A new species of Cyclotosaurus (Stereospondyli, Capitosauria) from the Late Triassic of Bielefeld, NW Germany, and the intrarelationships of the genus

A nearly complete dermal skull roof of a capitosaur stereospondyl with closed otic fenestrae from the middle Carnian Stuttgart Formation (Late Triassic) of BielefeldSieker (NW Germany) is described. The specimen is assigned to the genus Cyclotosaurus based on the limited contribution of the frontal to the orbital margin via narrow lateral processes. A new species, Cyclotosaurus buechneri sp. nov., is erected based upon the following unique combination of characters: (1) the interorbital distance is short so that the orbitae are medially placed (shared with C. robustus); (2) the region lateral to the orbitae is only slightly broader than the orbitae (shared with C. posthumus, C. ebrachensis, C. intermedius, and C. mordax); (3) the postorbital region is slender (shared with C. ebrachensis); (4) the preorbital projection of the jugal is shorter than half the length of the snout (shared with C. mordax, C. ebrachensis, C. intermedius, C. posthumus, and C. hemprichi). A phylogenetic analysis of seven Cyclotosaurus species, including C. buechneri, and eight further capitosaur taxa with the Rhinesuchidae as an outgroup finds a monophyletic Cyclotosaurus. In accordance with its stratigraphic occurrence, C. buechneri nests at its base but is more derived than C. robustus. Among the more derived Cyclotosaurus species, C. ebrachensis and C. intermedius, as well as C. posthumus and C. hemprichi, form sister groups, respectively. However, the phylogenetic position of C. mordax with respect to both groups remains unresolved. In the phylogenetic analysis presented here, Cyclotosaurus is the sister group of the Heylerosaurinae (Eocyclotosaurus+Quasicyclotosaurus). Cyclotosaurus buechneri represents the only unequivocal evidence of Cyclotosaurus (and of a cyclotosaur in general) in northern Germany.


Introduction
The largest amphibians in earth's history can be found among the capitosaur stereospondyls, a group of temnospondyls that reached body lengths of more than 5 m with large skulls and heavily ossified postcranial skeletons. Throughout the Triassic period, these superficially crocodile-like aquatic predators dominated the limnic and fluviatile ecosystems but sometimes also occurred in lagoonal and coastal marine strata (Schoch and Milner, 2000;Damiani, 2001;Steyer, 2003; . A cranial feature, present in some capitosaurs, is the posteriorly closed squamosal embayments (or "otic notches"), called the otic fenestrae. Capitosaurs bearing this character are often informally called "cyclotosaurs", literally meaning "round-ear lizards", although recent phylogenetic analyses suggest that this character may not be an indicator of a closer phylogenetic relationship (Damiani, 2001;Fortuny et al., 2011;Sidor et al., 2014). In Triassic strata of Germany, cyclotosaurs with closed otic  (Schoch and Milner, 2000). Cyclotosaurus is the best documented and most species-rich cyclotosaur genus. Its first remains were found more than 150 years ago in the southern German state of Württemberg. Furthermore, Cyclotosaurus is among the earliest temnospondyl genera ever described (Meyer and Plieninger, 1844;Quenstedt, 1850). Different species of Cyclotosaurus were later reported from Germany (Fraas, 1913;Kuhn, 1932Kuhn, , 1939Kuhn, , 1942, Poland (Sulej and Majer, 2005), Greenland (Jenkins et al., 1994;Sulej et al., 2014), Spitsbergen (Kear et al., 2015), and Thailand (Ingavat and Janvier, 1981). In Germany, Cyclotosaurus specimens are thus far known from the southern states of Baden-Württemberg and Bavaria, and an isolated finding was reported from central Germany (Saxony-Anhalt) (Jaekel, 1914; Kuhn, 1939Kuhn, , 1942. Remains of cyclotosaurs were almost unknown from northern Germany, with the exception of some dermal bones ("armor plates") from Keuperian sandstone layers (referrable to the Late Triassic Stuttgart Formation; Duchrow, 1984) north of the city of Melle (approximately 20 km north of Bielefeld, Fig. 1), briefly mentioned by Römer (1857, p. 682). These rare finds were later mentioned by von der Marck (1892) and Dienemann (1928Dienemann ( , 1939. Römer (1857) assigned these specimens to "Mastodonsaurus robustus" (Cyclotosaurus robustus) based on similarities to findings of "Mastodonsaurus robustus" from the Stuttgart Formation (Schilfsandstein) of Württemberg described by Quenstedt (1850). Unfortunately, the specimens from Melle were illustrated by neither Römer (1857) nor by von der Marck (1892) and Dienemann (1928Dienemann ( , 1939 and it can thus not be stated whether they belonged to the skull or the dermal pectoral girdle. They were part of the Oberbergrat Jugler collection and are believed to be lost. More than 40 years ago the skull of a cyclotosaur was discovered by Dr. Martin Büchner, then director of the Museum of Natural History in Bielefeld, during an excursion of the natural history society of Bielefeld (Naturwissenschaftlicher Verein für Bielefeld und Umgebung) in the Pape quarry in the Teutoburg Forest, which at that time served as a landfill for excavated soil. The sandstones that contained the skull originated from a construction site for a new bank building in Bielefeld-Sieker (Fig. 1). The find was brought to the Staatliches Museum für Naturkunde in Stuttgart, Germany, where it was prepared. It became part of the collection of the Museum of Natural History in Bielefeld and was assigned to Cyclotosaurus robustus. The skull became renowned locally in Bielefeld and surrounding areas through several media articles. A cast of the "ancient amphibian from Bielefeld" (Bielefelder Urlurch) is on display in a glass showcase mounted in the floor of an underground station in Bielefeld. In spite of this, the skull has never been described scientifically, with only brief mention in the scientific literature. In the Handbook of Paleoherpetology, Schoch and Milner (2000, p. 154) mentioned that C. robustus had been found in Bielefeld in addition to southern Germany, but they did not explicitly refer to the skull under study and neither de- scribed nor illustrated it. Büchner (2008a, b) mentioned and illustrated the skull and referred it to as Cyclotosaurus robustus. In this study, we provide a description of the specimen and a phylogenetic analysis of the genus Cyclotosaurus based largely on the data matrix of Damiani (2001) and . Our analysis includes the Bielefeld skull and -for the first time -all valid species of Cyclotosaurus. Because of the unique combination of characters present in the Bielefeld specimen, we erect a new species, Cyclotosaurus buechneri sp. nov.

Material
The specimen described herein is an almost complete skull roof in dorsal view. It shows a well-preserved dermal sculpture and is embedded in sandstone with the palate and occiput obscured (Fig. 2). The mandible and postcranial remains are unknown. The sutures between the skull roofing elements can be traced on the dorsal side of the skull, whereas, in the posterolateral part, the sutures of the ventral surface of the skull roof are preserved as imprints on the surface of the steinkern, i.e., consolidated sediment matrix that remained as a cast of the inner surface of the dermal skull roof after the original bones were lost. The specimen is housed in the collection of the Museum of Natural History in Bielefeld, Germany (inventory number Namu 1 ES/k 36053), and has so far been tentatively designated Cyclotosaurus robustus.

Locality and geological setting
The stereospondyl skull described in this study was found in September 1975 within the sandstone of the former excavation pit for a bank building in Bielefeld-Sieker (N 0 31.32 52 • E 34 1.68 8 • ), North Rhine-Westphalia, northwestern Germany (Fig. 1). An approximately 10 m thick succession of clay-rich sandstone and intercalated argillaceous marlstone of the Carnian (Late Triassic) Stuttgart Formation (Büchner, 1975;Mestwerdt and Burre, 1981) was temporarily exposed at this locality. The sandstone is especially rich in fragments of Equisetites arenaceus. These plant fossils are a characteristic feature of the sandstone facies of the Stuttgart Formation in general and are the reason for its name "Schilfsandstein" (literally "sandstone with reed") within the classical literature (e.g., Jaeger, 1827). In some cases, the overall bright-grey sandstone shows spots of intense green coloration in this locality and, in contrast to the marlstone, it contains only minor amounts of pyrite.
Associated sediments of the Stuttgart Formation are interpreted as predominantly fluviatile and lacustrine (Nitsch, 2005). Thick accumulations of sandstones are regarded as channel fills, which might apply for the former outcrop in Bielefeld-Sieker as well. In addition to the occurrence in Germany, a similar, time-equivalent facies is present in Poland ("piaskowiec trzcinowy") and northern France ("grès à roseaux") (Nitsch, 2008).

Shape and proportions of the skull
The skull has a parabolic outline with the lateral margins being straight to slightly convex rather than concave (Figs. 2, 3). Measured from the tips of the premaxillae to the preserved posterior end of the postparietals, the skull is 280 mm long. A sagittal line from the premaxillary tip to the level of the posterior end of the postotic bar measures 308 mm. The preorbital part of the skull (the "snout") is elongate and measures 175 mm in the midline. The posterior border of the skull table (formed by postparietals and tabulars) was apparently distinctly concave but its exact outline is not preserved.

Teeth
Marginal teeth of the labyrinthodont type are exposed in labial view in the region of the left premaxilla-maxilla boundary. They are barely visible because of a thick layer of lacquer coat that was applied to the skull during preparation. Other teeth are not exposed.

Openings of the skull roof
The line, with their longitudinal axis being oriented anteromedially. The maximum length / width ratio of the well-preserved left naris measures 21 to 9.9 mm. The nares are bordered anteriorly by the premaxilla and medially and posteromedially by the nasal. The maxilla forms the posterolateral boundary of the naris, but its anterior expansion lateral to the naris remains unclear. The orbitae are located in the posterior third of the skull roof. They are ovate in outline and their longitudinal axis is oriented anteroposteriorly. The maximum length/width ratio of the left orbit measures 38/27 mm, and that of the right orbit is 36/27 mm. The skull roof is dorsally concave between the orbitae. This concavity extends anteriorly along the median suture of the frontals. Here, two anteriorly running, shallow, broad ridges are formed, which extend from the anteromedial orbital margin. The interorbital concavity likewise extends posteriorly along the medial part of the postorbital skull table. The orbitae are framed by the prefrontal anteriorly, the frontal anteromedially, the postfrontal medially and posteromedially, the postorbital posterolaterally, and the jugal laterally. The parietal foramen (pineal opening) is located on the interparietal suture and well posterior to the orbital margin. Posteriorly the postotic bar of the tabular closes the squamosal embayment (or "otic notch") so that an otic fenestra is formed on each side. Posterolaterally, a squamosal-tabular contact is established. The preserved otic fenestra has an irregularly rounded outline with a maximum anteroposterior extension of 24 mm and maximum mediolateral extension of 27 mm.

Dermal sculpture
The dermal sculpture (or ornament) consists of furrows and polygons that are rather regular in outline, which is a common condition in many stereospondyls (Witzmann et al., 2010). Polygons are visible on the anterior half of nasal and lacrimal, the posterior two thirds of the prefrontal, the posterior third of the frontal, the ossification center of the jugal, and -where preserved -on the parietal, postparietal, postfrontal, postorbital, and supratemporal. Ridges and furrows, representing "zones of intensive growth" (Bystrow, 1935), are present on the anteriormost part and the posterior half of the nasal, the posterior half of the lacrimal, the anterior third of the prefrontal, the anterior two thirds of the frontal, the posterior, and posteromedial part of the jugal and -where preserved -on the squamosal. The sculpture is almost completely eroded on the premaxilla, the maxilla, and the tabular. The sizes of polygons differ; those on the postorbital skull table are distinctly smaller than those on the frontal, prefrontal, and jugal.

Lateral line sulci
In some regions of the skull roof, the lateral line sulci are deeply impressed and well visible, whereas in others they are largely or completely obscured by the "normal" der-mal sculpture. The lateral line sulci are most prominent on the medial part of the snout: the supraorbital sulcus extends posterolaterally from the ossification center of the nasal along the nasal-lacrimal and prefrontal-lacrimal boundary and then curves medially to the ossification center of the prefrontal. From here it extends towards the frontal, where it cannot be traced any longer. A Z-shaped flexure of the infraorbital sulcus is visible on the left lacrimal. In the posteromedial region of the jugal, a short portion of the suborbital part of the infraorbital sulcus is preserved that runs posteromedially towards the postorbital, where it is visible directly posterior to the orbit. Poor preservation of the bone surface precludes recognition of further parts of the lateral line system. A short, elongate, anteroposteriorly depression on the right side of the postorbital skull table might represent the preserved portion of the otic part of the infraorbital sulcus.

Particular bones of the dermal skull roof
The premaxilla is an anteroposteriorly short bone that forms the prenarial region and is shorter than the naris. Posteriorly the premaxilla contacts the nasal with a concave suture and constitutes the anterior (and probably anterolateral) border of the external naris. Its posterolateral extension is unknown.
There is no trace of a septomaxilla. The nasal is an elongate element that is comparatively shorter but wider than the frontal. The nasal reaches its maximum width on the level of the anterior tip of the lacrimal. Its triangular posterior projection nests between the prefrontal and frontal. Anterolaterally, it forms the medial and posteromedial margin of the naris.
The frontal participates in the formation of the anteromedial part of the orbital margin, but only via a narrow lateral process. Each frontal forms an anteromedially aligned suture with the nasal, so that the conjoint anterior portions of both frontals form an anteriorly directed wedge that nests between the nasals. The frontal reaches its maximum width between the orbitae, where it forms the lateral process with which it participates in the orbital margin.
The parietal is an anteroposterior short element that is wider than long. Its maximum transverse expansion is slightly larger than that of the frontal. The anterior border to the frontal is visible in the left parietal and is anteromedially inclined. The anterolateral margin of the parietal is distinctly concave to accommodate the postfrontal. Laterally, the parietal forms a straight border with the supratemporal. The posterior suture with the postparietal cannot be traced with certainty and is therefore stippled in Fig. 3. The parietal foramen is located slightly anterior to the midlength of the parietals. The interparietal suture in front of the foramen is well visible and only slightly meandering. In contrast, the suture is completely concealed by dermal sculpture posterior to the foramen.
The lacrimal is an anteroposteriorly elongate and transversally narrow element that does not participate in the orbital Foss. Rec., 19, 83-100 margin. Posteriorly a pointed, seemingly triangular portion is formed that nests between the prefrontal and jugal. Here, a slightly laterally convex lacrimal-nasal suture is visible. The anterior portion of the lacrimal is poorly preserved but appears to be pointed. The prefrontal forms the anterior orbital margin. The length of this bone accounts for almost the half of the preorbital skull length. It is a slender element whose lateral and medial margins are roughly parallel to each other but converge anteriorly to form a tip that wedges in between nasal and lacrimal. The prefrontal terminates anteriorly at the level of the anterior tip of the frontals. The postfrontal forms a convex medial border with the frontal and parietal, and a convex lateral border with the postorbital and supratemporal. It forms part of the posterior and -via an anterior process -posteromedial margin of the orbit. A contact to the prefrontal as in some specimens of Cyclotosaurus mordax (Schoch and Milner, 2000; is not established.
The postorbital is placed in the posterolateral portion of the skull roof. Its exact outline is unclear since the rear part of the specimen is not sufficiently preserved. The postorbital forms the posterolateral border of the orbit and, via a pronounced projection, large parts of the cranial portion lateral to the orbit. The named projection forms a suture with the jugal, which runs first nearly transversally and then turns abruptly in a posterolateral direction, forming a blunt wedge between jugal and squamosal. The posterior and posteromedial border of the postorbital cannot be traced because of the poor preservation.
A broad medial extension of the jugal forms most of the lateral orbital margin. The jugal is graded posteromedially to accommodate the aforementioned lateral projection of the postorbital. Its acute anterior margin reaches at least slightly farther anteriorly than the level of the posterior tip of the nasal and wedges in between the lacrimal and maxilla. The posterior extension of the bone is not clear, and also its lateral border with the maxilla is mostly not visible. However, the width of the jugal lateral to the orbit is virtually identical to the maximum orbital width.
The borders of the postparietal are mostly not visible, only on the left side of the skull, and an anterior suture to the parietal and a part of the medial suture between the postparietals are indicated. The posterior part of the postparietals is preserved as an imprint of the internal bone surface on top of the steinkern. It shows the posterior part of the internal sutures between the postparietals as well as the anteroposteriorly aligned suture of the right postparietal with the tabular.
Most of the sutures of the tabular with its surrounding elements are not or only poorly preserved (and thus questionable) on the dorsal bone surface, or they are visible as imprints on the steinkern. The tabular horn is laterally directed and forms a suture with the squamosal at the posterolateral side of the skull. Thus, the squamosal embayment is completely closed posteriorly by a robust postotic bar and a true postotic fenestra is formed that is encircled by the tabular posteromedially and by the squamosal anterolaterally.
The squamosal, supratemporal, quadratojugal, and maxilla are too poorly preserved to provide proper descriptions of their outlines and morphologies.
The analysis was performed with PAUP*4.0 (Swofford, 2003) using branch-and-bound search settings (multistate taxa interpreted as polymorphism) with all characters unordered except for characters 10 and 11, which were ordered. Three most parsimonious trees were recovered (tree length, TL = 114; consistency index, CI = 0.6754, CI excluding uninformative characters = 0.5978; homoplasy index, HI = 0.4035, HI excluding uninformative charac-  Fig. 5). The strict consensus of the three most parsimonious trees is shown with "cyclotosaurs" (i.e., forms with squamosal embayment closed to an otic fenestra) highlighted in green. Capitosauroids sensu  are highlighted in grey. Abbreviation: Mast., Mastodonsaurus. ters = 0.4022; retention index, RI = 0.6783; and rescaled consistency index, RC = 0.4581). The strict consensus of the three trees is shown in Fig. 4, and the intrarelationships of the genus Cyclotosaurus (based on the consensus tree) are shown in Fig. 5. In all trees Cyclotosaurus (corresponding to Cyclotosaurinae of Schoch and Milner, 2000) is shown to be monophyletic, with C. ebrachensis and C. intermedius as well as C. posthumus and C. hemprichi forming sister taxa, respectively. However, the relationship of these two groups with C. mordax is not resolved. Likewise, the Bielefeld specimen clusters as a sister to this clade in all trees, and C. robustus always forms the basalmost species (Fig. 5). Cyclotosaurus is supported by the following synapomorphies: #7 (state 0), lateral lines weakly impressed; #16 (0), vomerine plate short; #20 (1), anterior palatal vacuity medially subdivided by anterior process; #21 (0), pterygoid-parasphenoid suture as long as basal plate is wide; #24 (2), cultriform process slender with deep ventral crest; #25 (0), ectopterygoid excluded from lateral margin of interpterygoid vacuity; #48 (1), quadrate ramus of pterygoid laterally aligned and abbreviated; #66 (1), snout width wide parabolic. The post-C. robustus clade containing the Bielefeld specimen as its basalmost member is supported by one synapomorphy: #13 (0), preorbital projection of jugal shorter than half the length of the snout. The remaining grouping of Cyclotosaurus is supported by #44 (1), labial wall of adductor chamber dorsally convex; #68 (1), in-terorbital width to width of orbitae equal to or larger than 1.8. The sister-group relationship of C. ebrachensis and C. intermedius is supported by three synapomorphies: #32 (0), denticle fields on the palate; #67 (1), the laterally straight margin of the skull lateral to the otic fenestra; #69 (1), the straight to convex posteromedial margin of the tabular. The grouping of C. posthumus and C. hemprichi is supported by two synapomorphies: #1 (1), the tapering preorbital region; #66 (0), the elongate, parabolic snout width.

Generic assignment
The most striking feature of the Bielefeld skull is the presence of a rounded otic fenestra -i.e., the squamosal embayment is completely closed posteriorly by a postotic bar formed by tabular and squamosal. In stereospondyls, the presence of otic fenestrae is called the "cyclotosaur condition" and is characteristic of a group informally called cyclotosaurs . Fortuny et al. (2012) showed on the basis of finite-element analysis (FEA) that the otic fenestra in stereospondyls is an adaptation for the distribution of stress and deformation during skull raising. The known taxa with otic fenestrae are Procyclotosaurus from the Ladinian of England (Schoch and Milner, 2000); Cyclotosaurus from the Carnian and Norian of Germany, Poland, Greenland, and Thailand (Fraas, 1913;Kuhn, 1932Kuhn, , 1942Schoch and Milner, 2000;Sulej and Majer, 2005;Schoch et al., 2008); Kupferzellia (otic fenestra closed in adults) from the upper Ladinian of southern Germany (Schoch, 1997); Eocyclotosaurus from the Anisian of France, Germany, England, and North America (Heyler, 1969;Ortlam, 1970;Schoch, 2000a;Schoch and Milner, 2000); and Quasicyclotosaurus from the Anisian of North America (Schoch, 2000a. However, in contrast to earlier discussions (Welles and Cosgriff, 1965), it appears that this "cyclotosaur condition" evolved at least two times independently (Shishkin, 1980;Damiani, 2001 According to , there are two main phylogenetic hypotheses about cyclotosaur relationships: the monophyly scenario (or Pancyclotosauria hypothesis) and the convergence scenario (Eocyclotosauria and Paracyclotosauria hypothesis). In the monophyly scenario, Cyclotosaurus, Eocyclotosaurus, and Quasicyclotosaurus are closely related and the otic fenestrae therefore evolved once in their stem group (Schoch and Milner, 2000;Schoch, 2000b) and a sec-ond time in Kupferzellia and Procyclotosaurus. In the convergence scenario, Cyclotosaurus is more closely related to Procyclotosaurus and Kupferzellia (within a clade called Eucyclotosauria) than to Eocyclotosaurus and Quasicyclotosaurus (within a clade called Paracyclotosauria). Thus, in this scenario, the otic fenestra must have evolved once within the Eucyclotosauria and a second time within the Paracyclotosauria .

F. Witzmann et al.: A new species of Cyclotosaurus (Stereospondyli, Capitosauria)
The phylogenetic analysis of Schoch (2008) supported the convergence scenario, and it was further supported by the phylogenetic analyses of Fortuny et al. (2011) and Sidor et al. (2014), according to which Eocyclotosaurus and Quasicyclotosaurus (on the one hand) and Cyclotosaurus (on the other hand) were not closely related. In contrast, the phylogenetic analysis presented herein supports the monophyly hypothesis with Cyclotosaurus being the sister group of the Heylerosaurinae (Eocyclotosaurus + Quasicyclotosaurus). If this grouping is correct (it is admittedly based on a rather small number of taxa), it implies that the otic fenestrae evolved only once in its stemgroup. However, in this scenario the otic fenestrae also evolved more than once in capitosaurs: it may have evolved a second time in Kupferzellia (Tatrasuchus) and a third time in Procyclotosaurus (both of which are not closely related to Cyclotosaurus in the present analysis, in contrast to , where they form the Eucyclotosauria, and to Fortuny et al., 2011, where Kupferzellia (Tatrasuchus) is the sister taxon of Cyclotosaurus, and Procyclotosaurus is the sister taxon of the Heylerosaurinae). Alternatively, the otic fenestrae may have evolved in post-Eryosuchus capitosaurs with a reversal in post-Procyclotosaurus forms according to the present analysis (Fig. 4).
To which existing groups of capitosaurs with closed otic fenestra can the Bielefeld skull be attributed to? An assignment to Procyclotosaurus can be ruled out because this taxon has a triangular skull with broad cheeks and a slender, tapering snout (Schoch and Milner, 2000) that differs considerably from the parabolic outline of the Bielefeld specimen. Eocyclotosaurus and Quasicyclotosaurus can be excluded because of their more slender snouts with concave lateral margins and the sutured prefrontal-postfrontal contact medial to the orbitae (Schoch, 2000a. Kupferzellia (Tatrasuchus) has a much broader postorbital skull and a proportionally shorter snout, which is more tapering anteriorly (Schoch, 1997;Schoch and Milner, 2000). Instead, a diagnostic feature that unites the Bielefeld skull with Cyclotosaurs is the limited contribution of the frontal to the orbital margin via narrow lateral processes (Schoch and Milner, 2000). This assignment is supported by the present analysis in which the Bielefeld specimen constantly nests within a monophyletic Cyclotosaurus clade.

Species assignment
In the complete absence of palatal characters, it is more difficult to assign the Bielefeld specimen to a particular species of Cyclotosaurus. The stratigraphically oldest undisputed representative of Cyclotosaurus is C. robustus from the Schilfsandstein (middle Carnian) of southwestern Germany (Meyer and Plieninger, 1844;Quenstedt, 1850;Schoch and Milner, 2000;. Its autapomorphic characters are the much widened parabolic skull whose lateral margins are convex, plus the short interorbital width. The late Carnian taxa C. intermedius from the Opole region of Poland (Sulej and Majer, 2005) and C. ebrachensis (Kuhn, 1932) from Franconia (northern Bavaria) are the stratigraphically next youngest species. Schoch (2008, p. 214) pointed to the similarity between both species, "which apparently hardly differ at all". However, C. intermedius differs from all other known Cyclotosaurus species in the projection of the quadrate posterior to the hind margin of the skull roof (Sulej and Majer, 2005). Both species possess denticles on the palate (Kuhn, 1932;Sulej and Majer, 2005), but C. ebrachensis is characterized by the unique combination of an undivided, ovate anterior palatal vacuity and circular choanae (Kuhn, 1932). These taxa are stratigraphically followed by the middle Norian species C. mordax and C. posthumus (Fraas, 1913), both from southern Germany, and the late Norian C. hemprichi from Halberstadt in central Germany (Kuhn, 1939(Kuhn, , 1942. C. mordax is unique among Cyclotosaurus in bearing two apertures in the anteriormost part of the snout which accommodated the fangs of the lower jaw (Fraas, 1913). C. posthumus and C. hemprichi are similar in the outline of the skull since both have anteriorly tapering snouts. However, the margins of the snout are laterally concave in C. posthumus rather than straight as in C. hemprichi, and the orbitae are proportionally much larger and ovate in contrast to the relatively small, round orbitae in C. hemprichi (Fraas, 1913;Kuhn, 1942;Schoch and Milner, 2000). The diagnostic characters listed above for each of the six Cyclotosaurus species suggest that these species are valid.
However, some additional specimens may belong to Cyclotosaurus but their taxonomic state is unclear. A partial skull from the upper Ladinian (upper Muschelkalk) of southern Germany described as Cyclotosaurus papilio by Wepfer (1923) might represent the oldest Cyclotosaurus specimen known; however, according to , it shows no diagnostic features that warrant an assignment to that genus. Ingavat and Janvier (1981) described the posterior part of a skull from the Norian of Thailand, which they assigned to Cyclotosaurus cf. posthumus. Postcranial remains, mainly intercentra that resemble those of C. hemprichi from Halberstadt, have been reported from the Norian of Luxemburg (Milner et al., 1996) and the ?Carnian-Norian of Portugal (Witzmann and Gassner, 2008). A late Norian temnospondyl specimen from Jameson Land in Greenland was assigned to Cyclotosaurus cf. posthumus by Jenkins et al. (1994). Kear et al. (2015) tentatively referred the holotype of Capitosaurus polaris (Wiman, 1914) from the middle to late Carnian lower De Geerdalen Formation of Spitsbergen, Svalbard, to Cyclotosaurus sp.
Our comparison of the Bielefeld specimen focused on the six valid Cyclotosaurus species listed before. Apart from their later geological appearance (which in itself does not justify an exclusion), C. hemprichi differs from the Bielefeld specimen in its tapering, triangular snout and the small, circular orbitae, and C. posthumus differs by its laterally concave margins of the snout. C. mordax shows a marked differ-ence in the presence of the two apertures in the anteriormost part of the snout, which are clearly absent in the Bielefeld specimen. The two species C. intermedius and C. ebrachensis have a straight skull margin lateral to the otic fenestrae (Kuhn, 1932;Sulej and Majer, 2005), in contrast to the convex margin present in the Bielefeld specimen and all other known species of Cyclotosaurus. The remaining species, C. robustus, is similar to the Bielefeld specimen in the position of the orbitae, which are located more medially in contrast to the more lateral position in all other Cyclotosaurus species. Also, its stratigraphic age (middle Carnian Schilfsandstein, Stuttgart Formation) might be an argument in favor of a close relationship of the Bielefeld specimen and C. robustus. However, there are marked differences in proportions between the Bielefeld skull and C. robustus. First, the region lateral to the orbitae is much more slender in the Bielefeld specimen (with a ratio of width lateral to the orbit to orbital width of 1.1) than in all known specimens of C. robustus (ratio of 1.6-1.7). Second, the posterior skull region is more slender in the Bielefeld skull (ratio of maximum skull width to skull length of 0.65) than in C. robustus  2014), and the present study (but see . Therefore, the proportions of the Bielefeld skull are not regarded as juvenile here, and this is supported by the "adult" dermal sculpture of pits and ridges (sensu Bystrow 1935; see also Witzmann et al. 2010) that does not differ in its distribution on the dermal skull roof from that in C. robustus.
In conclusion, although there is no autapomorphic character present on the visible side of the Bielefeld specimen, the description and comparisons above reveal a unique combination of characters. This combination includes the short preorbital extent of the jugal, the narrow region lateral to the orbit, the narrow postorbital skull, and the medial position of the orbitae. For this reason, it is justifiable to erect a new species, Cyclotosaurus buechneri sp. nov., for this specimen.
The phylogenetic analysis suggests that C. buechneri sp. nov. is a basal member of the genus, which is in accordance with its stratigraphic age. It is more derived than C. robustus in the presence of a short jugal, but more plesiomorphic than all other Cyclotosaurus species (except for C. robustus) by the small interorbital width. Since the specimens from Melle have neither been described nor illustrated and their taxonomical referral is thus questionable, Cyclotosaurus buechneri sp. nov. is the only undoubted evidence of Cyclotosaurus (and of a cyclotosaur in general) from the northern part of Germany.