Cranial ornamentation in the Late Cretaceous nodosaurid ankylosaur Hungarosaurus

Abstract

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.

Keywords: Ankylosaur; Cranial elaboration; Cranial ornamentation; Hungarosaurus; Late Cretaceous; Osteoderm fusion; Sexual dimorphism; Skull.

Figure 1. Surface view and microCT cross sectional images (in level of the green line) of cranial ornamentation developed as either osteodermal fusion (A–F) or elaboration of skull bones (G–L) in squamates.

(A) Tiliqua scincoides skull (MDE R45) in dorsal view. (B) Partially fused polygonal osteoderms on the skull of T. scincoides. (C) Inner structure of the skull bones and covering osteoderms of T. scincoides. (D) Tiliqua nigrolutea skull (MDE R47) in dorsal view. (E) Partially fused polygonal osteoderms on the skull of T. nigrolutea. (F) Inner structure of the skull bones and covering osteoderms of T. nigrolutea. (G) Iguana iguana skull (MDE R20) in dorsal view. (H) Elaboration of skull bones in I. iguana. (I) Inner structure of the elaborated skull bones in I. iguana. (J) Chamaeleo calyptratus (MDE R43) skull in lateral view. (K) Inner structure of the elaborated skull bones in C. calyptratus. (L) Elaboration of skull bones in C. calyptratus. Abbreviations: cb, cancellous bone; cel, cranial elaboration; cob, compact bone; fr, frontal; glo, globular ornamentation; mx, maxilla; or, orbit, os, osteoderm; pa, parietal; pcr, parietal crest; pfos, partially fused osteoderms; plos, polygonal osteoderms; po, postorbital; uno, unossified part between osteoderm and skull bone; sp, small pits.

Figure 2. Cranial specimens of the Late Cretaceous (Santonian) nodosaurid ankylosaur, Hungarosaurus tormai in 3D reconstruction (for 3D reconstruction see Video S1–S6).

(A) holotype skull (MTM 2007.26.1.-2007.26.34.). (B) MTM PAL 2020.31.1. (C) MTM PAL 2020.32.1. (D) basicranium and partial skull roof MTM PAL 2013.23.1. Scal bar is for Fig. 2D, other skulls are in comparative scale.

Figure 3. Ontogenetic change of the cranial ornamentation on the premaxillae (A–D) and nasals (E–H) of Hungarosaurus.

Each element is visualized in surface view, three-dimensional surface rendering of microCT images, and microCT cross-sectional view. (A) Right premaxilla of MTM V 2003.12. (mirrored) in left lateral view. (B) left premaxilla of MTM PAL 2020.33.1. in left lateral view. (C) Premaxilla of MTM PAL 2020.31.1. in left lateral view. (D) Holotype premaxilla in left lateral view. (E) Nasals of MTM PAL 2020.32.1. in dorsal view. (F) right nasal from MTM PAL 2020.31.1. (G) Right nasal (MTM PAL 2020.34.1.) in doral view. (H) Holotype nasal fragment (mirrored). Abbreviations: cb, cancellous bone; cob, compact bone; den, dorsal margin of external nares; en, external nares; es, eroded surface; gr, groove; ins, internasal suture; lp, large pits; or, ornamentation; re, raised edge; sho, ornamentation in shingled arrangement; sp, small pits.

Figure 4. Ontogenetic change of the cranial ornamentation on the skull roof and orbital region of Hungarosaurus.

Each element is visualized in surface view, three-dimensional surface rendering of microCT images, and microCT cross-sectional view. (A) Skull roof of MTM PAL 2020.32.1. in dorsal view. (B) MTM 2007.27.1. left fragmentary frontal in dorsal view. (C) MTM PAL 2013.23.1. basicranium and partial sull roof in dorsal view. (D) Postorbital of MTM PAL 2020.32.1. (E) MTM 2007.28.1. left postorbital. (F) MTM 2010.1.1. left postorbital and jugal. (G) Holotype postorbital and jugal (mirrored). Abbreviations: cb, cancellous bone; cob, compact bone; efe, edge of frontal elaboration; gr, groove; lip, depressed “lip” at transition to softer skin; lp, large pits; npf, nasal process of frontal; orb, orbit; pa, parietal; po, postorbital; poc, postorbital crest; sp, small pits.