Cranial osteology of the ankylosaurian dinosaur formerly known asMinmisp. (Ornithischia: Thyreophora) from the Lower Cretaceous Allaru Mudstone of Richmond, Queensland, Australia

Abstract

Minmi is the only known genus of ankylosaurian dinosaur from Australia. Seven specimens are known, all from the Lower Cretaceous of Queensland. Only two of these have been described in any detail: the holotype specimen Minmi paravertebra from the Bungil Formation near Roma, and a near complete skeleton from the Allaru Mudstone on Marathon Station near Richmond, preliminarily referred to a possible new species of Minmi. The Marathon specimen represents one of the world’s most complete ankylosaurian skeletons and the best-preserved dinosaurian fossil from eastern Gondwana. Moreover, among ankylosaurians, its skull is one of only a few in which the majority of sutures have not been obliterated by dermal ossifications or surface remodelling. Recent preparation of the Marathon specimen has revealed new details of the palate and narial regions, permitting a comprehensive description and thus providing new insights cranial osteology of a basal ankylosaurian. The skull has also undergone computed tomography, digital segmentation and 3D computer visualisation enabling the reconstruction of its nasal cavity and endocranium. The airways of the Marathon specimen are more complicated than non-ankylosaurian dinosaurs but less so than derived ankylosaurians. The cranial (brain) endocast is superficially similar to those of other ankylosaurians but is strongly divergent in many important respects. The inner ear is extremely large and unlike that of any dinosaur yet known. Based on a high number of diagnostic differences between the skull of the Marathon specimen and other ankylosaurians, we consider it prudent to assign this specimen to a new genus and species of ankylosaurian. Kunbarrasaurus ieversi gen. et sp. nov. represents the second genus of ankylosaurian from Australia and is characterised by an unusual melange of both primitive and derived characters, shedding new light on the evolution of the ankylosaurian skull.

The most basal ankylosaurine dinosaur from the Albian–Cenomanian of China, with implications for the evolution of the tail club

“…Posteriorly, the nasal contacts the frontal in line with the midpoint of the orbit. The serrated suture with the frontal curves anteromedially, unlike the roughly transversely oriented suture in Pinacosaurus 14 , and Kunbarrasaurus 8 . The nasal contacts the prefrontal posterolaterally, and the long contact is oriented lateroanteriorly, and passes from the dorsal to the lateral side of the skull.…”

“…In contrast, the lacrimal separates the prefrontal from the maxilla in Pinacosaurus 11,14 . The prefrontal of Kunbarrasaurus seems to contact the dorsally strongly expanded maxilla 8 .…”

“…Some sutures of the dorsum posterior to the nasals are not fully fused. The sutural contacts of the skull are known from only a few taxa: Pinacosaurus grangeri from China and Mongolia, P. mephistocephalus , Liaoningosaurus and Chuanqilong from China, Kunbarrasaurus from Australia and Cedarpelta from the United States 7,8 . The dorsal surface of the skull has irregular, roughly textured skull ornamentation with some randomly directed non-vascular grooves, as in basal ankylosaurids such as Gobisaurus , Shamosaurus , Pinacosaurus and ‘ Zhongyuansaurus ’, rather than discrete caputegulae as in more derived ankylosaurids (Ankylosaurinae) like Ankylosaurus , Euoplocephalus or Saichania 6 .…”

“…The dorsal surface of the skull has irregular, roughly textured skull ornamentation with some randomly directed non-vascular grooves, as in basal ankylosaurids such as Gobisaurus , Shamosaurus , Pinacosaurus and ‘ Zhongyuansaurus ’, rather than discrete caputegulae as in more derived ankylosaurids (Ankylosaurinae) like Ankylosaurus , Euoplocephalus or Saichania 6 . In lateral view, the skull bears a nearly flat dorsal surface posterior to the nares with only slight arching of the maxillary rostrum, as in ‘ Zhongyuansaurus ’ 9 , and Kunbarrasaurus 8 . In contrast, Gobisaurus exhibits a domed dorsal surface 10 .…”

Unusual cranial and postcranial anatomy in the archetypal ankylosaur Ankylosaurus magniventris

“…Nevertheless, these remains indicate that Ankylosaurus had diverged from the Laramidian ankylosaurine bauplan to a surprising degree, particularly with regards to its narial anatomy and body size. Carpenter (2004) was the last to review Ankylosaurus anatomy in detail, but a bevy of recent ankylosaurid finds (e.g., Loewen et al 2013;Arbour et al 2014a;Arbour and Evans 2017), new insights into ankylosaur anatomy and relationships (Witmer and Ridgely 2008;Thompson et al 2012;Hill et al 2015;Leahey et al 2015;Arbour and Currie 2016), and novel information gleaned from historical specimens warrant a re-evaluation of the genus that we present here.…”