Saichania

Size comparison of GI SPS 100/151 and MPC-D 100/1305

Saichania was a medium-sized ankylosaur, measuring 5–7 metres (16–23 ft) in length and 1.4–2.0 metric tons (1.5–2.2 short tons) in body mass.[5][6][1] Finds of tail clubs of gigantic individuals suggest larger sizes but their reference to Saichania cannot be substantiated as the holotype, the only specimen sufficiently described, only consists of the front of the animal.[3]

Saichania shared the general ankylosaurid build, being a low-slung, broad, heavily armoured dinosaur, with short forelimbs. Even for an ankylosaurid however, Saichania is exceptionally robust, its rump strengthened by ossifications and fusions of the vertebral column, ribs, shoulder girdle and breast bones.[1]

Arbour in 2014 established a revised list of distinguishing traits. The osteoderms on the skull are bulbous. The first and second neck vertebrae are fused into a single element, a syncervical. The upper side of the humerus is very broad, equalling 70% of the total length of the bone. The rib shafts are expanded by intercostal ossifications, the cartilage connecting the ribs having been turned into bone sheets. The cervical halfrings, protecting the neck, have each an underlying continuous band of bone and the borders between the segments of these rings are covered by extra armour plates entirely hiding these connections from view.[3]

The skull of Saichania is broad, 455 millimetres long and 480 millimetres wide with the holotype.[1] The top of the snout is covered with strongly convex osteoderms. These armour tiles on the snout comprise a central large caputegula. A large "loreal" osteoderm covers much of the top edge and the side of the snout. The caputegula on the prefrontal is of moderate size and not strongly protruding sideways. The osteoderms on the upper eye socket rim are continuous, not forming two peaks. An extra osteoderm on the rear supraorbital, as in Tarchia, is lacking. The pyramid-shaped squamosal horns on the rear skull corners are broad, not narrow as with Tarchia. These horns have a uniform surface texture, not a division into a smooth and rough surface as in Zaraapelta.[3] On the cheek, large triangular quadratojugal horns are present.

Saichania skull cast mounted on the skeleton of cf. Pinacosaurus in Mongolia

The skull had very complex air passages. The main entrance of each external nostril consisted of a roomy "nasal vestibule". In each vestibule again two smaller entrances were present, vertically arranged. The lower hole allowed air to enter the hollow inside of the bone core of the beak. This premaxillary sinus had a little recess at the top, connected by a nerve channel to the mouth. Maryańska presumed this recess housed a Jacobson's organ, a secondary smelling organ. The main room of the premaxillary sinus was connected to behind with a sinus in the maxilla, which itself was partly divided in two by a transverse bone wall or septum. The nasal cavity was large, situated directly below the snout roof. It was divided into a left and right side by a thick vertical bone wall. It was also horizontally divided in two by high internal wings of the praemaxillae and the upper side of a crista maxilloturbinalis. This latter was a scroll-like structure, a turbinate bone serving with warm-blooded animals to condense and preserve exhaled moisture. Normally, in dinosaurs these turbinates are not ossified. Together with a crista nasoturbinalis, the crista maxilloturbinalis filled the lower half of the nasal cavity. Maryańska presumed it was connected with the underlying premaxillary sinus, allowing the animal to exhale air through the lower hole of the nasal vestibule. The upper half of the nasal cavity was the main respiratory tract, allowing air to enter via the upper hole of the nasal vestibule.[1] An unusually strongly ossified hard palate was present. The air passages may have allowed the animal to cool the air that it breathed and limit water loss. The hard palate allowed it to eat tough plants. All this suggested that it lived in a hot, arid, environment. There is even some evidence that the animal may have possessed a salt gland next to its nostrils, which would have further aided it in a desert habitat.[7]

Skeleton mount of cf. Pinacosaurus with Saichania skull cast.

The teeth were small and leaf-shaped. There are twenty-two of them in each maxilla, seventeen in the right and sixteen in the left lower jaw of the holotype. On the rear skull, the oval occipital condyle is obliquely pointing to below, indicating that the entire head was appending. A large hyoid bone apparatus was found, in 1977 the most complete discovered for any dinosaur. It is V-shaped with the central parts representing the basihyal and basibranchial, and the branches being the ceratobranchialia.[1] This bone apparently supported a long tongue.

The front skeleton shows some exceptional ossifications and fusions. The front neck vertebrae, the atlas and axis, are grown together. The cervical vertebrae have very long joint processes, zygapophyses, showing that thick intervertebral discs must have been present and that the neck was longer and more flexible than is often assumed. The short rib and the diapophysis of the first dorsal vertebra are fused with the coracoid, immobilising the entire shoulder girdle relative to the vertebral column. The coracoids are small but sharply curving to the inside below, almost meeting each other. From the fifth rib onwards, the rib shafts have intercostal plates on their rear edges, ossified cartilage sheets, overlapping the front edge of the next rib. The intercostal plate is positioned in a relatively high position in the fifth rib; more to the rear of the series it gradually descends towards the lower belly. These ribs also articulate at their lower ends with the breast bones, a condition which is rare in the Ornithischia. The breast bones are fully ossified and connect to form a sternal plate that is split in front and broadly forked at the rear.[1]

The humerus is very robust. Thirty centimetres long in the holotype, it has an upper side width of 212 millimetres due to a well-developed inner corner and a strong hatchet-shaped deltopectoral crest. The ulna, twenty-one centimetres in length, also is robust but has a relatively low olecranon. The metacarpus is short, in 1977 it was the shortest of any Asian ankylosaur known. The metacarpals were positioned vertically, closely connected into an arch. Below the first and second metacarpal small disc-shaped sesamoid bones were found.[1]

The holotype preserves the front body armour in articulation. The neck is protected by two cervical halfrings, each made of six rectangular segments positioned next to each other: two at the top, two at the upper sides and two at the lower sides. Each segment has a keel parallel to the long axis of the body. The keel of the lower side segments is the largest. The segments are connected to an underlying continuous band of bone, mainly by a broad fusion at the front edge, but also by a narrow strip at the rear. The seams between the segments are covered by a rectangular zone of small oval osteoderms. Between the upper and lower side segment a larger central osteoderm is present, forming a rosette. The front halfring is smaller than the rear one.[1]

A central row of symmetrical conical osteoderms is positioned on the back. On both sides of this median series, a parallel row of large thin osteoderms is present, featuring moderately high keels, their apexes pointing to behind. The vertical sides of the rump are covered by three rows of conical osteoderms: the upper rim is equipped with large plates and apexes pointing to the rear; at the middle side a similar row is present of even larger plates; the lower edge has a row of smaller plates, their keels to the contrary directed to the front. In general, the keels are sharp and narrower plates have higher and more asymmetrically placed keels. Some osteoderms have the shape of pure cones. The larger osteoderms are also ordered in transverse rows but are not fused into bands; small ossicles connect the larger elements. On the underside of the breast, osteoderms are present also.[1]