Transylvanosaurus platycephalus, Augustin, Bastiaans, Dumbravă & Csiki-Sava, 2022

Augustin, Felix J., Bastiaans, Dylan, Dumbravă, Mihai D. & Csiki-Sava, Zoltán, 2022, A new ornithopod dinosaur, <i> Transylvanosaurus platycephalus </ i> gen. et sp. nov. (Dinosauria: Ornithischia), from the Upper Cretaceous of the Haţeg Basin, Romania, Journal of Vertebrate Paleontology (e 2133610) 42 (2), pp. 1-23 : 4-14

TRANSYLVANOSAURUS PLATYCEPHALUS sp. nov.

Figs. 3–6

Holotype — LPB (FGGUB) R.2070 , a fragmentary skull comprising the articulated basicranium composed of the basioccipital, the exoccipital-opisthotic complexes, the basisphenoid-parasphenoid complex, the prootic and the laterosphenoid, as well as the articulated left and right frontals.

Etymology —‘Platys’ (Greek πλατύς) meaning wide, and ‘cephalos’ (Greek κέφαλος) meaning head. The specific name refers to the exceptionally wide skull of the new dinosaur compared with that of other rhabdodontids.

Type Locality —The holotype materialwas found in the Barbat River Valley section, near Pui, eastern Hateg Basin, Hunedoara County, Romania. The bones of the basicranium and the paroccipital processes were found in articulation, directly below and behind the articulated frontals ( Fig. 2 ).

Type Stratum —LPB (FGGUB) R.2070 was recovered in 2007 from the middle part of the uppermost Cretaceous continental succession from the Barbat River Valley section, informally also referred to as the ‘Barbat Formation’ ( Therrien, 2005) or the ‘Pui Beds’ ( Csiki-Sava et al., 2016). The ‘Pui Beds’ have been estimated to be ‘middle’ Maastrichtian in age, i.e., close to the early to late Maastrichtian boundary ( Van Itterbeeck et al., 2005); the locality yielding specimen LPB (FGGUB) R.2070 is located slightly southwards of (i.e., stratigraphically above) the level sampled for palynology by Van Itterbeeck etal. (2005).

Diagnosis —A small- to medium-sized rhabdodontid ornithopod dinosaur characterized by the following autapomorphies: (1) proportionately wide frontals with an anteroposterior length to mediolateral width ratio of 1.38; (2) presence of a well-developed, anteriorly placed transverse frontal crest that distally bounds the confluent nasal-prefrontal articulation facets; (3) very long, straight and thin paroccipital processes that make only a gentle lateral curve, and direct mostly posterolaterally and slightly dorsally; (4) very prominent and massive prootic processes that extend mainly anterolaterally and ventrally; (5) mediolaterally wide, crest-like basal tubera that meet the long axis of the braincase, which is parallel to the orientation of the endocranial floor, at a very flat angle of approximately 140°; (6) widely splayed basipterygoid processes that extend mainly ventrolaterally and slightly anteriorly, diverging approximately 25° from the sagittal plane; (7) a well-developed, anteroventrally inclined notch on the lateral side of the basicranium, just anterior to the basal tubera, that is continuous, straight, and semi-circular in cross section.

Inaddition, the taxondiffers fromall other rhabdodontids by the followinguniquecombination of characters: abasioccipital condyle thatishighlyconvex and trapezoidalinventral view; a heart-shaped foramen magnum that is wider mediolaterally than it is high dorsoventrally; a flat and straight endocranial floor that constantly widens posteriorly; a weakly developed crista tuberalis; an anteroposteriorly elongated basisphenoid; a dorsoventrally deep basisphenoid-parasphenoid complex; a wrinkled posterior surface of the basal tubera with a prominent midline process that does not extend for the entire dorsoventral height of the basal tubera.

DESCRIPTION

The holotype specimen of Transylvanosaurus platycephalus, LPB (FGGUB) R.2070, comprises the articulated basicranium ( Figs. 3 , 4 , 5 ) composed of the basioccipital, the exoccipitalopisthotic complexes, the basisphenoid-parasphenoid complex, the prootic, and the laterosphenoid, which were found in the field associated with the articulated left and right frontals ( Fig. 6 ). Aside from the missing parts, the specimen is well-preserved with small processes and foramina still present and largely undistorted. The frontals were found slightly above and anterior to the basicranium in their roughly correct anatomical position ( Fig. 2D, E ). No additional skull bones or remains thereof have been found between the basicranium and the frontals nor in their close proximity. This peculiar state of preservation indicates that originally, some soft tissues were probably still connecting the basicranium with the frontals when the specimen was embedded into the sediment. Also, the pattern of surface exposure of the specimen when identified in the field ( Fig. 2D ), together with the dorsally damaged margins of the basicranium as currently preserved, suggests that other parts of the occipital section of the skull may also have been preserved during burial, but were most probably removed by fluvial erosion in this very dynamic, actively eroding riverbed site, prior to the discovery of the specimen.

Basioccipital

The basioccipital contributes to the posterior and ventral parts of the braincase ( Fig. 3A–D ). It is kidney-shaped in posterior view, as well as trapezoidal and markedly convex in ventral view. The posterior articular surface for the atlas is slightly convex and directed posteroventrally. The dorsal aspect of the basioccipital is concave, forming the ventral part of the foramen magnum and the posterior part of the endocranial floor ( Fig. 4A–D ). Asmall part of the bone near the right posterolateral margin is missing. The basioccipital is fused to the exoccipitals dorsolaterally and to the basisphenoid-parasphenoid complex anteriorly ( Fig. 5A–D ). The suture between the basioccipital and the exoccipitals is hardly visible and only a faint suture is present on the left side, whereas a crack largely obliterates the sutural contact on the right side. In posterior view, the suture between the basioccipital and the exoccipitals extends dorsomedially. In lateral view, the suture between the basioccipital and the exoccipital extends anteriorly and to a lesser degree ventrally. Together, the basioccipital and the ventromedial extremities of the exoccipitals form the occipital condyle, although the former contributes to a much greater extent. In ventral view, the basioccipital is connected to the basisphenoid anteriorly through a short but distinct neck ( Fig. 5C, D ). The suture between the basioccipital and the basisphenoid is not discernible in ventral nor in dorsal view. On the ventral aspect of the basicranium, a large crack runs anterolaterally, extending almost for the entire diagonal width of the basicranium.

Exoccipital-Opisthotic Complex

The exoccipital-opisthotic complex contributes to the posterior and the lateral parts of the braincase ( Fig. 3A–D ). It is formed by the exoccipitals ventromedially and by the opisthotics dorsolaterally. The exoccipitals are roughly ellipsoidal and convex in posterior view, having a knob-like morphology. The posteroventral part participates in the formation of the occipital condyle, although to a much lesser degree than the basioccipital. Additionally, the exoccipitals form the ventrolateral margin of the foramen magnum ( Fig. 4C, D ). Ventrally, the exoccipitals are fused to the basioccipital and dorsally to the opisthotics along a well-discernible suture. The suture between the exoccipitals and opisthotics extends anteroventrally, subparallel to the suture between the basioccipital and the exoccipitals, but is inclined slightly more ventrally than the latter. In lateral view, three large foramina are visible that lie approximately on the suture between the exoccipital and the opisthotic ( Fig. 3A, B ). The posterior-most and largest of these represents the opening for cranial nerve XII or hypoglossal nerve. The two foramina that are located more anteriorly are much smaller and represent the openings for cranial nerve XI or accessory nerve, as well as the opening for cranial nerve X or vagus nerve, respectively.

The opisthotic forms the rod-like paroccipital process that extends mainly posterolaterally and dorsally ( Figs. 3A–D , 4C, D ). The paroccipital process is relatively thin both anteroposteriorly and dorsoventrally. It has a roughly ellipsoidal cross section being higher dorsoventrally than wide anteroposteriorly. In posterior view, the paroccipital process makes a gentle dorsolateral curve and meets the exoccipital at a wide angle. The distal parts of the paroccipital processes are missing. In addition to this dorsolateral and posterior development, the opisthotic also extends dorsomedially, forming the curved dorsolateral part of the foramen magnum. The dorsal and anterior faces of the paroccipital processes are slightly damaged but still exhibit the sutural contacts with the (not preserved) supraoccipital and squamosal, respectively. The foramen magnum is wide and slightly heartshaped in posterior view, although the dorsal margin is unknown due to the missing supraoccipital, which would apparently be wedged in between the two opisthotics along a rather straight and vertical contact ( Fig. 4C, D ). From what is preserved, however, the foramen magnum seems to have been wider mediolaterally than high dorsoventrally. The anterolateral part of the opisthotic forms a weakly developed ridge or crest, the crista tuberalis, which connects to the prootic anteriorly and lies directly ventral to the fenestra ovalis (see below).

Basisphenoid-Parasphenoid Complex

The basisphenoid-parasphenoid complex contributes to the ventral part of the braincase ( Figs. 3A–D , 5C, D ). It is actually composed of two bones that, however, are seamlessly fused to each other. The basisphenoid-parasphenoid complex comprises the concave ventral part of the endocranial floor dorsally ( Fig. 5A, B ), as well as the prominent crest-like basal tubera and the large wing-like basipterygoid processes anteroventrally ( Fig. 5C, D ). The basisphenoid-parasphenoid complex is fused to the basioccipital posteriorly, as well as to the prootic and laterosphenoid dorsally. In dorsal view, the basisphenoid-parasphenoid complex forms the middle and anterior part of the endocranial floor. In general, the endocranial floor is completely straight and flat with a semi-circular cross section that progressively widens posteriorly. The dorsum sellae is located on the anterior portion of the endocranial floor and exhibits two small, hardly visible foramina, the openings for the paired cranial nerve VI or abducens nerve. Anterior to the dorsum sellae, the endocranial floor sharply slopes down ventrally. The basisphenoid-parasphenoid complex is broken anterior to this section, exposing the ellipsoidal pituitary fossa in anterior view that lies ventral to the endocranial floor and houses two canals for the paired internal carotid arteries ( Fig. 4A, B ).

In ventral view, the basisphenoid is connected to the basioccipital posteriorly through a distinct neck. The suture between the basisphenoid and the basioccipital is not discernible. The region between the basioccipital and the basisphenoid shows a large crack that continues anterodorsally through the basisphenoidparasphenoid complex. Anterior to the basioccipital neck, are the prominent and well-developed basal tubera that project mainly anteroventrally and together form a wide, mediolaterally extending ridge with a crest-like morphology ( Figs. 3A–D , 5C, D ). The basal tubera meet the long axis of the braincase, which is parallel to the orientation of the endocranial floor, at an angle of about 140°, which is best seen in lateral view. The posterior face of the basal tubera has a wrinkled appearance, especially near its ventral margin. This surface likely was the attachment site for the m. rectus capitis ventralis ( Weishampel et al., 2003). Moreover, the posterior surface of the basal tubera bears a prominent transverse midline process, which projects mainly posteriorly and is dorsoventrally elongated but does not extend for the entire dorsoventral height of the basal tubera.

In lateral view, a well-developed deep notch is located just anterodorsal to the basal tubera that extends anteroventrally at an angle of about 45° relative to the long axis of the braincase ( Fig. 3A, B ). This notch is bordered by the crest-like lateral expansion of the basal tubera (i.e., the crista transversalis) posteroventrally and by the alar process anterodorsally. It is relatively straight and completely continuous, ending in a semicircular opening both dorsally and ventrally. In the ventral third of this notch lies the entrance for the carotid artery. On the right side of LPB (FGGUB) R.2070, the notch is slightly damaged by the large crack that runs through the basisphenoid-parasphenoid. The alar process is a thin ridge that extends posterolaterally and borders the deep notch on the lateral aspect of the basisphenoid. Ventrally, the alar process merges with the basipterygoid process that projects ventrolaterally, being inclined at an angle of about 25° relative to the sagittal plane, and also slightly anteriorly ( Figs. 3A, B , 4A, B ). The lateral part of the basipterygoid process is slightly rugose, likely indicating the (cartilaginous) contact with the pterygoid ( Holliday and Witmer, 2008). Only the left basipterygoid process is preserved. The surface between the basipterygoid processes is roughly triangular in ventral view, smooth and slightly anterodorsally inclined. The lateral surface of the basisphenoid-parasphenoid complex, dorsal to the basipterygoid processes, is roughly triangular and dorsomedially inclined, parallel to the orientation of the basipterygoid processes, resulting in a wing-like morphology of this area. Aslight depression in this area probably corresponds to the attachment site of the m. protractor pterygoideus ( Holliday, 2009).

Prootic and Laterosphenoid

The prootic and the laterosphenoid bones contribute to the lateral parts of the braincase ( Fig. 3A–D ). The prootic is sutured to the exoccipital-opisthotic complex posteriorly, to the laterosphenoid anteriorly, and to the basisphenoid ventrally. The laterosphenoid, in its turn, is sutured to the prootic posteriorly, and to the basisphenoid-parasphenoid complex ventrally. The suture between the prootic and the laterosphenoid is not discernible in the holotype specimen, however, and thus they are here described as a single complex, unless indicated otherwise. In lateral view, the prootic-laterosphenoid complex is a roughly rectangular to trapezoidal block-like element. Between the opisthotic and the prootic, there is a large opening, the fenestra ovalis ( Fig. 3A–D ). Anterior to this opening, the prootic becomes markedly thicker mediolaterally and contributes to the dorsal part of the deep notch extending across the lateral side of the braincase, which is bordered by the basal tubera posteroventrally and the alar process anterodorsally (see above). A prominent ventral process of the prootic forms the posteroventral margin of this notch. This ventral prootic process has a knob-like morphology and extends mainly anterolaterally and also somewhat ventrally.

The suture between the prootic and the basisphenoid is situated on the ventral aspect of this prootic process and extends approximately anteroposteriorly. In ventral view, there is a large cleft between the prootic process and the basal tubera of the basisphenoid. Anterior to the deep notch, the prootic-laterosphenoid complex becomes thinner and curves slightly medially. A large indentation is located anterior to the conspicuous swelling of the prootic-laterosphenoid complex, probably representing the opening for cranial nerve V, or trigeminal nerve. The dorsal margin of the prootic-laterosphenoid complex is imperfectly preserved and it gently slopes down anteroventrally ( Fig. 5A, B ). The sutural contact with the supraoccipital is partly visible in the posterior part of the complex, although the supraoccipital itself is missing.

Frontals

The left and right frontals are well-preserved, undistorted, and almost complete ( Fig. 6 ). They are nearly symmetrical, although the left frontal seems to have been somewhat larger. The frontals are not fused to each other but were found next to each other in articulation, separated by a narrow gap filled with sediment ( Fig. 2D–F ). Both frontals are relatively flat dorsoventrally and have a trapezoidal to sub-triangular outline in dorsal and ventral views, being only slightly longer anteroposteriorly than wide mediolaterally. The length to width ratio of the frontals is approximately 1.38, based on the dimensions of the slightly more complete left frontal. The width of the frontals is greatest near their anterior margin, and then it stays relatively constant for more than half of their length before becoming narrower posteriorly. The anterior width of the frontal bone is over four times larger than its posterior width near the parietal facet. The frontals are sutured to each other along midline, to the parietal posteriorly, to the postorbital laterally as well as to the nasal and prefrontal anteriorly. The suture between the frontals is relatively straight and extends anteroposteriorly.

Posteriorly, the frontals form a broad triangular projection medially that shows a well-developed sutural contact on its ventral aspect for articulation with the parietal, which they seem to have considerably overlapped. Along their lateral margins, the frontals show a suture with the postorbital that extends anteroposteriorly at the lateral segment of the frontal and anterolaterally at the posterolateral segment, respectively. The sutural contact with the cranial elements lying anterior to the frontal (the nasal medially and the prefrontal laterally) occurs along the mediolaterally oriented wide transversal anterior margin of the frontals ( Fig. 6A, B ). These two contacts cannot be identified as clearly separate facets and they appear to have been confluent within a joint naso-prefrontal-to-frontal sutural facet. This naso-prefrontal-frontal suture is extensive and coarsely ridged, covering the entire wide anterior margin of the frontals, and is visible primarily on their dorsal aspect, which seem to have been overlapped by the nasals and prefrontals accordingly. This joint suture is bordered posteriorly by a low but angular, clearly visible ridge that extends mainly mediolaterally. Similarly, a low ridge also borders the posterolateral margin of the frontals in dorsal view. The surface between these raised rims is markedly concave and in medial view, the anterior and posterior margins of the frontals are somewhat dorsally curved. Other than these ridge-like features, the dorsal surface of the frontals is very smooth.

The ventral aspect of the frontals is much better preserved in the right frontal than in the left one, in which this side is locally damaged. In ventral view, the frontal shows three distinct concave depressions (one anterior, one lateral, and one posterior), which are separated from one another by low ridges ( Fig. 6C, D ). The thickest part of the frontals is at the center of the bone, near the ventral ridge that separates the anterior depression from the posterior one. The anterior depression likely represents the impression of the olfactory bulb of the brain. It has a roughly triangular shape, with the tip directed posteromedially, and is bordered medially by an anteroposteriorly extending ridge, and laterally by an anterolaterally extending ridge. The lateral depression is round and represents the medial part of the roof of the orbit. It is separated from the posterior depression by a very shallow rim that extends in a posterolateral direction. The posterior depression is elliptical to subtriangular and represents the impression of the cerebral part of the endocranium. The ridge that separates the anterior depression of the olfactory bulb roof from the lateral depression of the orbital roof likely represents the sutural contact of the frontal with the sphenethmoid plate.

COMPARISONS

Transylvanosaurus platycephalus is clearly referable to the Rhabdodontidae , as it exhibits the characteristic basicranial morphology of the group, i.e., a distinct and well-developed neck connecting the occipital condyle with the basal tubera anteriorly, as well as a mediolaterally wide and crest-like basal tubera (for a discussion contrasting the basicranial morphology in rhabdodontids, other basally branching iguanodontians and hadrosauroids, see Augustin et al. in press). Furthermore, two sets of phylogenetic analyses performed by us also consistently recovered Transylvanosaurus as being firmly nested within Rhabdodontidae (see below). As such, in the following section, the holotype of Transylvanosaurus platycephalus is compared extensively to rhabdodontid cranial material previously reported from the Upper Cretaceous of the Transylvanian area, which until now has exclusively been referred to the genus Zalmoxes . In addition, we compare the holotype partial skull described herein with the only other rhabdodontid for which substantial parts of the braincase and the frontals had been described, i.e., the genus Rhabdodon from southern France. In order to make the comparisons with the currently existing rhabdodontid cranial material from Romania and France as clear and meaningful as possible, and because there have been uncertainties as to the taxonomic affinities of some specimens ( Osi et al., 2012), we specifically refer to individual specimens instead of simply referring to Zalmoxes and Rhabdodon in the case of the Romanian and, respectively, the French material.

An Overview of the Braincase Material referred previously to Rhabdodontidae

In total, four more or less complete rhabdodontid basicrania have been reported until now from the Upper Cretaceous of the Transylvanian area, all recovered from the Hateg Basin (see also Augustin et al., in press). The first two of these, NHMUK R.3408 and NHMUK R.3409, were excavated more than a century ago from the stratotype Sînpetru Formation along the Sibisel Valley, in the south-central part of the basin ( Fig. 1B ). These specimens were described and figured by Nopcsa (1904), who referred them initially to the rhabdodontid Mochlodon robustus , later transferred to Zalmoxes robustus by Weishampel et al. (2003). Specimen NHMUK R.3408 comprises the complete basioccipital and most of the basisphenoid ( Nopcsa, 1904:fig. 2, pl. 1), whereas NHMUK R.3409 only preserves the anterior-most part of the basioccipital and the posterior-most part of the basisphenoid, i.e., the region around the basal tubera ( Nopcsa, 1904:pl. 1). Athird rhabdodontid basicranium, LPB (FGGUB) R.1629, was recovered much later, in 1998, from the middle part of the Densus-Ciula Formation at the Tustea-Oltoane nesting site, in the northwestern part of the Hateg Basin ( Fig. 1B ). The specimen consists of a complete basioccipital that was mentioned by Weishampel et al. (2003:78), and was subsequently illustrated and briefly described by Augustin et al. (in press:fig. 5). Alargely complete left exoccipital-opisthotic complex, LPB (FGGUB) R.1591, was found in close proximity to, and shows a perfect fit with, LPB (FGGUB) R.1629, and thus almost certainly belongs to the same individual ( Botfalvai et al., 2017: fig. 8). The last known rhabdodontid basicranium from the Hateg Basin, LPB (FGGUB) R.1723, was also found at the same Tustea locality in 2000 ( Fig. 1B ). It comprises the complete basioccipital and most of the basisphenoid, and has been described and figured by Weishampel et al. (2003:fig. 11). Two other Transylvanian braincase specimens that have been referred to Zalmoxes in the past, UBB NVZ1-42 ( Godefroit etal., 2009) from Nalat-Vad and NHMUK R.3401A ( Weishampel et al., 2003) from Sânpetru ( Fig. 1B ), were recently re-assigned to the hadrosauroid dinosaur Telmatosaurus (Augustin et al. in press), and are thus not considered in our comparisons.

Several more or less well-preserved rhabdodontid frontals have been described in the past from the Upper Cretaceous deposits of Romania, the most complete ones of which are used in the comparisons below. The first specimen, NHMUK R.3400, has been recovered from the Sînpetru Formation of the Sibisel Valley section ( Fig. 1B ) and was originally described by Nopcsa (1904), who referred it to Mochlodon (= Zalmoxes ). This specimen comprises the fused left and right frontals ( Nopcsa, 1904:pl. 1). Later, Nopcsa (1929b:fig. 1) figured and described another pair of fused frontals, MBFSZ v.13528, from the Densus-Ciula Formation near Valioara ( Fig. 1B ), which he assigned to the hadrosauroid Orthomerus (= Telmatosaurus ). Later, this specimen was first referred to an indeterminate arctometatarsalian theropod by Jianu and Weishampel (1997), before Weishampel et al. (2003) re-assigned it to Zalmoxes robustus ( Weishampel et al., 2003:fig. 8). Anearly complete left frontal fused to the postorbital, LPB (FGGUB) R.1616, was recovered much later from the Tustea-Oltoane site of the Densus-Ciula Formation ( Fig. 1B ). The specimen was described and figured by Weishampel et al. (2003:fig. 10), who referred it to Zalmoxes robustus . Alargely complete frontal from the Râul Mare River section near Nalat-Vad ( Fig. 1B ), UBB NVZ1-38, was figured and described by Godefroit et al. (2009:fig. 6). Based on its association within the same site with other, more diagnostic material, these authors referred UBB NVZ1-38 to Zalmoxes shqiperorum ( Godefroit et al., 2009) . Most recently, an almost complete left frontal from the lowermost part of the Maastrichtian Sebes Formation cropping out at Petresti-Arini, in the southwestern Transylvanian Basin (and about 70 km to the northeast of the Hateg Basin localities; Fig. 1A ), MMIRS 680, was described and figured by Vremir et al. (2014:27–28, fig. 10), who referred it to Zalmoxes sp.

Four rhabdodontid braincase specimens have been described to date from the Upper Cretaceous of southern France and all have been assigned to the genus Rhabdodon . Two of these specimens, MC-M4 and MC-MN25, both from the Upper Cretaceous (upper Campanian–lower Maastrichtian; Buffetaut et al., 1999) of southern France near Cruzy (Languedoc), were described in detail by Pincemaille-Quillevere et al. (2006). MC-M4 comprises a largely complete braincase including the basioccipital, the exoccipital-opisthotic complex, the basisphenoid-parasphenoid complex, the prootic, the laterosphenoid, and the supraoccipital ( Pincemaille-Quillevere et al., 2006:figs. 1–4), whereas MC-MN25 is more incompletely preserved and includes only the distorted posterior part of the braincase. Due to the poor preservation of MC-MN25, we mostly excluded it from the comparisons below. More recently, two additional rhabdodontid braincase specimens have been reported from the Upper Cretaceous of southern France, CM-669 from the late Campanian–early Maastrichtian locality Fox-Amphoux (Provence), and MC-M1575 also from Cruzy ( Chanthasit, 2010). They both preserve the majority of the braincase, including the basioccipital, the exoccipital-opisthotic complex, the basisphenoid-parasphenoid complex, the prootic, the laterosphenoid, the supraoccipital, and the parietal ( Chanthasit, 2010:45–49). Until now, no reasonably complete frontal has been described for the genus Rhabdodon ; the only currently known referred specimen is an incomplete right frontal, MC-QR8, from the Upper Cretaceous of southern France ( Chanthasit, 2010).

The holotype of Transylvanosaurus platycephalus, LPB (FGGUB) R.2070, is one of the most complete rhabdodontid skulls composed of associated elements that are undoubtedly referable to a single individual that has been reported so far from the Upper Cretaceous of Romania, despite previous claims of several associations of rhabdodontid cranial elements by Nopcsa (1904; see also Dumbravaet al., 2017). Notably, it is very similar in size to the other rhabdodontid basicrania from the Hateg Basin, especially to LPB (FGGUB) R.1629 and R.1723, and is only slightly larger than NHMUK R.3408 and R.3409. The rhabdodontid braincases from the Upper Cretaceous of France show a larger variation in size, ranging from close in size to those from Romania (as in MC-M4), to somewhat larger (up to a third larger, as in CM-669, MC-M1575), and even to significantly (more than a third) larger, as in MC-MN25, in agreement with previous assessments regarding a similar amount of overall body size difference between the latest Cretaceous Romanian ( Zalmoxes ) and French ( Rhabdodon ) rhabdodontids (e.g., Weishampel et al., 2003). Although being of a roughly similar size, the basicranium morphology of Transylvanosaurus differs considerably from all other rhabdodontid basicrania of the Hateg Basin as well as from those of southern France. The rhabdodontid frontals known from the Upper Cretaceous of Romania show a much higher size disparity than that noted for the basicrania, LPB (FGGUB) R.1616 and MMIRS 680 being at least one-third larger than Transylvanosaurus . Furthermore, just as for the braincase, the frontals of Transylvanosaurus also show several remarkable morphological differences from these other known Romanian rhabdodontid frontals.

Basioccipital and Endocranial Floor

The basioccipital is largely similar among the rhabdodontid basicrania from the Hateg Basin and southern France, but some differences are nevertheless noteworthy. The basioccipital is reniform in posterior view, as well as trapezoidal and convex in ventral view in all these rhabdodontid specimens preserving the occipital condyle, although the ventral convexity is most pronounced in Transylvanosaurus , which has an almost round basioccipital in ventral view. Specimen LPB (FGGUB) R.1629 differs from Transylvanosaurus and the other rhabdodontid basicrania in that the occipital condyle is demarcated from the basioccipital neck anteriorly by a well-developed rim. In LPB (FGGUB) R.1723, a well-developed notch is present on the anterolateral part of the basioccipital, which is absent or at most weakly developed in Transylvanosaurus, LPB (FGGUB) R.1629, NHMUK R.3408, and all of the French specimens. Like the other rhabdodontids, Transylvanosaurus has a well-developed neck connecting the occipital condyle with the basisphenoid.

Notably, theholotype of Transylvanosaurus differs from allother Romanian rhabdodontid specimens in having a straight endocranial floor. In contrast, the endocranial floor in LPB (FGGUB) R.1723 curves slightly dorsally anterior to the foramen magnum reaching a dorsal peak in the anterior half of the basioccipital, before sloping sharply ventrally to a ventral peak approximately at the level of the opening for the internal carotid artery; anterior to this ventral peak, the endocranial floor curves dorsally again. In LPB (FGGUB) R.1629 and NHMUK R.3408, the endocranial floor is relatively straight posteriorly, up until mid-length of the basioccipital, and then curves down ventrally reaching the deepest point approximately at the level of the opening for the internalcarotid artery. Therefore, the endocranial floor ismarkedly sinuous in LPB (FGGUB) R.1723, as well as, to a lesser extent, in LPB (FGGUB) R.1629 and NHMUK R.3408, as opposed to the completely straight endocranial floor in Transylvanosaurus . The orientation of the endocranial floor is not visible in the specimens from southern France as the endocranium is filled with sediment in CM-699, crushed in MC-MN25, or fully concealed by the braincase itself in MC-M4 and MC-M1575.

Exoccipital-Opisthotic Complex

The exoccipital-opisthotic complex of Transylvanosaurus differs markedly from that of LPB (FGGUB) R.1591, the only other reasonably complete element known from Transylvania, as well as from those preserved in specimens MC-M4, MC-M1575, and CM-699 from southern France. Generally, the ventromedial corner of the exoccipital in all of these basicrania is knob-like and participates in the formation of the occipital condyle in the form of a condylid, thus resembling the exoccipital of Transylvanosaurus . Additionally, in both LPB (FGGUB) R.1591 and MC-M4, the openings for cranial nerves X–XII are positioned on a relatively straight line extending roughly anteroposteriorly between the exoccipital condylid and the paroccipital process, just as in Transylvanosaurus .

However, the morphology of the paroccipital processes is completely different in Transylvanosaurus as compared with that of the other rhabdodontids. In Transylvanosaurus , the paroccipital process makes only a gentle dorsolateral curve proximally and is completely straight otherwise. In contrast, the paroccipital process of LPB (FGGUB) R.1591 makes a much sharper dorsolateral curve and its ventral margin is curved over the entire length of the process. In the specimens from southern France referred to Rhabdodon , the paroccipital process curves slightly dorsomedially before it turns sharply dorsolaterally and then extends only laterally at about the level of the skull roof. Consequently, the paroccipital processes in these French specimens resemble that of Transylvanosaurus in that they are relatively straight for most of their length, differing from the highly arched paroccipital process seen in LPB (FGGUB) R.1591 that laterally curves downward (i.e., ventrally). In general, however, the paroccipital processes of Transylvanosaurus extend much more laterally but less dorsally than do those of LPB (FGGUB) R.1591 as well as MC-M4, MC-M1575, and CM-699, therefore being overall straighter. Moreover, the paroccipital processes are also somewhat longer and considerably thinner dorsoventrally in Transylvanosaurus than in all other rhabdodontid specimens. Nevertheless, it more closely resembles specimens MC-M4, MC-M1575, and CM- 699 in this regard, too, whereas LPB (FGGUB) R.1591 has much thicker paroccipital processes. Due to the highly arched paroccipital processes of LPB (FGGUB) R.1591 as well as to their greater dorsoventral thickness and shorter length, the skull of this animal seems to have been somewhat narrower but relatively higher than that of Transylvanosaurus and the French rhabdodontids.

The medial margin of the exoccipital-opisthotic process that forms the lateral wall of the foramen magnum is also dorsoventrally higher in LPB (FGGUB) R.1591, MC-M4, MC-M1575, and MN-25, compared with LPB (FGGUB) R.2070. Accordingly, the foramen magnum is higher dorsoventrally than wide mediolaterally in these specimens, whereas it is wider mediolaterally than high dorsoventrally in Transylvanosaurus . Furthermore, the crista tuberalis is only weakly developed in Transylvanosaurus , while it is much more pronounced in all the other known rhabdodontid braincases. Although the supraoccipital is missing in the holotype specimen of Transylvanosaurus , based on the morphology of the opisthotic, it must have been very narrow mediolaterally. Additionally, the suture between the opisthotic and the supraoccipital is nearly vertical (extending dorsoventrally) in Transylvanosaurus , whereas it is oblique (extending dorsolaterally) in LPB FGGUB) R.1591, CM-699, MC-M1575, and MC-M4.

Prootic

In Transylvanosaurus , the ventral part of the prootic forms a well-developed and massive process that extends mainly anterolaterally and to a lesser degree also ventrally. This process is completely absent in MC-M4 and MC-M1575, while this region is preserved neither in LPB (FGGUB) R.1723 and R.1629, nor in NHMUK R.3408 and R.3409. But even so, it is nonetheless highly probable that the prootic must have had a slightly different morphology in these specimens when compared with Transylvanosaurus . In Transylvanosaurus , the prootic process participates in the formation of the groove on the lateral side of the braincase that houses the entrance for the internal carotid artery, whereas in all the other rhabdodontid braincases, this groove ends in a small chamber dorsally on the lateral aspect of the basisphenoid-parasphenoid complex and thus cannot reach the prootic process (if present). Asmall crest-like extension of the prootic in CM-699 might correspond to the prootic process seen in Transylvanosaurus , although it is much more weakly developed and appears to represent more likely a continuation of the crista transversalis of the basal tubera. Consequently, it differs completely from the massive knob-like process seen in Transylvanosaurus that is almost completely separated from the crista transversalis.

Basisphenoid-Parasphenoid Complex

The basisphenoid-parasphenoid complex of Transylvanosaurus shows several significant differences from those of all other currently known rhabdodontid basicrania. Arguably, the most important difference is that the transverse, crest-like basal tubera meet the long axis of the braincase, which is parallel to the orientation of the endocranial floor, at an angle of approximately 140° in Transylvanosaurus as opposed to 120° in NHMUK R.3408 and R.3409, as well as LPB (FGGUB) R.1723, 125° in MC-699 as well as 130° in MC-M4 and MC-M1575. Consequently, Transylvanosaurus resembles more closely the rhabdodontid specimens from southern France in this regard. Partly due to the flat angle between the basal tubera and the long axis of the braincase, the basisphenoid is also much more elongated anteroposteriorly in Transylvanosaurus compared with the other rhabdodontid basicrania.

Moreover, the basal tubera display different morphologies in the different rhabdodontid braincase specimens. The dorsoventral extension (or height) of the basal tubera and of the entire basisphenoid-parasphenoid complex is much greater in Transylvanosaurus , in the different French rhabdodontid basicrania, and in LPB (FGGUB) R.1723, compared with the condition seen in NHMUK R.3408 and R.3409. In addition, the anterior part of the basisphenoid-parasphenoid complex (just anterior to the basal tubera) is anterodorsally inclined in Transylvanosaurus , the French rhabdodontid basicrania, and LPB (FGGUB) R.1723, while it is completely straight and extends only anteriorly in NHMUK R.3408 and R.3409. The two London specimens further differ from Transylvanosaurus in that the basal tubera extend not only anteroventrally but also laterally and thus encircle the ventral portion of the basicranium up until the level of the endocranial floor in a semicircular manner. Therefore, the basal tubera are very wide mediolaterally in NHMUK R.3408 and R.3409 and well visible in dorsal view, lateral to the endocranial floor. Although a similar condition can also be noted in MC-M4, MC-M1575, and CM-699, it is much more pronounced in NHMUK R.3408 and R.3409. In contrast, the basal tubera of LPB (FGGUB) R.1723 project mostly anteroventrally, just as in Transylvanosaurus . Transylvanosaurus differs, however, from LPB (FGGUB) R. 1723 in having basal tubera that are much wider mediolaterally and thus visible in dorsal view as well. In all rhabdodontid basicrania from the Hateg Basin, the posterior face of the basal tubera seems to have a slightly wrinkled appearance and a prominent midline ridge, albeit only a fractured surface marks its position in NHMUK R.3408. Both the wrinkles and the midline ridge are, however, much more strongly developed in Transylvanosaurus than in the other specimens. The French rhabdodontid basicrania lack both the wrinkled appearance on the posterior face of the basal tubera and the midline ridge.

Another striking difference between Transylvanosaurus and the other rhabdodontids concerns the morphology of the groove on the lateral aspect of the basisphenoid housing the entrance for the internal carotid artery. In all rhabdodontids except Transylvanosaurus , this groove is oriented roughly dorsoventrally and terminates in a rounded chamber, well below the level of the endocranial floor. In contrast, this groove displays a completely different morphology in Transylvanosaurus , where it is oriented anteroventrally and forms a continuous canal that extends above the level of the endocranial floor. The basipterygoid processes also have a unique morphology and orientation in Transylvanosaurus , differing markedly from the condition seen in LPB (FGGUB) R.1723, MC-M4, and MC-M1575. In Transylvanosaurus , these processes direct ventrolaterally and anteriorly, whereas they project ventrolaterally and posteriorly in the other rhabdodontid specimens. In addition, the basipterygoid processes diverge from the sagittal plane at a wider angle in Transylvanosaurus and their lateral surface is much broader anteroposteriorly, giving them a wing-like morphology. The ventral surface between the basipterygoid processes is narrower and somewhat more steeply inclined in LPB (FGGUB) R.1723 and MC-M1575 than in Transylvanosaurus . Unlike the condition seen in Transylvanosaurus , the region anterior to the basal tubera, on the ventral aspect of the basicranium, shows a straight and elongated groove extending anteroposteriorly in specimens NHMUK R.3408 and R.3409; in the first of these two specimens, two triangular fractured surfaces mark the position of the missing basipterygoid processes lateral to this groove. Although imperfectly preserved in Transylvanosaurus , the pituitary fossa is apparently much shorter dorsoventrally than in LPB (FGGUB) R.1723 and thus resembles the tube-like and round pituitary fossa present in NHMUK R.3409.

Frontals

Although the frontals of Transylvanosaurus generally resemble those that have been previously referred to the Rhabdodontidae from the Upper Cretaceous of Romania, some notable differences are present. Most importantly, the frontals of Transylvanosaurus are very wide mediolaterally, having an anteroposterior length to mediolateral width ratio of 1.38, which represents the lowest value recorded among the rhabdodontid frontals that have so far been described. This ratio can be reliably measured for three other frontals that are reasonably complete, all from the Hateg Basin. Of these, MBFSZ v.13528 has a length to width ratio of 1.46 and thus is relatively close to the value seen in Transylvanosaurus . The other two frontals however, LPB (FGGUB) R.1616 and NHMUK R.3400, have much higher values of this ratio, of 1.69 and 1.93, respectively, more in line with the general diagnosis of the frontal of Zalmoxes as given by Weishampel et al. (2003). Moreover, the frontals remain relatively broad for almost their entire length in Transylvanosaurus and MBFSZ v.15328, whereas they evenly and markedly taper posteriorly in LPB (FGGUB) R.1616 and NHMUK 3400. Accordingly, the outline of the frontals is rather trapezoidal (short and broad) in Transylvanosaurus and MBFSZ v.13528, as opposed to the more triangular (long and narrow) outlines of LPB (FGGUB) R.1616 and NHMUK R.3400. Although imperfectly preserved, specimen MMIRS 680 from the southwestern Transylvanian Basin seems to have been relatively broad as well, with a length to width ratio of approximately 1.51, thus more closely resembling Transylvanosaurus in this regard. However, unlike Transylvanosaurus , this frontal also tapers posteriorly giving it a triangular outline, also seen in the frontal UBB NVZ1-38 from Nalat-Vad, the only such specimen referred to Zalmoxes shqiperorum by Godefroit et al. (2009). All of these ratios were calculated with measurements of the left frontal, which is more complete in both Transylvanosaurus and NHMUK R.3400, as well as being the only side preserved in LPB (FGGUB) R.1616 and MMIRS 680.

Aside from their variable overall outline and relative dimensions, the known rhabdodontid frontals also differ in other aspects of their general morphology. In Transylvanosaurus , the dorsal surface of the frontals is concave, just as in MMIRS 680 and MBFSZ v.13528, whereas it is rather flat or even slightly convex in NHMUK R.3400 and LPB (FGGUB) R.1616. Additionally, a well-developed transverse crest, placed closely behind and parallel to the unique naso-prefrontal suture of the frontal, is present in Transylvanosaurus and some other rhabdodontid frontals from Romania, including MMIRS 680 and MBFSZ v.13528, but it is absent in LPB (FGGUB) R.1616 (where a very slightly raised posterior margin of these two non-coalesced sutural facets is present, nevertheless) and in NHMUK R.3400. The unique naso-prefrontal suture extends primarily mediolaterally in Transylvanosaurus, MBFSZ v.13528, and MMIRS 680, and the frontals are overlain anteriorly by the nasals and prefrontals along their entire width (although the sutural contacts between the frontal and the nasal medially, respectively the prefrontal laterally, cannot be identified as clearly separate facets, see above). In contrast to this condition, the frontal-nasal and frontal-prefrontal sutures are clearly divided, posteriorly pointed triangular facets in NHMUK R.3400 and UBB NVZ1-38. Specimen LPB (FGGUB) R.1616 exhibits still another configuration of this sutural relationship, in which the two facets are partly confluent (as noted by Weishampel et al., 2003), although they are still clearly discernible, with a less posteriorly projected and smaller prefrontal facet laterally and a larger, more posteriorly extended nasal facet medially. Consequently, the fronto-nasal suture is somewhat oblique in LPB (FGGUB) R.1616, NHMUK R.3400, and UBB NVZ1-38 and the nasals overlie the frontals mostly in the medial part, giving the nasals a triangular shape in dorsal view with the posteriorly pointed tip inserted between the paired frontals. Interestingly, the frontal specimens in which a well-developed transverse frontal crest is present also seem to have a concave dorsal surface, a relatively wider overall shape and a roughly similar, confluent and transversely oriented frontal/ nasal-prefrontal suture morphology. The general pattern presented by the ventral surface of the frontals, housing the impressions of the olfactory bulb and the cerebrum, as well as the orbital roof, is very similar in all rhabdodontid frontals.

PHYLOGENETIC ANALYSES

Two phylogenetic analyses were performed in order to assess the phylogenetic relationships of Transylvanosaurus platycephalus (for details on the two datasets and the settings used for the analysis, see above). We added Transylvanosaurus to the first dataset of Dieudonné et al. (2021) and, given the nature of its holotype, restricted to the partial posterior skull, were able to score a total of 18 characters (representing only 5% of the total dataset) for the new taxon (the complete data matrix can be found in the Supplementary material). The analysis recovered 2508 equally parsimonious trees with 1422 steps. Consistency (CI) and retention indices (RI) were calculated for the whole tree (CI = 0.296 and RI = 0.615) using the script available in TNT. Adding Transylvanosaurus to the matrix of Dieudonné et al. (2021) resulted in an overall much poorer resolution of the tree topology compared with the original analysis. In the strict consensus tree, Transylvanosaurus was recovered at the base of Iguanodontia in a polytomy with Fostoria , the ‘Vegagete ornithopod,’ as well as the Rhabdodon , Mochlodon , and Zalmoxes ( Fig. 7 ).

In addition, we added Transylvanosaurus to the second matrix of Madzia et al. (2018) in order to test the results of the first analysis and were able to score 15 characters for it in total, representing about 6% of the dataset (the complete data matrix can be found in the Supplementary material). The second analysis recovered 362 equally parsimonious trees with 904 steps. Consistency (CI) and retention indices (RI) were again calculated for the whole tree (CI = 0.344 and RI = 0.640) using the script available in TNT. Just as in the case of the first analysis, adding Transylvanosaurus to the matrix of Madzia et al. (2018) resulted in an overall much poorer resolution of the tree topology compared with the original analysis, which was to be expected given the large amount of missing data for the new Romanian taxon. In the strict consensus tree of the second analysis, Transylvanosaurus was recovered at the base of Iguanodontia in a polytomy with Mochlodon and Zalmoxes , these taxa together forming the sister group to Rhabdodon (thus recovering a monophyletic Rhabdodontidae including all traditionally assigned genera as well as the new taxon from Pui), with Muttaburrasaurus placed in a more basal position ( Fig. 8 ).

Figures

FIGURE 1. Locality information for the holotype of Transylvanosaurus platycephalus gen. et sp. nov. A, Location of the type locality of Transylvanosaurus platycephalus gen. et sp. nov. south of Pui, in the eastern Hateg Basin, western Romania, alongside with that of other rhabdodontid posterior cranial remains (frontals and basicrania listed above, respectively below the horizontal line) discussed in the text; the holotype is LPB (FGGUB) R.2070, in bold (for details on specimen numbers, see text). Key: 1, uplifted pre-Alpine crystalline basement rocks bordering the Hateg Basin; 2, pre-uppermost Cretaceous sedimentary units of the Hateg Basin (mainly marine beds); 3–5, vertebrate-bearing uppermost Cretaceous (Maastrichtian) continental deposits: 3, Sînpetru Formation (spf); 4, Sînpetru Formation-correlative units (‘Râul Mare Beds’ in the central part of the basin, ‘Pui Beds’ in the eastern part); 5, Densus-Ciula Formation (dcf), with v—volcanoclastic ‘lower member’; 6, Cenozoic (mainly Quaternary) sedimentary cover; 7, main fossiliferous localities with rhabdodontid posterior cranial material. B, Inset shows the position of the Hateg Basin within Romania (rectangle), as well as the approximate location of the rhabdodontid frontal MMIRS 780 in the southwestern part of the Transylvanian Basin (star).

FIGURE 2. The type locality of Transylvanosaurus platycephalus gen. et sp. nov. at the Barbat River Valley section, near Pui, eastern Hateg Basin. A, General overview of the riverbed outcropping condition of the uppermost Cretaceous continental ‘Pui Beds’ along the Barbat River, south of Pui; in the background, flat-lying coarse cobbly-sandy Quaternary deposits covering the reddish uppermost Cretaceous rocks. B, Details of the superposed greenish coarser-grained channel deposits and red fine-grained floodplain sediments with well-developed whitish pedogenic calcrete horizons, characteristic of the ‘Pui Beds.’ C, View of the ‘Pui Beds’ looking southward, with the type locality and bed (a red silty mudstone) of Transylvanosaurus platycephalus gen. et sp. nov. exposed in the middle ground; the type specimen, LPB (FGGUB) R.2070, was discovered near the left edge of the photograph (white arrow). D, Partial posterior cranium of Transylvanosaurus platycephalus gen. et sp. nov., specimen LPB (FGGUB) R.2070 (exposed paired frontals, above, and partly buried basicranium, below) in the moment of its discovery, July 2007; chisel for scale. E, Specimen LPB (FGGUB) R.2070 completely exposed during excavation. F, Block containing specimen LPB (FGGUB) R.2070 after completed excavation and before plaster jacketing.

FIGURE 3. Transylvanosaurus platycephalus gen. et sp. nov., holotype basicranium, FGGUB (LPB) R.2070, in lateral view. A, photo and B, drawing of the basicranium in left lateral view. C, photo and D, drawing of the basicranium in right lateral view. Abbreviations: alp, alar process; boc, basioccipital; bpt, basipterygoid process; bsp, basisphenoid; btu, basal tubera; cn, cranial nerve; ctr, crista transversalis; ctu, crista tuberalis; exo, exoccipital; fov, foramen ovalis; ica, opening for the internal carotid artery; lgr, lateral groove; lsp, laterosphenoid; opi, opisthotic; pap, paroccipital process; pro, prootic; prp, prootic process.

FIGURE 4. Transylvanosaurus platycephalus gen. et sp. nov., holotype basicranium, FGGUB (LPB) R.2070, in anterior and posterior view. A, photo and B, drawing in anterior view. C, photo and D, drawing in posterior view. Abbreviations: boc, basioccipital; bpt, basipterygoid process; bsp, basisphenoid; btu, basal tubera; exo, exoccipital; fom, foramen magnum; lsp, laterosphenoid; opi, opisthotic; pap, paroccipital process; pit, pituitary fossa; pro, prootic; prp, prootic process.

FIGURE 5. Transylvanosaurus platycephalus gen. et sp. nov., holotype basicranium, FGGUB (LPB) R.2070, in dorsal and ventral view. A, photo and B, drawing of the basicranium in dorsal view. C, photo and D, drawing of the basicranium in ventral view. Abbreviations: boc, basioccipital; bpt, basipterygoid process; bsp, basisphenoid; btu, basal tubera; cn, cranial nerve; exo, exoccipital; fov, foramen ovalis; mri, midline ridge on the basal tubera; lsp, laterosphenoid; opi, opisthotic; pap, paroccipital process; pit, pituitary fossa; pro, prootic; prp, prootic process.

FIGURE 6. Transylvanosaurus platycephalus gen. et sp. nov., holotype frontals, FGGUB (LPB) R.2070. A, photo and B, drawing of the frontals in dorsal view. C, photo and D, drawing of the frontals in ventral view. Note that the ventral side of the left frontal is damaged and thus does not preserve the impressions of the orbital roof and the olfactory bulb. Abbreviations: cer, impression of the cerebrum; nps, confluent nasal-prefrontal suture; olf, impression of the olfactory bulb; orb, orbital roof; pas, parietal suture; pos, postorbital suture; sph, sutural contact with the sphenethmoid plate; tfc, transverse frontal crest.

FIGURE 7. Strict consensus tree of the first phylogenetic analysis performed by us using the matrix of Dieudonné et al. (2021), showing the relationships of Transylvanosaurus platycephalus within Ornithischia and Ornithopoda.

FIGURE 8. Strict consensus tree of the first phylogenetic analysis performed by us using the matrix of Madzia et al. (2018), showing the relationships of Transylvanosaurus platycephalus within Ornithischia and Ornithopoda. Notably, the phylogenetic relationships within Rhabdodontidae as shown herein differ from those reconstructed based on our thorough morphological comparisons (i.e., a particularly close relationship between Transylvanosaurus and Rhabdodon). Due to the scarcity of relevant braincase characters in the original dataset and the poor resolution of Rhabdodontidae, we regard the hypothesis derived from the morphological comparisons as more likely.