Saichania

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Saichania
Temporal range: Campanian
Saichania chulsanensis.jpg
Cast of holotype skull GI SPS 100/151
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Order: Ornithischia
Family: Ankylosauridae
Subfamily: Ankylosaurinae
Genus: Saichania
Maryańska, 1977
Species: † S. chulsanensis
Binomial name
Saichania chulsanensis
Maryańska, 1977

Saichania (Mongolian meaning "beautiful one") is a genus of herbivorous ankylosaurid dinosaur from the Late Cretaceous period of Mongolia and China.

History of discovery[edit]

Skull PIN 3142/250 formerly referred to Tarchia

In 1970 and 1971 a Polish-Mongolian expedition found ankylosaurian fossils in the Gobi Desert near Chulsan, or Khulsan.

The type species Saichania chulsanensis was named and described by the Polish palaeontologist Teresa Maryańska in 1977, along with the related species Tarchia kielanae.[1] The generic name mean "the beautiful one" in Mongolian, referring to the pristine state of preservation of the type specimen. The specific name refers to the provenance near Chulsa.

The holotype of Saichania chulsanensis, specimen GI SPS 100/151, was found in a layer of the Barun Goyot Formation, dating from the late Campanian, about seventy-three million years old. It consists of a skull and the anterior part of the postcranial skeleton: seven neck vertebrae, ten back vertebrae, the left shoulder girdle, the left forelimb, the two cervical halfrings and extensive armour in life position. The holotype is largely articulated. Referred specimens include ZPAL MgD-I/114 consisting of an undescribed fragmentary skull roof and associated armour, and an undescribed, almost complete skeleton with skull, specimen PIN 3142/251.

Later, also the juvenile specimen MPC 100/1305 was referred and extensively described in 2011, seeming for the first time to provide complete information on the postcranial skeleton.[2] However, in 2014 Victoria Megan Arbour concluded that the describers had been misled by the skeleton having been completed with a skull cast of GI SPS 100/151, and that the remainder of the fossil belonged to some other ankylosaur, possibly Pinacosaurus. On the other hand, Arbour added to the number of possible Saichania specimens by referring PIN 3142/250, a skull previously seen as a Tarchia exemplar. This would imply that Saichania, formerly thought to occur solely in the Barun Goyot Formation at Khulsan, is also known from the Nemegt Formation at Khermeen Tsav. Saichania would then be the only ankylosaur definitely known from the Nemegt, its occurrence thus spanning the time of the Campanian–Maastrichtian transition, and early Maastrichtian (Nemegtian) period. Arbour also considered the Chinese taxa Tianzhenosaurus youngi Pang & Cheng 1998 and Shanxia tianzhenensis Barrett, You, Upchurch & Burton 1998 to be junior synonyms of Saichania.[3]

Description[edit]

Front view of PIN 3142/250

Saichania was a large ankylosaurid. Maryańska estimated its length at seven meters.[1] Other estimates roughly confirmed this, stating a maximum length of about 6.6 metres (22 ft) long.[4] However, Gregory S. Paul in 2010 gave a lower estimate of 5.2 metres, with a weight of two tonnes.[5] 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]

Left side view of PIN 3142/250

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.

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 extensive secondary hard palate was present. The air passages may have allowed the animal to cool the air that it breathed, and to eat tough plants, suggesting 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. The armour on the top of its head and along its back and flanks were studded with large spikes, and it plausibly had a club-shaped tail.[6]

Classification[edit]

Maryańska classified Saichania as an ankylosaurid related to Pinacosaurus and observed that these two dinosaurs differ from all others in the structure of their nasal cavities. Maryańska provided a differential diagnosis that showed that the two genera were distinct based on morphological differences observed in the bones of the skull and braincase.

Saichania skull cast mounted on the skeleton of Pinacosaurus in Mongolia

The following cladogram is based on the phylogenetic analysis of the Ankylosaurinae conducted by Arbour and Currie (2013):[7]

Ankylosaurinae

Scolosaurus (=Oohkotokia horneri)





Ankylosaurus




Anodontosaurus



Euoplocephalus







Dyoplosaurus



Talarurus



Tsagantegia





Minotaurasaurus



Nodocephalosaurus



Tianzhenosaurus





Pinacosaurus grangeri



Pinacosaurus mephistocephalus





Tarchia



Saichania










References[edit]

  1. ^ a b c d e Maryańska, T. (1977). "Ankylosauridae (Dinosauria) from Mongolia". Palaeontologia Polonica 37: 85–151. 
  2. ^ Carpenter, K., Hayashi, S., Kobayashi, Y., Maryańska, T., Barsbold, R., Sato, K., and Obata, I., 2011, "Saichania chulsanensis (Ornithischia, Ankylosauridae) from the Upper Cretaceous of Mongolia", Palaeontographica, Abteilung A, 294(1-3): 1-61
  3. ^ a b c d Arbour, Victoria Megan, 2014. Systematics, evolution, and biogeography of the ankylosaurid dinosaurs. Ph.D thesis, University of Alberta
  4. ^ Seebacher, F. (2001). "A new method to calculate allometric length–mass relationships of dinosaurs." Journal of Vertebrate Paleontology, 21(1): 51–60.[1]
  5. ^ Paul, G.S., 2010, The Princeton Field Guide to Dinosaurs, Princeton University Press p. 231
  6. ^ Palmer, D., ed. (1999). The Marshall Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals. London: Marshall Editions. p. 161. ISBN 1-84028-152-9. 
  7. ^ Arbour V.M. and Currie P.J., 2013, "Euoplocephalus tutus and the Diversity of Ankylosaurid Dinosaurs in the Late Cretaceous of Alberta, Canada, and Montana, USA", PLoS ONE 8(5): e62421. doi:10.1371/journal.pone.0062421

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