Cranial ornamentation in the Late Cretaceous nodosaurid ankylosaur Hungarosaurus

Bony cranial ornamentation is developed by many groups of vertebrates, including ankylosaur dinosaurs. To date, the morphology and ontogenetic origin of ankylosaurian cranial ornamentation has primarily focused on a limited number of species from only one of the two major lineages, Ankylosauridae. For members of the sister group Nodosauridae, less is known. Here, we provide new details of the cranial anatomy of the nodosaurid Hungarosaurus from the Santonian of Europe. Based on a number of previously described and newly identified fragmentary skulls and skull elements, we recognize three different size classes of Hungarosaurus. We interpret these size classes as representing different stages of ontogeny. Cranial ornamentation is already well-developed in the earliest ontogenetic stage represented herein, suggesting that the presence of outgrowths may have played a role in intra- and interspecific recognition. We find no evidence that cranial ornamentation in Hungarosaurus involves the contribution of coossified osteoderms. Instead, available evidence indicates that cranial ornamentation forms as a result of the elaboration of individual elements. Although individual differences and sexual dimorphism cannot be excluded, the observed variation in Hungarosaurus cranial ornamentation appears to be associated with ontogeny.

interorbital part of the skull roof in ventral, K, dorsal view. Abreviations: alv, alveoli; apm, articular surface for premaxilla; bus, buccal shelf; ch, channel; cm, cutting margin; den, dorsal margin of external nares; dmo, dorsal margin of the orbit; en, external nares; fo, foramen; fr, frontal; ipm, interpremaxillary suture; lmch, lateral margin of choana; mpr, maxillary process of nasal; np, nasal process; or, orbit; pcm, posterior end of cutting margin; pmp, premaxillary process of nasal; prs, preocular shelf; sor, supraorbital. premaxillary alveoli are also featues shared with Hungarosaurus. The posterior end of the nasal process is completely preserved on the left premaxilla having a triangular, very pointed tip (Suppl. Fig. 1C, D) that slightly overlaps the anterodorsomedial margin of the left nasal.
Close to the basal part of the nasal process both premaxillae have a ventrally, anteroventrally oriented, laterally opened channel that breaks thorough the premaxilla (Suppl. Fig. 1B) and on the ventral side, it opens into a relatively large (3-4 mm in diameter) foramen. This feature cannot be seen on the holotype of Hungarosaurus, and as such might be an ontogenetic or sexual difference. However, it seems to be present on an even smaller, isolated premaxilla (MTM V 2003.12., Ősi andMakádi 2009) suggesting that it might have had the function to supply the ontogenetically growing bone.
Of the two nasals, the right one is more complete being more than twice as long as wide.
Anteriorly it becomes narrow to connect to the premaxilla, laterally it curves ventrally to connect with the maxilla and posteriorly it is widest with some slightly eroded scarf joints on its medioventral surface (Suppl. Fig. 1H, I). The internasal suture is a straight, slightly waving, dorsoventrally ca. 3 mm thick surface. Its outer surface is strongly weathered. The anterior part of the left nasal is better preserved having a rounded anterolateral margin bordering dorsally the external nares. The smaller specimen (Suppl. Fig. 1H) is almost identical with the left nasal of the holotype of Hungarosaurus.
The fragmentary right maxilla has an anteriorly slightly widening morphology (Suppl. Fig.   1F, G). Anterolaterally the ventral side of the maxilla forms the posteriorly widened cutting margin of the premaxilla. Eight alveoli are preserved in the maxilla, but the posterior half of the alveolar row is missing suggesting at least the double of the number of alveoli. This bone is also compressed dorsomedially, but has a significant buccal shelf (1.5-2 cm wide lateromedially) lateral to the alveolar row, as is present in most ankylosaurs (Vickaryous et al. 2004). Mediodorsal to the anterior alveoli, is a 1 cm long process which curves anterodorsally. This may representthe anteriormost margin of the choana.
A large part of the anterior skull roof between the orbits is preserved. The outer surface is strongly weathered (and relatively thin, max. thickness of 4-6mm) and thus not overly informative, but laterally and anterolaterally it preserves the dorsal and anterodorsal margin of the orbit. No crest or other distinctive cranial ornamentation is present on this part of the orbital margin. The posterior section of the orbit, present in the holotype of Hungarosaurus, is not preserved for MTM PAL 2020.31.1. (Suppl. Fig. 1K), thus the presence or absence of a postorbital crest is unkown. Ventrally the preocular shelf extends into the anteroventrally bending anterodorsal corner of the orbital rim (Suppl. Fig. 1J). Sutures between the individual cranial elements on this piece of bone cannot be observed, but according to the position of this element most of the right prefrontal-supraorbital-frontal complex is preserved.
Since there are no corresponding elements with any of the cranial material of Struthiosaurus spp. (Seeley 1881, Nopcsa 1929, Pereda-Suberbiola and Galton 1994, 2001 This specimen represents the smallest among the four skulls. It consists of the partial basicranium, most of the skull roof between and posteriorto orbits, the two nasals, the left postorbital, left squamosal, most of the left quadrate and the distal end of the right quadrate (Suppl. Fig. 2).
In contrast to MTM PAL 2020.31.1., this specimen is better preserved showing many details of the cranial ornamentation on the dorsal elements. The specimen is slightly compressed dorsoventrally thus the basicranium was pushed into braincase. Posterior to the frontal all bones of the skull roof, braincase and orbital region are completely fused and no sutures can be recognized (Suppl. Fig. 2A, B). Although the two nasals are preserved together, the bones are not fused to each other. Neither are they fused to the maxillae and the bones posterior to the nasal, as it can be seen by the overlapping articulation surfaces preserved on the nasals.
The nasals are at least twice as anteroposteriorly long than wide (although anteriorly they are not completely preserved), dorsally highly ornamented (for details see main text), trapezoid elements. At the nasal-frontal contact, the nasals overlap the anterior process of the frontals (Suppl. Fig. 2A, B).Similar sutural contact can be assumed between the nasal and prefrontal, despite the prefrontals not being preserved, as the posterolaterally facing, wedge-like articulation surfaces of the nasals suggest this condition (Suppl. Fig. 2B). An interfrontal suture may be present between the anterior part of the bones  (also seen weekly on the ventral side), but cranial ornamentation mostly masks it. The anterior part of the nasals are not preserved preventing comparison with that of the holotype of Hungarosaurus.
The dorsal surface of the skull roof is highly ornamented. The parietal region is highly vaulted, as seen in Struthiosaurus spp., a referred specimen of Hungarosaurus (MTM PAL 2013.23.1.) and Pawpawsaurus (Lee 1996). The supraoccipital is strongly fused with the parietal and exoccipitals, it bears a short saggital crest. Laterodorsal to the foramen magnum the exoccipital bears an oval, dorsally-posterodorsally facing protuberance, similar to that of MTM PAL 2013.23.1. The condylus occipitalis is wider than high but rather triangular and heart-shaped in posterior-posteroventral view. This is slightly different from the ventrally rounded condyle of the holotype of Hungarosaurus and MTM PAL 2013.23.1. It also differs from that of Struthiosaurus. The exoccipital-squamosal-quadrate region is a massively fused, L-shaped block. In dorsal view, the squamosal is laterally-posterolaterally oriented and bears no significant ornamentation. Only some rugose texture can be seen on its mediodorsal surface (Suppl. Fig. 2E  Anterior to the postorbital crest is a slightly ventrally bending dorsally ornamented rim. In lateral view, this margin extends far anteriorly resulting in an abruptly long dorsal margin and would result an enormous orbit. Though the skull was certainly compressed dorsoventrally, the pre-and postocular shelfs are at a near vertical angle (ca. 85-90°) further suggesting large sized orbits.
One of the quadrates shows the typical L-shape in posterior view separating proximally the lateral temporal fenestra from the posttemporal fenestra (Suppl. Fig. 3B). Mandibular condyle is complete on the right and fragmentary on the left quadrate. The mandibular articulation surface, though much smaller than the type of Hungarosaurus or that of Struthiosaurus spp.
(PIUW 2349, BMNH R 4966), it is more similar to that of Hungarosaurus in having a rather rhomboidal than oval shape in distal view (Suppl. Fig. 3C, E). Furthermore, the lateral condyle of the quadrate is separated by a distinct neck from the quadratojugal process (Suppl. to Hungarosaurus. However, the very slight differences in the morphology of these characters in conjunction with the relatively large orbits, unfused preorbital bones and the relatively small size of the skull suggests a subadult ontogenetic stage. Thus herein MTM PAL 2020.32.1. is considered as an ontogenetically immature form of Hungarosaurus tormai.