomorphs (Witzmann 2006; Schoch & Witzmann 2009), rhinesuchids (van Hoepen 1915), or plagiosaurids (Witzmann & Schoch 2013).

5.6.Referred postcranial material

Only few postcranial elements are known of P. nasutus, because the material consists of isolated bones or skulls rather than articulated skeletons. Comparison with close relatives is further rendered difficult, because among the Parotosuchidae, there is not a single species known by an articulated skeleton.

The bulk of the available postcranial elements from Merkel's Quarry may be referred to Trematosaurus brauni, which is readily identified because of its elongated and slender clavicle and interclavicle. Two elements, a clavicle and a partial interclavicle at GIH, differ from the mentioned bones in their broader and shorter proportions (Fig. 9A, B). These are generally consistent with elements in other capitosauroids and are here referred to P. nasutus. Several rib fragments are consistent in shape, especially the outline and position of uncinate process, with capitosaurs, such as Mastodonsaurus giganteus (Fig. 9C, D; Schoch 1999). The bodily proportions, structure of the vertebrae, relative size of limbs, trunk and tail, remain unknown.

5.7.Ontogeny and variation

Diagnostic skulls of P. nasutus cover a wide size range (Fig. 3), with the smallest (GG 22.163, 14 cm long) less than half as long as the largest complete specimen (GIH-BP1, 34.7 cm). The bulk of the material falls in the 25–30 cm size range.

In general, stereospondyl skulls grew at a remarkably isometric rate. In particular, the elongate preorbital region was established early (Warren & Hutchinson 1988; Warren & Schroeder 1995). In P. nasutus, the most conspicuous changes in the skull roof were identified in the posterior skull table. The strongest allometry is the broadening of the interorbital and internarial region, driven by widening of frontal and nasal elements.

Fig. 9.

Postcranial material referred to Parotosuchus nasutus. A, right clavicle (GPS.MLU.2016.35). B, interclavicle (GPS. MLU.2018.09). C, posterior thoracal rib (GPS.MLU.2016.10). D, anterior thoracal rib (GPS.MLU.2018.08).

As mentioned in the taxonomical section, individual variation in the interorbital distance is considerable. Ontogenetically, the ratio interorbital distance to skull length ranges from 0.13 in juveniles to 0.21 in adults. Individually, large specimens vary between slender-skulled (0.17, Fig. 5C) and wide-skulled versions (0.21, Fig. 4B, C), with the type holding an intermediate position (0.19, Fig. 2). Although the fragmentary nature of most skulls precludes exact measuring and statistical tests, it is more likely that there is a continuous range rather than two clusters.

The frontais are narrower than the prefrontals in the juveniles, whereas in adults they have nearly equal width. The postfrontals at least double their width proportionally, both expanding medially as well as accommodating the negative allometry of the orbits. The parietais and supratemporals are each extended along their lateral margins, whereas the tabular horn becomes wider both along the occipital margin and the rim of the otic notch. The squamosal develops a large falciform crest along its otic margin.

In contrast, there are no significant changes in dermal ornament or the proportional width of the lateral line sulci. Even the ornamented ventral side of the pterygoid was established in small specimens. The orbit margins are slightly raised in juveniles, getting much more pronounced in adults. Both the raised orbit margins and the thickened tabular horns are features of P. orenburgensis, which appears to extend the ontogenetic series of P. nasutus rather than forming a distinct morphology.

Dermal ornament also underwent some changes. Small specimens (Fig. 5A) have a more finely pitted skull roof and more densely set elongated ridges on the squamosal, jugal, prefrontal, and lacrimal. Larger specimens such as the type (Fig. 2D) have polygonal rather than rounded pits. In the largest specimens, the pitted ornament is extended at the expense of areas with elongated grooves and ridges (Fig. 5D).

In the palate, the interpterygoid vacuity is slightly wider and posteriorly more rounded in large specimens, whereas the choana is narrower and proportionally longer in adults. The ornament on the ventral side of the pterygoid is present already in the smallest specimens.

6.1. State of the art

Capitosaurs form a speciose clade spanning the entire Triassic with almost worldwide distribution, and new taxa are discovered on a regular basis. The relationships of capitosaurs were studied by Watson (1919, 1951, 1958, 1962), Säve-Söderbergh (1935), Romer (1947, 1966), Welles & Cosgriff (1965), and Kamphausen (1989). First cladistic analyses in which some capitosaurs were considered were conducted by Yates & Warren (2000), Schoch (2000b) and Damiani (2001a). All subsequent papers made use of variants of the Damiani's (2001a) matrix, such as Schoch (2008), Maganuco et al. (2009), Fortuny et al. (2011a), Sidor et al. (2014), Liu (2016), Witzmann et al. (2016), and Marzola et al. (2017).

As for the position of Parotosuchus, Damiani (2001a), Steyer (2003), Schoch (2008), Maganuco et al. (2009), and Liu (2016) all found this genus to nest in (or form) the basalmost clade of capitosauroids, consistent with the traditional hypothesis (Watson 1962; Schoch 2000b). However, Fortuny et al. (2011) and Sidor et al. (2014) retained Parotosuchus higher within their Capito sauria clade, more closely related to Mastodonsaurus and Cyclotosaurus than to Paracyclotosaurus and Edingerella.

6.2. Phylogenetic analysis

In the present study, the four best-known species of Parotosuchus were included in the matrix of Schoch (2008), in addition to the two more recently described capitosauroids Calmasuchus (Fortuny et al. 2011a) and Antarctosuchus (Sidor et al. 2014). In a phylogenetic analysis supported by the TNT 1.0 package, a total of 24 taxa and 69 morphological characters was analyzed under the New Technology Search option (1000 replicates). The analysis gave five most parsimonious trees (140 steps), with a consistency index of 0.56 and a retention index of 0.78 (Fig. 10).

Here, the basal position of Parotosuchus within the Capitosauria is essentially corroborated, with Parotosuchus nesting above the basal capitosaurians Edingerella and Watsonisuchus (Fig. 10) and below Cherninia, Xenotosuchus, Eryosuchus, Kupferzellia, and the “higher” capitosauroids (Stanocephalosaurus, Yuanansuchus, Antarctosuchus, Calmasuchus, Mastodonsaurus, Eocyclotosaurus, Quasicyclotosaurus, and Cyclotosaurus). This is in line with many previous findings in which capitosauroids were monophyletic, contrasting the more recently proposed alternative (Fortuny et al. 2011a; Sidor et al. 2014) of a diphyletic Capitosauroidea, in which Edingerella and Watsonisuchus nest at the base of a large “Paracyclotosauria” clade. The closure of the otic notch (squamosal embayment) had long been rated highly as a phylogenetic signal, but biomechanical analyses indicate that this pattern probably emerged convergently as a response to changes in modes of prey capture and biting (Fortuny et al. 2011b, 2012).

When included, the position of the incompletely known genus Odenwaldia is ambiguous, depending on the inclusion of Cherninia. When both taxa are considered, they form sister taxa, supported by their shared possession of a medially divided anterior palatal vacuity, which is a rather rare charcater-state in capitosaurs. When Odenwaldia is omitted, Cherninia nests at the base of post-parotosuchid capitosauroids. Finally, without Cherninia, Odenwaldia forms an unresolved polytomy with (1) Wetlugasaurus and Watsonisuchus, (2) Parotosuchidae, and (3) all higher capitosauroids. Arguments for a higher, post-parotosuchid position of Cherninia are the laterally directed tabular horn and the elongated basicranial suture (Damiani 2001b). Odenwaldia, in turn, is still more plesiomorphic than parotosuchids in retaining the prefrontal-postfrontal contact, more consistent with Wetlugasaurus, trematosauroids, and more basal stereospondyls. This indicates conflicting evidence with Edingerella and Watsonisuchus, as highlighted by the results of some recent analyses, where both these two taxa nested higher within capitosauroids (Fortuny et al. 2011a; Sidor et al. 2014).

6.3. Parotosuchidae and capitosaur evolution

The consistent morphology (otic notch, basicranial region, posterior skull table, choana) of the four Parotosuchus species agrees with the existence of a clade Parotosuchidae, and P. nasutus is here confirmed as the basalmost species of the clade, as found already by Schoch (2008). There may be a size-related effect contributing to this phylogenetic interpretation, because most specimens of P. nasutus are smaller than the types of the three other species, and many differences between P. nasutus and the other three are related to the relatively lower level of ossification in the former. Conceivably, the specimens from Bernburg represent a time-averaged population, as they were all found in the same horizon. This suggests that adult specimens should be present in that sample, which indicates that the poorer ossification of P. nasutus is not a result of immaturity of available specimens, but that the differences to other Parotosuchus species were indeed established in adults.

The geological age of the four taxa does not match their phytogeny, but stratigraphical resolution is poor, highlighted by the still open question of the exact age of the Bernburg Fossillagerstaette. In the Germanic Basin, diagnostic remains of Parotosuchus are confined to only two localities, Bernburg (P. nasutus) and Heligoland (p. helgolandicus). The only roughly similar find, a fragmentary anterior palate from the Middle Buntsandstein of the Black Forest in southwestern Germany, was shown to be more consistent with Wetlugsaurus than Parotosuchus proper (Schoch 2011).

A range of potential parotosuchids has been described in the literature, which Welles & Cosgriff (1965) and Schoch & Milner (2000) had already discussed in some detail. In most cases, more diagnostic material is required from these taxa to permit their placement in a phytogeny and to assess their affinities to Parotosuchus. The Parotosuchidae may well have been more speciose and widely distributed across Pangaea, as suggested by Sidor et al. (2007), who reported Parotosuchus sp. from Antarctica, based on a palate fragment with a slit-like choana; as discussed in Sidor et al. (2014), this find differs from , Antarctosuchus and may stem from a parotosuchid. In 2009, Shishkin & Sulej named and described P. spelaeus, based on exquisite but isolated elements from Czatkowice in Poland. They reported numerous isolated bones from that fissure-fill deposit, which also yielded important reptile material (Dzik 2009). The Polish specimen bears some resemblance to parotosuchids, but as shown in their restoration, the skull material stems from juvenile capitosaurs or small forms such as Edingerella, and apomorphies of Parotosuchus are either not apparent (choana) or not preserved (basicranium). A referral of this taxon to the Parotosuchidae is therefore impossible without more complete material. The case is more clear-cut for Parotosuchus ptaszynskii, based on a posterior palate and mandible from Olenekian deposits at Wióry at the foothills of the Holy Cross Mountains, southeastern Poland (Sulej & Niedžwiedzki 2013). The preserved parts are quite consistent especially with P. orenburgensis and P. helgolandicus, thus bridging the gap between the Germanic Basin and the Southern Urals. Finally, the material of P. orientalis (Ochev 1966) is very fragmentary, but generally consistent with that of P. orenburgensis, so that this taxon probably forms part of the Parotosuchidae.

Fig. 10.

Phylogeny of capitosauroids with special emphasis on the position of Parotosuchus nasutus and the Parotosuchidae. The position of Cherninia megarhina critically depends on the inclusion of Odenwaldia.