Plagiosaurus depressus Jaekel 1914
Creators: name: Witzmann, Florian
name: Schoch, Rainer R.
Publisher: name: Zenodo
Year: 2024
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Stereospondyli
Plagiosauridae
Plagiosaurus
Plagiosaurus depressus

description: Genus Plagiosaurus depressus Jaekel, 1914
Type and only species: Plagiosaurus depressus Jaekel, 1914.
Lectotype: MB.Am.637, a posterolateral portion of the skull (Fig. 1), designated by Hellrung (2003).
Type horizon and locality: Knollenmergel, middle Keuper, Norian, Late Triassic. Tongrube Baerecke-Limpricht near Halberstadt, Saxony-Anhalt, Germany.
Referred specimens: Apart from the lectotype, 20 specimens from the type locality are stored in the MB and one in the NHMS collection. Two specimens illustrated by Kuhn (1939) and stored at the municipal museum Halberstadt are lost.
Revised diagnosis [characters (1)–(2) Ƒom Hellrung (2003) and Schoch and Milner (2014)]: (1) Postorbital in broad contact with parietal; (2) tabular much abbreviated; (3) postfrontal absent, meaning that the parietal participates in the orbital margin; and (4) at least presacral neural spines with broad longitudinal dorsal groove that slopes in a posteroventral direction between the postzygapophyes. Characters shared with some, but not all specimens of Gerrothorax: (5) tusk on ectopterygoid present; (6) posttemporal fossa present; and (7) sculptural tubercles arranged on radial ridges on dermal skull bones. Differences from Gerrothorax: (8) posterior skull region deeper; (9) orbit proportionally larger; and (10) cheek region less expanded and thus skull narrower.
Description of the skull
General overview
From the skull, only the right posterolateral portion, the lectotype MB.Am.637, is known (Figs 1, 2). It is three-dimensionally preserved, and some of the bones are slightly displaced, exposing the bony shelves that were overlain by neighbouring bones. The skull roof consists of a partly preserved parietal, the postparietal, postorbital, supratemporal, tabular, squamosal, quadratojugal, and jugal. The sutures are, for the most part, gently undulating and not zigzag-like serrated. The preserved endocranial bones encompass the exoccipitals and the quadrate, and the palate is represented by the pterygoid, a fragment of the parasphenoid basal plate, and the ectopterygoid. The specimen measures 152 mm in width (measured from the medial margin of the postparietal to the lateral margin of the quadratojugal) and 100 mm in length (measured sagittally from the level of the preserved anterior tip of the jugal to the level of the posterior end of the sculptured part of the postparietal). The posterolateral rim of the right orbit is preserved and formed by the parietal, postorbital, and jugal; the postfrontal is absent (see description of skull roof below). Compared with Gerrothorax, the dimensions of the preserved orbital margin suggest a proportionally larger orbit in Plagiosaurus, and the cheek region (formed by the quadratojugal, squamosal, and posterior part of the jugal) is much more slender in the latter, resulting in a narrower skull. The rear margin of the dermal skull roof is straight overall, with a posteriorly straight tabular connecting the slightly convex posterior margins of the postparietal and squamosal. Laterally, the posterior margin is framed by a prominent posterolateral horn of the quadratojugal. The dorsal surface of the dermal skull-roofing bones bear a distinct tubercular sculpture (or ornament). Most of the tubercles are of similar size and outline but are more pronounced (higher) on the posterolateral edge of the quadratojugal. On most preserved bones, isolated tubercles are present in the region of the ossification centre, from which ridges with rows of tubercles radiate outwards to the bone periphery. Low ridges connect some of the tubercles, which gives the sculpture a vermicular pattern. The lateral line sulci are traceable on the jugal, postorbital, and quadratojugal (infraorbital sulcus), in addition to the supratemporal (otic part of infraorbital sulcus). Although the sulci are smooth, they can bear tubercles in places, as in Gerrothorax (Schoch and Witzmann 2012), especially in the ossification centres of the jugal and postorbital, a pattern that has so far not been reported in other temnospondyls.
Skull roof in dorsal view
The longest bone of the preserved skull roof is the jugal, which has a pointed anterior end, the tip of which is broken off (Fig. 2). The portion posterior to the orbit constitutes the deepest part of the jugal. Its posterior end is wedged between the squamosal and quadratojugal, and medially it sutures with the postorbital anteriorly and the squamosal posteriorly. However, the exact morphology of the sutures with these bones is unknown because pieces of bone are missing in this region. The dermal sculpture consists of isolated and coalesced tubercles radiating outwards from the ossification centre in the anterior half of the bone. There, the infraorbital sulcus, which runs from the postorbital to the jugal, branches into an anterior portion and into the posterior jugal sulcus that continues posteriorly on the quadratojugal. Lateral to the jugal sulcus is a parallel sulcus-like furrow on the jugal that becomes increasingly wider in the posterior part of the jugal, and lateral to it is a second furrow. These are not lateral lines, but instead represent pronounced radial sculpture, with tubercles emplaced on high ridges with broad, smooth furrows between them.
The postorbital is a triangular bone with a concave anterior margin forming the posterolateral rim of the orbit. Anterolaterally, it forms a short, striated shelf that was overlain by the jugal laterally. The posterior tip of the postorbital originally wedged between the supratemporal and squamosal but was slightly displaced postmortem. Medially, it forms a short suture with the parietal. The sculptural pattern is radial, with the most pronounced ridges in the posterior half of the bone. The sculptural tubercles extend to the immediate edge of the orbital rim, such that some tubercles point into the orbit. A similar pattern has been described only in Gerrothorax (Schoch and Witzmann 2012), where some tubercles can even be found on the ventral side of some circumorbital bones. This, however, cannot be ascertained in Plagiosaurus because the ventral surface of the circumorbital bones is not exposed. The postorbital part of the infraorbital sulcus runs surprisingly close to the orbital margin. In the region of the ossification centre, the otic part of the infraorbital sulcus branches off and extends onto the supratemporal. This bone has an elongate, rounded rectangular shape, with the longitudinal axis being anteromedially aligned. It is wedged between the parietal and postorbital anteriorly, and it contacts the postparietal medially and the squamosal and tabular laterally and posteriorly, respectively.
The supratemporal forms a broad, striated shelf anterolaterally that was overlapped by the postorbital. Anterolateral to this shelf, the supratemporal was overlying a posterior shelf of the postorbital. The dermal sculpture is radial-tubercular. The otic part of the infraorbital line terminates on the ossification centre, which lies exactly in the middle of the bone. From the parietal, only the posterolateral part is preserved, which has a pentagonal outline. In our interpretation, the postfrontal is absent in Plagiosaurus depressus (see discussion below), hence the parietal forms the posteromedial rim of the orbit anteriorly. This bone is wedged between the postorbital and supratemporal laterally, and it forms a rather straight, slightly oblique boundary with the postparietal posteriorly. The medial margin of the parietal is not preserved, hence nothing can be said about the location and size of the parietal opening. The complete parietal was probably L-shaped, as in Gerrothorax specimens with short postfrontals (e.g. Hellrung 2003: fig. 5). The sculptural pattern is poorly preserved on the parietal, but coalesced tubercles are visible on the lateral part, which are aligned posterolaterally. Using Gerrothorax as a reference, approximately the right two-thirds of the postparietal is preserved. The preserved part is broader than long, with the lateral part tapering towards the anteroposteriorly straight suture with the tabular. The posterior margin is slightly convex, with a shallow, concave embayment laterally, similar to Gerrothorax (Hellrung 2003: figs 5, 6). The posteriorly located ossification centre is covered with isolated tubercles, from where ridges of coalesced tubercles radiate anteriorly, laterally, and medially (less well preserved on the lateral side). The posteriormost part of the dorsally exposed bone forms a narrow, shallow rim without dermal sculpture. From the rear margin, the supraoccipital lamina extends posteroventrally and forms the dorsal margin of the foramen magnum and the posttemporal fossa (see below). The tabular of Plagiosaurus depressus has a characteristic wedge-shaped morphology. It is anteroposteriorly shortened, being>2.5 times wider than long, with a short, straight medial suture contacting the postparietal. Its anteromedially aligned anterior border sutures with the supratemporal and squamosal. The tabular terminates with a pointed tip on the posteromedial edge of the squamosal. The posterior margin is straight and slightly overhangs the occipital face of the bone. The poorly preserved sculpture consists of scattered tubercles. The occipital face of the tabular forms the dorsal margin of the subtympanic fossa, and the paroccipital process of the tabular descends ventromedially to contact the exoccipital and forms the ventrolateral margin of the posttemporal fossa (see description of skull in occipital view below).
The squamosal is by area the largest preserved bone of the dermal skull roof. The bone is roughly pentagonal in shape and posteriorly slightly convex. In contrast, the squamosal of Gerrothorax is posteriorly slightly concave (Hellrung 2003, Jenkins et al. 2008, Schoch and Witzmann 2012). The squamosal is the bone with the most pronounced sculpture in Plagiosaurus depressus, having rows of (partly coalesced) tubercles sitting on distinct ridges that radiate outwards from the posteriorly located ossification centre. There is no lateral line sulcus discernable on the squamosal. A medial striated shelf of the squamosal is overlapped by the supratemporal. Posteroventrally, the squamosal forms a deep descending lamina (see description of skull in occipital view below). In posterior view, the squamosal is directed ventrolaterally with an angle of slightly <30° with respect to the horizontal skull table. This stands in contrast to Gerrothorax, in which the squamosal and the skull table lie in the same plane (Schoch and Witzmann 2012). The only exception is the large Gerrothorax skull from Pfaffenhofen, in which the lateral part of the squamosal and the quadratojugal form a faintly inclined cheek region (von Huene 1922: fig. 31; Hellrung 2003: fig. 37). The quadratojugal forms a pointed tip anteriorly and a prominent posterolateral horn that is sculptured both on the dorsal side and on the slightly concave ventral side. A pronounced sculpture of higher tubercles is present on the lateral flank. On the dorsal surface, the sculptural pattern is radial tubercular, with the ossification centre located in the posterolateral region of the bone. The jugal sulcus runs on the medial part of the quadratojugal directly adjacent to the quadratojugal–squamosal suture, as described in Gerrothorax (Hellrung 2003, Jenkins et al. 2008). In posterior view, the quadratojugal forms a concave posteroventral flange that contacts the quadrate (see description of skull in occipital view below). It is also worth mentioning here that the maxilla is lost, meaning that the posterior portion of the ectopterygoid is exposed in dorsal view. Interestingly, Hellrung (2003) noted that the maxilla is frequently missing in Gerrothorax skulls, indicating that this bone was probably not firmly sutured with the skull. This was obviously the also case in Plagiosaurus depressus.
Skull in palatal view
In palatal view, the following bones of the partial skull can be discerned (Fig. 3): the left pterygoid, a fragment of the parasphenoidal basal plate, the posterior part of the left ectopterygoid, the ventral process of the left jugal (insula jugalis), the left quadratojugal, and the right exoccipital. The pterygoid is the largest preserved bone of the palate and can be divided into three discrete portions, the anteromedially directed palatine ramus, the corpus of the pterygoid with the medially directed basipterygoid ramus, and the posterolaterally directed quadrate ramus. The ventral surface of the palatine ramus is partly eroded, but as far as can be discerned, it probably bore dermal sculpture, as in many but not all specimens of Gerrothorax (Schoch and Witzmann 2012). No denticles can be identified on the palatal ramus, similar to most specimens of Gerrothorax except for one specimen that bears denticles on the dermal sculpture (Schoch and Witzmann 2012). The palatal ramus contacts the insula jugalis via an oblique, anteromedially aligned suture and contacts the posteromedial part of the ectoperygoid via a transverse suture. A similar transverse suture is also present in Gerrothorax (Hellrung 2003, Jenkins et al. 2008, Schoch and Witzmann 2012). The palatal ramus does not contact the palatine, as in Gerrothorax (Hellrung 2003, Jenkins et al. 2008, Schoch and Witzmann 2012) and Plagiosuchus (Damiani et al. 2009), but unlike Plagiosternum (Gastou 2007). The corpus of the pterygoid is gently concave ventrally, and its basipterygoid ramus is the longest of the three rami, proportionally longer and more slender than in Gerrothorax, in which the palatal ramus is the longest (Schoch and Witzmann 2012). The ventral surface of the basipterygoid ramus is slightly convex, and its medial part is incomplete. The quadrate ramus is steeply oriented and was probably more vertically oriented in life. Its posterolateral end forms a broad contact with the quadrate. The quadrate ramus gives rise to the ascending lamina that constitutes the lateral margin of the subtympanic fossa in occipital view (see description of skull in occipital view below). The dorsal surface of the pterygoid is not exposed.
A plate-like bone fragment is preserved anterolateral to the basipterygoid ramus. Its left side bears two striated, roughly semicircular facets, which are separated by smooth, finished bone. This bone represents the posterior part of the parasphenoidal basal plate in dorsal view, and the two facets received the exoccipitals (compare Schoch and Witzmann 2012: fig. 4g –i). The preserved part of the ectopterygoid is slender but expands posteromedially, where it forms a transverse suture with the pterygoid. At its anteriormost preserved part is a replacement pit for a large fang (see also Hellrung 2003), followed by a row of seven teeth whose size decreases in a posterior direction. The dorsal surface of the bone bears longitudinal striae. The ectopterygoid of Gerrothorax as described by Hellrung (2003), Jenkins et al. (2008), and Schoch and Witzmann (2012) differs from that of Plagiosaurus depressus in being proportionally wider posteriorly. Furthermore, according to Hellrung (2003) and Schoch and Witzmann (2012), fangs are absent on the ectopterygoid of Gerrothorax, whereas Jenkins et al. (2008: fig. 2b) illustrated a fang and replacement pit on the anterior part of the ectopterygoid.
From the jugal, the insula jugalis is visible, which faces ventrally and laterally. It contacts the pterygoid medially and the ectopterygoid anteriorly and thus excludes the latter bone from the adductor fossa. Posterior to the insula jugalis, the ventral margin of the jugal with a narrow, concave shelf is exposed, to which the posterior portion of the maxilla was aligned and which probably contacted the quadratojugal. This bone forms the lateral margin of the adductor fossa (Figs 3, 4A). The ventrolateral side of the bone bears dermal sculpture, similar to Gerrothorax (Witzmann et al. 2012), but the sculptured portion is more slender than in the latter. Ventral to it, the non-sculptured, ventromedially directed supraquadrate flange or pars paraquadrata sensu Shishkin (1987) is visible, which is partly broken: the quadrate was shifted dorsally and posteriorly such that the supraquadrate flange of the quadratojugal broke in parts; a large fragment (the part that forms a broad sutural contact with the quadrate dorsolaterally) was shifted laterodorsally (Fig. 4A). Thus, in the skull fragment as preserved, the original depth of the flange is considerably reduced and the quadrate is displaced dorsally. The broken nature of this region precludes recognition of a paraquadrate or accessory paraquadrate foramen. Posterior to the laminar process of the quadrate (see description of skull in occipital view below) is a broken, squamose fragment of the occipital lamina of the quadratojugal. From the quadrate, the large, screw-shaped quadrate trochlea and the anterodorsally directed laminar process are visible in ventral view. The trochlea is aligned transversally and slightly anterolaterally. It possesses an unfinished surface and consists of two spindles, a larger median one and a smaller lateral one. The laminar process extends ~ 15 mm dorsally but does not approach the ventral side of the skull roof, i.e. the dorsal part of quadratojugal and squamosal (Fig. 4A). There is also no bony medial extension of this process on the quadrate ramus of the pterygoid. Thus, the quadrate is distinctly less ossified than that in the Gerrothorax specimen described by Witzmann et al. (2012) and in Plagiorophus (Plagiosternum) danilovi (Shishkin (Shishkin, 1986) (Shishkin 1987). From the right exoccipital, the occipital condyle and the dorsolateral process (paroccipital process) are visible in ventral view. Both structures are described below in occipital view. The ventral face of the exoccipital shows a striated articular facet that in life contacted the above-described posterodorsal facets on the parasphenoideal basal plate.
Skull in occipital view
Although most bones that compose the occipital face of the skull are preserved, their often fragmentary state and numerous cracks make the determination of their particular outline difficult, and in some places their exact morphology must remain hypothetical (Fig. 4B). The complete occipital region is considerably compressed dorsoventrally, and the different bones slid over each other. The preserved bones encompass the posteroventral laminae (descending flanges) of the postparietal, tabular, squamosal, and quadratojugal, the exoccipital, the quadrate ramus of the pterygoid with its ascending lamina, and the quadrate. The posterior rim of the postparietal, tabular, squamosal, and quadratojugal forms a bulge that overhangs the posteroventrally directed, non-sculptured laminae of these bones. These occipital laminae are distinctly concave, especially in their dorsal parts. The occipital lamella of the postparietal contacts the exoccipital ventrally and frames the posttemporal fenestra or fossa dorsally (see below). Laterally, it connects with the occipital lamella of the tabular via an oblique suture. The occipital lamella of the tabular forms the dorsal rim of the large, mediolaterally elongate subtympanic fossa. The occipital lamella of the squamosal constitutes the deepest part of the occipital region, which sutures laterally with the quadratojugal, medially with the tabular, ventrally with the ascending lamina of the pterygoid, and posteroventrally with the quadrate. It might contribute to a minor degree to the dorsal rim of the subtympanic fossa, but this cannot be stated with certainty owing to poor preservation. The ventrolateral part of the squamosal occipital lamella that contacted the quadrate ventrolaterally is broken apart. In the middle part of the lamella and slightly dorsally is a transversely elongate, elevated area that was also described in Gerrothorax (Jenkins et al. 2008, Witzmann et al. 2012), Plagioscutum ochevi Shishkin, 1986 (Shishkin 1987), and Plagiosternum granulosum (Fraas, 1889) (Gastou 2007) and which can be interpreted as origin of the m. depressor mandibulae. The distinctly concave occipital lamella of the quadratojugal is located posteroventral to the quadratojugal horn. Its medial suture with the squamosal is traceable, but its ventral extent and the suture with the quadrate are difficult to discern. The dorsal displacement of the quadrate led to breakage of the quadratojugal lamella, and a large quadratojugal fragment was shifted on the posterior face of the quadrate, which considerably reduced the original height of this region. Furthermore, as described above, a lateral piece of the supraquadrate flange was pushed dorsally. When reconstructed, the quadratojugal occipital lamella probably extended further ventrally than that of the squamosal. The paraquadrate foramen is located in the right dorsal corner of the occipital lamella and appears to be divided into a dorsal part and a ventral part.
The exoccipital has a broad dorsal process or vertical column, with a blade-like appearance in posterior view rather than being a true column. Its outer surface is partly eroded, and the bone is pervaded by numerous cracks attributable to dorsoventral compression, meaning that foramina (e.g. for nerves X and XII) cannot be determined. The dorsal process of the exoccipital forms a broad suture with the occipital lamella of the postparietal. The dorsolaterally directed, strikingly short paroccipital process of the exoccipital contacts the ventromedially aligned paroccipital process of the tabular. At the junction of these three bones, a small but distinct foramen, the posttemporal fenestra, is located. This fenestra is variably developed in Gerrothorax; it can be present on both lateral sides or missing on one or even both sides (Hellrung 2003, Jenkins et al. 2008, Schoch and Witzmann 2012). In Plagiosaurus depressus, the dorsal margin of the fenestra is formed by an emargination in the postparietal occipital lamella, whereas most of the concave ventral margin is formed by the paroccipital process of the exoccipital. Only a small ventrolateral part is formed by the tabular. The lateral margin of the dorsal process of the exoccipital constitutes the medial margin of the subtympanic fossa, but because of poor preservation the exact outline of this margin cannot be ascertained. Ventrally, the subotic process of the exoccipital forms a firm, overlapping suture with the pterygoid. The occipital condyle is displaced and was shifted ventrally and ~ 10 mm anteriorly relative to the dorsal process of the exoccipital. The articular surface of the occipital condyle is saddle-shaped and has a roughened surface. It is gently convex dorsally and ventrally, almost straight laterally and slightly concave dorsomedially. Compared with the condyle of Gerrothorax, it is proportionally broader and lower. In the living animal, it probably faced posteromedially and slightly dorsally. A part of the submedullar process, as described in Gerrothorax (Witzmann et al. 2012), is preserved medial to the occipital condyle. Anterolateral to the condyle are distinct tuberosities, as described for Gerrothorax and the basal stereospondylomorph cf. Glanochthon (Boy 1971).
The pterygoid is exposed in occipital view, with its basipterygoid ramus, which forms the floor of the subtympanic fossa, and with the ascending lamina of the quadrate ramus, which constitutes the lateral and anterolateral boundary of the fossa. The ascending lamina contacts the tabular and squamosal dorsally and the quadrate laterally, but owing to compression, the occipital laminae of the tabular and squamosal slid a considerable distance ventrally on the posterior face of the ascending lamina, such that the dorsal margin came to lie dorsal to the level of the insertion site of the depressor mandibulae.
Reconstruction
A reconstruction of the skull fragment in dorsal view is provided in Figure 5A. Comparison with the cheek region of Gerrothorax (Fig. 5B) illustrates the proportional differences between both taxa. Where the dermal skull-roofing bones are slightly dislocated, they were graphically pushed together for reconstruction, corresponding to the preserved areas of overlap. Breakage and fragmentation of certain bones made the reconstruction of the sutural morphology difficult and in parts impossible. This applies especially to the contact between the postorbital and jugal, between the jugal and squamosal, and between the jugal and quadratojugal. These sutures are shown partly with dashed lines. Also, the lateral line sulci cannot be reconstructed throughout because erosion of parts of the dermal bone surface obliterated their course, especially on the postorbital and quadratojugal. In posterior view, the skull is reconstructed distinctly deeper than preserved (Fig. 5C). This interpretation is based on the following taphonomic alterations, which were corrected for the reconstruction: (i) breakage of the supraquadrate flange by dorsal movement of the quadrate; (ii) ventral shift of the descending lamina of the squamosal relative to the ascending lamina of the pterygoid; and (iii) compression of the dorsal process of the exoccipital. However, owing to the fragmentation mentioned above and incompleteness of particular parts of the specimen, the reconstructions have to be regarded as tentative in nature.
Description of the lower jaw
General overview
From the lower jaw, a larger fragment and two smaller ones are stored under inventory number MB.Am.638. Jaekel (1914) and Hellrung (2003) gave brief descriptions of the larger fragment, and Jaekel’s figure 27 is, so far, the only illustration of a part of the lower jaw of Plagiosaurus depressus. The larger fragment will be described in the following (Fig. 6). It encompasses slightly more than the anterior third of the right mandibular ramus and preserves the dentary, pre- and postsplenial, parts of the angular, and coronoids 1 and 2. It is strongly arcuate in dorsal and ventral view, respectively.
Dentition
The mandibular fragment has space for 32 marginal tooth loci on the dentary. Most of them are broken, such that only their bases are visible, which are round in cross-section and sometimes slightly compressed mesiodistally (Fig. 6B, C). They show simple labyrinthodont infolding, with primary folds that are not folded secondarily, as described by Warren and Davey (1992) in a tooth of Plagiosuchus. The tooth bases are attached to a thin labial crest (pleurodont condition). The dentary forms a lingual shelf at the symphysis that bears two smaller parasymphyseal teeth lingual to the marginal tooth row (Fig. 6B, C). No symphyseal fangs are present. Teeth are also located on the dorsal edges of the preserved coronoids, but preservation is incomplete (Fig. 6B, C). All coronoid teeth except for the smallest ones show folded dentine. There appears to be a single row of teeth on coronoid 1, starting anteriorly with a tiny tooth, followed by a middle-sized tooth. The next two following teeth have approximately the size as the two parasymphyseal teeth, and the next following teeth are smaller again. Coronoid 2 bears a patch of teeth that are all smaller than the parasymphyseal teeth. The width of coronoid 2 bears space for up to three adjacent teeth. In dorsal view, the coronoid teeth and the parasymphyseal teeth appear to form a functional row of smaller teeth lingual to the marginal tooth arcade on the dentary. The distribution and size of teeth is very similar to Gerrothorax, which has two to five parasymphyseal teeth (Hellrung 2003, Jenkins et al. 2008, Schoch and Witzmann 2012). All three coronoids bear teeth in Gerrothorax, but individual variation ranges from single rows of teeth to patches or multiple rows (Hellrung 2003, Jenkins et al. 2008, Schoch and Witzmann 2012).
Configuration and morphology of dermal bones
The symphysis is formed by the dentary dorsally and the presplenial ventrally in equal parts (Fig. 6B). On the lingual side of the symphysis, both bones are separated by a broad hollow, which was also described in Gerrothorax and named the foramen mentale by Hellrung (2003). This foramen extends to the anterior part of coronoid 1 and probably contained the anterior part of Meckel’s cartilage (Hellrung 2003). The foramen mentale continues posteriorly as a groove between the presplenial and dentary but is covered by the ventral part of the coronoid bones. Coronoid 1 extends from the level of the seventh dentary tooth position to the 13th tooth position, where the bone is broken off. The next coronoid fragment is present at the level of tooth positions 19–22. This fragment is interpreted here as the posterior portion of coronoid 1, whereas the middle part of the bone had broken off. The anterior end of coronoid 2 is located at the level of dentary tooth locus 23 and preserved until locus 30. The dorsal margin of the preserved coronoids reaches approximately the level of the dorsal margin of the dentary. The coronoids are approximately as deep as the splenials in lingual view and overlap a large part of the lingual surface of the dentary. This stands in contrast to Gerrothorax, in which the coronoids are much narrower than the splenials (Schoch and Witzmann 2012).
In slightly more than the anterior half of the fragment, the dentary forms approximately the dorsal half of the mandible in labial view and is constant in height (Fig. 6A). Ventrally, it contacts the pre- and postsplenial by a longitudinal suture. More posteriorly, the dentary forms an oblique, posterodorsally directed suture with the angular and thus diminishes in height posteriorly. The posterior termination of the dentary is unknown. On the labial side, the dentary forms a conspicuous lateral bulge (Fig. 6A) that constitutes the ventral two-thirds of the lateral face in the anterior part and about its ventral half in the posterior preserved part of the bone. In the region of the symphysis, the bulge forms a dorsolaterally facing concavity, and until tooth position 7, the dorsal part of the bulge is sloping ventrolaterally. Posterior to this, the dorsal surface of the bulge is horizontally aligned and slightly concave and forms a prominent plateau. It extends posteriorly until tooth position 23, but diminishes in width from position 18 onwards. The lateral face of the dentary dorsal to the bulge is inclined slightly medially in the anterior portion of the bone, becoming steeper and eventually attaining a vertical orientation in the region of the plateau. More posteriorly, the dorsolateral face becomes inclined laterally (i.e. is overhanging laterally) from about tooth location 19 onwards. The surface of the bulge possesses a very much subdued dermal sculpture of irregular, roughly anteroposteriorly oriented ridges and depressions in its anterior part. More posteriorly, the sculpture becomes weaker until it is hardly discernable. A lateral line sulcus is not visible. The part of the dentary dorsal to the bulge is largely smooth, except for numerous tiny pits close to the dorsal margin. In dorsal view, the dentary forms a narrow, dorsally slightly concave shelf lingual to the arcade of marginal teeth before it extends vertically downwards where it is overlapped by the coronoids (see description of coronoids above) (Fig. 6C).
The presplenial extends from the symphysis to about the 11th tooth position, where it forms an oblique, anterodorsally directed suture with the postsplenial. Lingually, the bone is smooth and has approximately the same height as on the labial side. A sharp crest is visible on the ventral side of the foramen mentale. The labial surface of the presplenial bears a pronounced tubercular sculpture. The tubercles are arranged in anteriorly and slightly dorsally directed rows. The presplenial is the only preserved bone of the lower jaw that bears a lateral line sulcus (Fig. 6A). It is well developed and located in the anterior part, close to the symphyseal region. It is directed anterodorsally at an angle of ~40°. Hellrung (2003: fig. 15) indicated a comparable sulcus on the presplenial of Gerrothorax but did not mention it in the text. To the best of our knowledge, a lateral line sulcus on the presplenial has not been described in any other temnospondyl. In stem tetrapods, a mandibular canal (which can be partly open or enclosed in bone) runs down from the surangular to the angular and on the ventral margin of the mandible, where it crosses the post- and presplenial (Ahlberg and Clack 1998). In temnospondyls, the sulcus marginalis (sensu Bystrow and Efremov 1940) that passes on the surangular and angular can be regarded as a remnant of the mandibular canal in temnospondyls; accordingly, the sulcus on the presplenial in Plagiosaurus and Gerrothorax could be interpreted as the anterior part of the mandibular canal. Its presence might be a synapomorphy of the two plagiosaurine taxa.
The posteriorly following postsplenial has approximately the same length as the presplenial and is bordered posteriorly by an oblique, anterodorsally directed suture with the angular. On the labial side, it shows similar pronounced tubercular sculpture to the presplenial, but the tubercles do not show a specific arrangement. The smooth, lingual part of the postsplenial bears three small foramina and a larger posterior one arranged in line. The last one might represent the foramen for the arteria mylohyoidea anterior (sensu Shishkin 1973). From the angular, only the anterior and anterodorsal parts are preserved on the labial side (Fig. 6A). Its labial surface bears a pattern of tubercles arranged on radial ridges.
In its general proportions, the bone configuration and dentition, the deep sulcus on the presplenial, and the presence of a large foramen mentale, the lower jaw of Plagiosaurus closely resembles that of Gerrothorax. It differs, however, in the presence of the conspicuous lateral bulge of the dentary that is absent in Gerrothorax.
Description of postcranial elements
Clavicle
Only four, mostly fragmentary clavicles are preserved from Plagiosaurus depressus. The most complete specimens are MB.Am.636, with an almost intact ventral blade, and MB.Am.631, with a well preserved cleithral process (dorsal process) that articulates with the cleithrum, whereas the anterior and posterior parts of the ventral blade are missing. MB.Am.629 and 630 are fragments of the posterior region. A fifth specimen, which was probably lost in World War II, was illustrated by Kuhn (1939: pl. 17, fig. 1a, b). As revealed by MB.Am.636, the shape of the clavicle is elongate, with a distinctly concave medial and convex lateral margin (Fig. 7A). It is slender anteriorly and widens continuously in a posterior direction, attaining its maximal width immediately anterior to the convex posterior end.
The cleithral process is located in the posterior half of the clavicle. It is oriented vertically in all four preserved specimens. In dorsal view, its posterior portion is curved medially, such that the cleithral process is distinctly concave medially and convex laterally (Fig. 7A, B). This is again visible in all four specimens and in the clavicular fragment illustrated by Kuhn (1939). MB.Am.631 (Fig. 7C) shows that the free lateral surface of the cleithral process is completely covered by tubercular sculpture and forms an interdigitating, posteroventrally directed suture with the cleithrum, as first recognized by Nilsson (1939).
Immediately posterior to the cleithral process is a well-defined depression on the dorsal surface of the clavicular blade (Fig. 7B), which was named the posterior fossa (fossa posterior) by Nilsson (1939). It is a deep groove rather than a fossa, extending horizontally on the clavicular blade and turning vertically on the cleithral process of the clavicle, where it is confluent with the infracleithral fossa of the cleithrum (see below). However, for historical reasons, Nilsson’s term ‘posterior fossa’ will be retained here. This fossa is bordered laterally by the lamina posterior, anteriorly by the cleithral process, and medially by a crest termed the vertical lamina (vertikale Lamina) by Nilsson (1939), which contacts the ventrolateral part of the clavicular process of the cleithrum (see below). There is a second, smaller fossa that is located medial to the vertical lamina and the posterior fossa. Nilsson (1939) regarded this second fossa as part of the posterior fossa (he spoke of the paired posterior fossa), but it is probably more appropriate to regard this second fossa as a distinct structure, hence it is termed the medial fossa here (Fig. 7B). This medial fossa is also preserved in MB.Am.631 (Fig. 7D, E). The posterior fossa was interpreted by Nilsson (1939) as a muscle insertion site, probably for the pectoralis clavicularis muscle. This will be discussed below. Immediately posterolateral to the posterior end of the cleithral process and medial to the vertical lamina is an elevated surface, with conspicuous crests and furrows oriented anterolaterally (Fig. 7B). Because the vertical lamina was deep (see MB.Am.631), these rugosities must have constituted the floor of a narrow groove that received the clavicular process of the cleithrum.
The anterior lamina (sensu Shishkin 1987), which extends from the cleithral process in an anterior direction, diminishes abruptly in height in an anterior direction.It has a flattened, broad dorsal edge in MB.Am.631, with a roughened surface. It was interpreted by Nilsson (1939) as a muscle attachment site, most likely for the trapezius muscle, including the cleidomastoideus. A similar dorsal edge, albeit poorly preserved, is also present in MB.Am.636. The posterior lamina (sensu Shishkin 1987), which extends posteriorly from the cleithral process, is much shorter than the anterior lamina and is located lateral to the posterior fossa. Nilsson (1939) pointed to the fact that the dermal sculpture of its lateral surface extends on its dorsoposterior edge and on parts of its medial surface.
A deep fossa in the ventral blade immediately adjacent to the concave medial margin of the cleithral process was termed the anterior fossa (fossa anterior) by Nilsson (1939). It bears conspicuous rugosities in MB.Am.636 that extend anteriorly (Fig. 7A), and in MB.Am.631 the fossa is divided by a crest that continues anterior to the fossa as rugosities (Fig. 7E). Nilsson (1939) assumed the insertion of hyobranchial muscles in this fossa. Medial to the posterior fossa is the large triangular surface for articulation with the interclavicle (Fig. 7A).
In the following, the clavicular morphology of Plagiosaurus depressus is compared with SMNS 83087, which is an excellently preserved clavicle of G. pulcherrimus (Fig. 8A–C). The clavicular morphology of Gerrothorax has so far not been described comprehensively and is thus treated in more detail here. The basic morphology of the clavicle is very similar in both taxa, but some distinct differences do exist, which are worth mentioning. First, the shape of the clavicular blade is different in dorsal and ventral view, respectively. In Plagiosaurus, the medial margin of the blade is clearly concave, whereas it is slightly convex in Gerrothorax, with a sinusoidal margin (Fig. 8A), leading to a proportionally greater width of the clavicle in Gerrothorax. Further differences can be found in the morphology and orientation of the cleithral process. In contrast to Plagiosaurus, the process is not bent medially in its posterior part but is rather straight in Gerrothorax in dorsal view, and its orientation is not vertical as in Plagiosaurus, but is inclined dorsolaterally at an angle of ~75° (Fig. 8B). In medial view, the shape of the cleithral process is triangular with a rounded dorsal tip in Plagiosaurus, whereas it is merely lozenge-shaped in Gerrothorax. Although the vertical lamina is well visible in Gerrothorax, it does not attain the height as in Plagiosaurus, and the floor of the groove for accommodation of the clavicular process of the cleithrum in Gerrothorax does not bear the strongly developed rugosities as in Plagiosaurus (Fig. 8B). In Gerrothorax, the articular facet that received the interclavicle lacks the distinct, sharp ridges that are present in Plagiosaurus. As in Plagiosaurus, the anterior lamina has a rugose dorsal margin, probably for muscle attachment, albeit proportionally shorter and less distinct (Fig. 8A). In summary, the clavicle of Gerrothorax appears proportionally wider by the medially expanded ventral blade and the laterally inclined cleithral process.
Cleithrum
Only one right cleithrum is known from Plagiosaurus depressus, which is firmly sutured with the clavicle (MB.Am.631) and which was described in detail by Nilsson (1939). The specimen was newly prepared for the present study, revealing some new details concerning the infracleithral fossa of the cleithrum and the region of the posterior fossa of the clavicle. In Plagiosaurus, the sculptured, lateral surface of the cleithrum (lateral lamina sensu Nilsson 1945) is vertically oriented and almost as high as the clavicle in lateral view (Fig. 7C). It forms a posteroventrally sloping, serrated suture with the cleithral process of the clavicle. The dorsal, likewise sculptured surface (dorsal lamina sensu Nilsson 1945) is almost horizontally aligned (Fig. 7D), forming a right angle with the lateral surface (see also Nilsson 1939).
The medial surface or medial lamina sensu Nilsson (1945) forms the vertically oriented, ventrally tapering clavicular process, which bears a medial crest (Fig. 7D, E). This crest subdivides the medial lamina into an anterior portion, the area clavicularis, and a posterior portion, the area scapularis, which has a distinctly roughened surface (Nilsson 1939, 1945, Shishkin 1987). The area clavicularis articulates firmly with the cleithral process of the clavicle and is thus completely obscured. The clavicular process of the cleithrum articulates ventrally with the roughened cleithral groove of the clavicle (well visible in MB.Am.636; Fig. 7B) and is ventrolaterally embraced by the vertical lamina of the clavicula (MB.Am.631; Fig. 7D). This lamina extends far dorsally and reaches approximately half the height of the cleithral process of the clavicle. The area scapularis of the medial lamina is medially convex and, as described by Nilsson (1939), bears a small posterior ventral process. Between the area scapularis and the sculptured lateral wall formed by the cleithrum dorsally and clavicle ventrally is a hollow that is roofed by the ventrally concave infracleithral fossa. The walls of the fossa are smooth; it is strikingly deep and has the shape of a cone that is anterodorsally directed.
In contrast to Plagiosaurus, a number of isolated cleithra are known in G. pulcherrimus. In the following, the similarities and differences between the cleithra of Plagiosaurus and Gerrothorax are outlined, based on Gerrothorax cleithra SMNS 83167 and 83248 and the literature (von Huene 1922, Nilsson 1945). The basic morphology of the cleithra in both taxa is similar, although striking differences in proportions and orientation do exist. As already pointed out by Nilsson (1945), the lateral wall of the cleithrum is distinctly lower in Gerrothorax, and the dorsal surface is not horizontally inclined, but lateroventrally.In specimens SMNS 83167 and 83248, no discrete dorsal and lateral surfaces can be distinguished; both merge into each other and form a homogeneous, laterally convex sculptured surface (Figs 8D, 9). In the Gerrothorax cleithrum described by Nilsson (1945: fig. 2), however, a boundary between the oblique dorsal surface and the low vertical wall is visible, indicating individual variation in this character. The dorsolateral orientation of the cleithral process in the Gerrothorax clavicle (see above) indicates that the clavicular process of the cleithrum was directed ventromedially when articulated, rather than vertical as in Plagiosaurus. Thus, when articulated with the clavicle, the sculptured part of the Gerrothorax cleithrum has virtually no distinct dorsal surface; instead, almost the complete sculptured surface faces laterally. The medial crest on the medial lamina in SMNS 83167 and 83248 is slightly sigmoidal and courses from posterodorsal to anteroventral (Fig. 9A), whereas it is nearly straight in Plagiosaurus (Fig. 7E). As in Plagiosaurus, the medial crest subdivides the medial lamina in the anterior area clavicularis and the posterior area scapularis. Both areas have a conspicuously roughened surface, with crests radiating outwards from the region of the dorsal end of the median crest (well visible in SMNS 83248). Nilsson (1945) noted that the dorsal lamina in Gerrothorax is more elongated posteriorly compared with Plagiosaurus. This applies to the specimens described by him and von Huene (1922), but not to SMNS 83167 and 83248, again pointing to a high degree of individual variation. Both SMNS 83167 and 83248 possess a small but distinct medial fossa in the posterodorsal part of the area clavicularis, directly anterior to the dorsal end of the medial crest. Also, Nilsson (1945) described this fossa in Gerrothorax, whereas in Plagiosaurus it is merely a slight concavity rather than a distinct fossa. The fossa infracleithralis of the Gerrothorax cleithrum is much shallower than that of Plagiosaurus. It is mostly smooth walled but bears scattered rugosities.
Scapulocoracoid of Gerrothorax
The scapulocoracoid of Plagiosaurus depressus is unknown, but the comparatively well-ossified scapulocoracoid of Gerrothorax (Hellrung 2003) suggests that it was likewise ossified in Plagiosaurus but has simply not been found. Nilsson (1939) suggested that the scapulocoracoid of Plagiosaurus was either accommodated dorsally and anteriorly by the fossa infracleithralis of the cleithrum or that it was attached to the medial lamina of the cleithrum, posterior to the median crest. He favoured the second interpretation and thus coined the name area scapularis for this part of the medial lamina and regarded the fossa infracleithralis as the insertion point for the musculature of the forelimb. Nilsson (1945) extended this interpretation also to Gerrothorax. Nilsson’s (1939, 1945) interpretation was followed by Shishkin (1987) and Hellrung (2003). Because the scapulocoracoid of Gerrothorax was described only briefly by Hellrung (2003), it will be described based on SMNS 83388 in the following to reconstruct its mode of articulation with the dermal shoulder girdle in Gerrothorax. The inferences for the morphology of the pectoral girdle in Plagiosaurus will be dealt with in the Discussion below.
In accordance with the generally flattened skull and postcranium of Gerrothorax, the scapulocoracoid is strikingly low in comparison to other temnospondyls (Fig. 9C–F). This is reflected by the fact that the large, high-ovate supraglenoid foramen is located anterior and not dorsal to the glenoid, as is common in temnospondyls (Pawley and Warren 2006). The scapular blade is distinctly concave posteriorly, but its overall shape is not crescentic as in most other temnospondyls; instead, as already outlined by Hellrung (2003), an anterodorsal outgrowth gives the element the shape of an X. This outgrowth forms a laterally concave shelf and has a rugose surface. It is limited posteriorly by a posterodorsally aligned conspicuous crest that extends to the anteroventral corner of the bone. The dorsal margin of the scapulocoracoid is dorsally concave and is highly rugose dorsally and laterally. The described dorsolateral rugose area and the anterodorsal shelf perfectly match the area scapularis of the cleithrum, and the median crest of this bone was attached to the posterodorsally aligned crest of the scapulocoracoid. In this way, a tight connection between the scapulocoracoid and cleithrum was established and, together with the firm, sutural articulation between the cleithrum and clavicle, indicates a rigid shoulder girdle in Gerrothorax. There is no evidence that the scapulocoracoid attached the cleithral process of the clavicle. Ventrally, the scapulocoracoid is unfinished and indicates a mainly cartilaginous coracoid region. Owing to spatial constraints, this region must have been small; Hellrung (2003: fig. 28c, d) illustrated a scapulocoracoid of Gerrothorax with an ossified portion of the coracoid posteroventral to the glenoid. There are no clearly defined facets on the dorsal surface of the clavicular blade for articulation with the ventral margin of the scapulocoracoid. Interestingly, the scapulocoracoid was not roofed by the cleithrum in Gerrothorax, in contrast to most other temnospondyls (Pawley and Warren 2006). Medial articular surfaces at the dorsal edge of the cleithrum indicate the presence of osteoderms medial to the cleithrum that overlaid the scapulocoracoid. The glenoid is trapezoidal in shape, with a concave ventral margin. The posterior edge of the scapular blade bears distinct rugosities dorsal to the glenoid, probably representing muscle attachment sites. The exact orientation of the glenoid in the living animal is difficult to ascertain because no specimen is known in which the cleithrum, scapulocoracoid and clavicle are articulated (Hellrung 2003). However, if the orientation of the scapular blade was approximately parasagittal (as indicated by the articulated cleithrum and clavicle), then the glenoid was oriented posterolaterally.
A striking difference from most other temnospondyls is the presence of the fossa infracleithralis dorsolateral to the scapulocoracoid in Gerrothorax, which was interpreted as a muscle attachment site for the forelimb (Hellrung 2003). This otherwise smooth fossa bears rugosities supporting this interpretation (Fig. 9B). As visible in the Gerrothorax clavicle (SMNS 83087), the fossa infracleithralis is continued ventrally on the dorsal surface of the clavicle as the posterior fossa (see description above and Fig. 8B).
Interclavicle
Nilsson (1937: fig. 7) provided a reconstruction of the interclavicle of Plagiosaurus depressus based on two partly preserved specimens, MB.Am.619 and 678. However, the anterior part of the interclavicle (anterior to the anterior wings) is either not preserved (MB.Am.619) or incomplete and distorted (MB. Am.679) (Fig. 10A, B). Nilsson could not know that a third, almost complete interclavicle existed, which was later illustrated by Kuhn (1939) as a photograph in dorsal view and which is now lost (see above). We attempted a new reconstruction of the Plagiosaurus interclavicle based on both MB.Am.619 and 678 and Kuhn’s (1939: p. 18, fig. 1) illustration (Fig. 10C). However, because our new reconstruction is a composite of three specimens, of which one is available only as a photograph, some uncertainty remains. Our reconstruction differs from that of Nilsson (1937) in the anterior part of the interclavicle being longer and wider. Interestingly, it is more similar to Jaekel’s (1914: fig. 28) reconstruction (which has to be turned by 180°); it cannot be ruled out that one or both MB specimens were more complete in Jaekel’s time.
Compared with Gerrothorax, the interclavicle of Plagiosaurus has a proportionally longer and wider anterior portion (contra Nilsson 1937) (Fig. 10C–H). Furthermore, the posterior wings are strikingly short. However, the relative length and width of the anterior portion of the interclavicle in Gerrothorax is highly variable, and this also applies to the size of the posterior wings (Fig. 10E–H). However, Hellrung (2003: p. 109) pointed to an apparently unique character of Plagiosaurus: the posterior wings do not exceed the width of the sculptured surface on the level of the anterior wings. As in Gerrothorax, a posterior shelf for articulation with osteoderms is present, indicating that Plagiosaurus possessed a similar pattern of dermal armour. According to Kuhn’s (1939) photograph of the dorsal side of the interclavicle, a conspicuous bulge extends from the presumed ossification centre towards the anterior wings, as described also in Gerrothorax (Nilsson 1937, Hellrung 2003). It cannot be assessed based on Kuhn’s photograph whether a second bulge was also present that was directed towards the posterior wings, as in Gerrothorax.
Humerus
Apart from Plagiosuchus pustuliferus (Fraas, 1896), from which humeri are preserved but have yet to be described, Plagiosaurus and Gerrothorax are the only plagiosaurids with known humeri (Fig. 11). In Plagiosaurus, Nilsson discovered an almost complete humerus (now MB.Am.632) in the matrix next to the clavicle and cleithrum of specimen MB.Am.631. Because Nilsson (1939) has already given an excellent description of the Plagiosaurus humerus, it is only briefly recapitulated here with reference to the more recent literature. Then it will be compared with the so far undescribed humerus of Gerrothorax.
The distal and proximal ends (condyles) of the Plagiosaurus humerus are poorly ossified and rotated in relationship to one another at an angle of 90°, giving the bone a tetrahedral morphology, with a short but rod-like shaft (Fig. 11A–F). In cross-section, the proximal head is elongate ovate, with an offset anterior part. As described by Nilsson (1939), a distinct deltopectoral crest (e.g. like in Eryops Cope, 1878a) is absent; instead, an anterolateral elevation on the shaft is visible, which was interpreted by Nilsson (1939) as a possible insertion site for the deltoideus muscle. The supinator process is prominent and points anteriorly. Also, the processus latissimus dorsi is well developed and situated more proximally than in Sclerocephalus Goldfuss, 1847 (Meckert 1993, Schoch and Witzmann 2009) or Eryops (Pawley and Warren 2006). The entepicondyle is a distinct, slightly curved plate; its proportions are comparable with Sclerocephalus, but it is less expanded than in Trimerorhachis Cope, 1878b (Pawley 2007) and Eryops (Pawley and Warren 2006). The radial condyle is small and not convex as in Eryops (Pawley and Warren 2006). In contrast, the ectepicondyle is poorly developed and represented by a low, elongate elevation.
The morphology of the humerus of Gerrothorax is basically similar to that of Plagiosaurus (Fig. 11G–L). It is briefly described in the following based on SMNS 81400. The supinator process is likewise robust and oriented anteriorly but also slightly medially. The processus latissimus dorsi is less pronounced in Gerrothorax such that the proximal head is more slender in anterior and posterior view. The shaft is overall less robust than in Plagiosaurus, and a distinct insertion site for the deltoideus muscle is not developed. This could be attributable to the smaller size of the Gerrothorax humerus, which might be derived from a less advanced juvenile. In contrast to Plagiosaurus, the cross-section of the proximal head in Gerrothorax is rounded rectangular rather than elongate oval.
Apart from the humerus, Nilsson (1939) mentioned a radius associated with the dermal shoulder girdle MB.Am.631 and the humerus MB.Am.632. New preparation of MB.Am.631 revealed that the element in question is a broken part of the clavicular blade and has a tubercular ventral surface.
Vertebrae
Two neural arches were illustrated by Jaekel (1914: fig. 27) but not described. These specimens were subsequently destroyed, and only small fragments of the neural arches are left that bear little information. However, an almost complete neural arch is left among several vertebral centra with the collective inventory number MB.Am.621. Because of the rather cursory treatment of neural arches by Jaekel (1914), this specimen is described in the following (Fig. 12A–E). As already mentioned by Jaekel (1914), the neural spine is strikingly low and has a truncated appearance. In its anterior half, the neural spine is expanded and bears a dorsally and slightly laterally facing facet on either side. These facets probably articulated with overlying osteoderms. Posteriorly, these facets send out two ridges on either side, a dorsomedial sharp ridge and a more lateral broader one, the surface between which is deeply concave (Fig. 12C, E). The ridges terminate at the anterodorsal edge of the postzygapophyses. In contrast to the illustrations of Jaekel (1914), the neural spine bears a broad longitudinal groove that is smooth walled and extends on its complete dorsal side between the above-described lateral facets and ridges; in its posterior part, the groove slopes in a posteroventral direction between the postzygapophyes. The prezygapophyes are proportionally very large and form slightly concave articular facets. They are almost vertically aligned and face anteromedially with an angle of ~45° to the sagittal plane. Accordingly, the slightly convex facets of the postzygapophses are nearly vertical and face posterolaterally. The transverse processes are short and bear laterally and slightly posteriorly directed, reniform diapophyses with a concave surface. The neural canal is not closed ventrally. The ventral surface of the neural arch bears elongate anteroventral and short posteroventral facets for articulation with the centrum on either side of the neural canal, indicating an intervertebral position of the neural arch, as is common in plagiosaurids (Jaekel 1914, Nilsson 1937, Shishkin 1987, 1989, Hellrung 2003). The medial margin of the anterior half of the anteroventral neurocentral facets bears ventrally directed ridges (Fig. 12D) whose significance is unknown.
Comparison with Gerrothorax shows that its presacral neural spines are very similar, being very low and bearing lateral processes to support the overlying osteoderms (von Huene 1922, Warren and Snell 1991, Hellrung 2003). However, a dorsal groove on the neural spine has not been described in Gerrothorax either by Nilsson (1937) or by Hellrung (2003); only von Huene (1922: p. 46) mentioned an isolated neural arch of Gerrothorax (‘ Plagiosaurus striopustulatus ’) that ‘consists … clearly of a right and left half ’. It is not evident whether this refers to a dorsal groove separating the spine into two lateral halves, and also his figure (von Huene 1922: fig. 43) does not clarify this. Only Witzmann and Soler-Gijón (2010) described a longitudinal groove on a neural arch of the plagiosaurid Plagiosuchus pustuliferus; however, this groove is located on the anterior and dorsal edge of the proportionally much taller neural spine and served for accommodation of the ventral keel of a medial osteoderm. Likewise, the neural arches of dissorophids, such as Dissorophus Cope, 1895 and Broliellus Williston,191 Williston, 1914, possess ‘split spines’ with grooves on their anterior, dorsal, and posterior edges, which received ventral processes of overlying osteoderms (DeMar 1966, 1968). However, the low neural spine of Plagiosaurus in combination with its dorsal groove and the lateral processes most closely resembles the ‘partially divided’ morphology of the neural spine in the rhytidosteid Acerastea (Warren and Hutchinson 1987: p. 296, fig. 8). Nilsson (1937: p. 18) noted that the diapophyses of Gerrothorax are larger than those of Plagiosaurus, and this is supported by the illustration of a neural arch of Gerrothorax (‘ Plagiosaurus sp’.) by Warren and Snell (1991: fig. 3e).
Jaekel (1914) gave a brief description of the vertebral centra of the trunk but did not illustrate them. He interpreted the centra as completely ossified (i.e. spool-shaped) intercentra, which left no space for ossified or cartilaginous pleurocentra. Von Huene (1922) and Nilsson (1937), in describing the vertebrae of Gerrothorax, agreed with Jaekel’s (1914) interpretation. Nilsson (1937) stated that Plagiosaurus centra have the same proportions as those of Gerrothorax, but like Jaekel, he did not provide a figure of Plagiosaurus vertebrae. Therefore, centrum morphology is described briefly in the following (Fig. 12F–H).
The dorsal aspect of the vertebral centrum is distinctly incised by the floor of the neural canal, which is lined by finished bone. Two neurocentral facets are present on either side, with a small area of finished bone between them. In each of the preserved centra, one laterally opposed pair is shorter than the other. Based on the morphology of the neural arch facets, the side with the shorter pair of facets is the anterior one, and von Huene (1922) came to the same conclusion in Gerrothorax based on a series of articulated vertebrae. The anterior facets are directed anterodorsally and the posterior ones face posterodorsally; therefore, the centrum forms a dorsal gablelike elevation in lateral view. Parapophyseal facets are located at the anterior and posterior edges of each centrum, facing anterolaterally and posterolaterally, respectively, meaning that each parapophysis is formed by two successive centra, as described in other plagiosaurids (von Huene 1922, Nilsson 1937, Shiskin 1987, Warren and Snell 1991, Hellrung 2003). The anterior and posterior articular surfaces of the centrum are slightly amphicoelous to platycoelous, and the centra are only slightly concave (waisted) laterally and ventrally. In their general morphology, at least the trunk vertebral centra of Plagiosaurus cannot be distinguished from those of Gerrothorax (von Huene 1922, Nilsson 1937).
Ribs
Jaekel (1914) mentioned a number of double-headed rib fragments of Plagiosaurus depressus and noted that the articulation between the capitulum and parapophysis was intervertebral. Otherwise, he did not describe or illustrate the ribs. In his description of Gerrothorax, Nilsson (1937: fig. 4) illustrated a proximal rib fragment of Plagiosaurus.
The Plagiosaurus material contains eight proximal fragments of trunk ribs with the collective inventory number MB.Am.623. The rib heads are well separated (Fig. 13A–G). The tuberculum is rounded or square-shaped and approximately as wide as high. It is located immediately dorsal to the capitulum. The latter is anteroposteriorly elongate and consists of two facets, an anterolateral one and a posterolateral one, which articulated with the parapophyses of the anterior and posterior centrum, respectively. The rib shaft is straight, with the exception of a slight ventral curvature in its proximal part, and becomes increasingly flattened in the distal direction. The anterior and posterior edges of the shaft or only one of them may bear a sharp crest (Fig. 13B, C, E). The size of the crests may vary, but owing to the lack of articulated material, the morphological variation of ribs along the vertebral column cannot be determined. The morphology of the trunk ribs in Plagiosaurus corresponds to that described for Gerrothorax (von Huene 1922, Nilsson 1937, 1946, Warren and Snell 1991, Hellrung 2003).
One rib (MB.Am.622) differs from all other preserved ribs of Plagiosaurus in having a single-headed proximal end, a distinctly curved shaft, and an enlarged posterior crest reminiscent of an uncinate process in the proximal third (Fig. 13H, I). The proximal articular facet is vertically ovate and probably consists of a fused capitulum and tuberculum. The rib is interpreted here as a left caudal rib. Caudal ribs of Gerrothorax are likewise curved (Nilsson 1946: figs 2, 4; plate II, fig. 2; Hellrung 2003: p. 66) and possess a single proximal articular facet, as visible in SMNS 83552. However, the caudal ribs in Gerrothorax lack the pronounced posterior crest.
Osteoderms
The osteoderms of Plagiosaurus that were mentioned but not illustrated by Jaekel (1914) cannot be detected anymore. However, new preparation of MB.Am.631 revealed a number of broken osteoderms on the dorsal surface of the clavicle (Fig. 7D). The external surface of the osteoderms is tubercular, as in Gerrothorax (Witzmann and Soler-Gijón 2010), but owing to their broken state nothing can be said about their outline.
Phylogenetic analysis
In order to elucidate the phylogenetic position of Plagiosaurus depressus and its bearing on plagiosaurid phylogeny, we used a modified version of the matrix used by Schoch et al. (2014). Multistate characters were treated throughout as unordered. Characters 1 and 84 of Schoch et al. (2014) were replaced by the following characters: character 1, ornament (elements): reticulate ridges of various sizes (0), isolated tubercles (1), or tubercles enlarged to bulbous pustules of irregular height and width (2); and character 84, interclavicle (posterior margin): with posterior process (0), without posterior process and slightly convex (1), or without posterior process and transversely straight (2).
To the 114 characters used in the analysis by Schoch et al. (2014), we added and additional nine characters: character 115, cranial flattening, skull table markedly higher than preorbital region (0, yes; 1, no); character 116, basicranial suture foreshortened (0, no; 1, yes); character 117, dorsal process of clavicle short (0, no; 1, yes); character 118, naso-frontal suture emplaced further posterior (0, no; 1, yes); character 119, ornament with tubercles at junction of ridges (0, no; 1, yes); character 120, postorbital expanding anterolaterally along orbit margin (0, no; 1, yes); character 121, postfrontal absent (0, no; 1, yes); character 122, tabular without posterior horn-like projection (0, no; 1, yes); and character 123, cleithrum shape (0, elongate rod-like; 1, shortened shaft). For the list of characters and the character–taxon matrix, please see the Supporting Information (Appendices S1 and S2).
Three successively more derived outgroups were included to root the ingroups: (i) Dendrerpeton acadianum Owen, 1853; (ii) Trimerorhachis insignis Cope 1878b; and (iii) Sclerocephalus haeuseri Goldfuss, 1847. Plagiosaurus depressus was added to the analysis for the first time. Furthermore, two plagiosaurid taxa from the East European Platform, Plagioscutum ochevi and Plagiorophus danilovi (Shishkin 1986, 1987), were included in a phylogenetic analysis, and we rescored the character codings for all other plagiosaurids according to our present observations. This analysis gave four alternative most parsimonious trees requiring 283 steps (consistency index,.473; and retention index,.745).
Plagiosauridae was found to be monophyletic, forming the sister group of Laidleria gracilis, together forming the Plagiosauroidea Kitching, 1957 sensu Yates and Warren (2000) (Fig. 14). A monophyletic Brachyopoidea, consisting of the chigutisaurid Siderops kehli Warren and Hutchinson, 1983 and the brachyopids Bothriceps australis Huxley, 1859 and Bathignathus watsoni (Haughton, 1925), constitute the sister group of Plagiosauroidea plus Laidleria. Within Plagiosauridae, the subdivision into three monophyletic groups, the Plagiosuchinae (Plagioscutum + Plagiosuchus), Plagiosaurinae (Plagiosaurus + Gerrothorax), and Plagiosterninae (Plagioroph us + (Plagiosternum + Megalophthalma )), is confirmed by our present analysis (Fig. 15). However, with the exception of the Plagiosaurinae, the bootstrap values of the plagiosaurid subclades are rather low (Fig. 15), and bootstrap does not find any support for a monophyletic Plagiosuchinae. Instead, Plagioscutum and Plagiosuchus form successive sister taxa of a clade consisting of Plagiosaurinae plus Plagiosterninae.
The Plagiosauridae are supported by seven synapomorphies (characters 5, 7, 26, 41, 76, 90, and 118), the Plagiosuchinae by one (character 1), the more crownward plagiosaurids (Plagios aurinae + Plagiosterninae) are based on three synapomorphies (characters 83, 115, and 116), the Plagiosaurinae by three characters (characters 1, 67, and 117), and the Plagiosterninae by two (characters 31 and 119). Plagiosaurus depressus is characterized by two autapomorphies (characters 112 and 121), and the Lower Keuper plagiosternines Megalophthalma and Plagiosternum by two synapomorphies (characters 26 and 121).
description: Published as part of Witzmann, Florian & Schoch, Rainer R., 2024, Osteology and phylogenetic position of Plagiosaurus depressus (Temnospondyli: Plagiosauridae) from the Late Triassic of Germany and the repeated loss of dermal bones in plagiosaurids, pp. 1-29 in Zoological Journal of the Linnean Society 202 (3) on pages 4-23, DOI: 10.1093/zoolinnean/zlae014, http://zenodo.org/record/14423591