Gigantspinosaurussichuanensis

Introduction: Stegosaurs are famous for their ornate plates and spikes, but few could compete with the spikiness of Gigantspinosaurus, a moderately sized stegosaur with a name that means "Giant Spined Lizard". This creature, discovered in the late 20th Century in the Late Jurassic Shaximiao Formation of Sichuan, China, not only had plates and spikes on its back and tail like other stegosaurs, but it also had a pair of absolutely enormous spikes on its shoulders. A pair of spikes over the shoulder or hips is not unheard of in stegosaurs, but it is the sheer size of the shoulder spikes that earns Gigantspinosaurus its name. This heavy ornamentation probably would have been used for display to members of its species and for warding off predators like the giant allosaur Yangchuanosaurus.

Discovery: Gigantspinosaurus sichuanensis was preserved in early Late Jurassic (Oxfordian) rocks of the Shaximiao Formation near Zigong, Sichuan Province, China. The holotype (ZDM 0019), consisting of a partial skeleton missing the skull (but including the lower jaws), hind feet, and the end of the tail, was discovered in 1985 by Ouyang Hui, and first reported as a specimen of Tuojiangosaurus in 1986. It was described and named by Ouyang in 1992 in an abstract of a lecture. He named the genus in Latin and Greek, Latin giganteus, meaning "giant” or “enormous", and spina, "spine", and Greek sauros, meaning “lizard”, named for its enormous shoulder spines. The species name means “from Sichuan” referring to the Chinese province in which it was found. So the whole name means “Giant Spined Lizard from Sichuan”. A very fitting name. A second specimen (ZDM 0156), consisting of a pelvis, was reported in 2005.

Evolution: Gigantspinosaurus is an ornithischian (“bird hipped” dinosaur) and a member of Thyreophora, a group of quadrupedal ornithischians that generally had small heads and armor. The two main clades within Thyreophora were Stegosauria which were more ornate and dominant in the Jurassic, and Ankylosauria which were more heavily armored and dominant in the Cretaceous. Gigantspinosaurus was a member of the former. According to some authors, it is a primitive member of the group, and as such was closely related to Tuojiangosaurus and Huayangosaurus with which it shared its environment, and a more distant relative of the famous American Stegosaurus. However, according to another more recent study, Gigantspinosaurus may be more derived, closer related to Stegosaurus and Kentrosaurus than to Tuojiangosaurus and Huayangosaurus.

Physical Description and Ecology: Like other stegosaurs, Gigantspinosaurus was stout and quadrupedal, with a small head and relatively short legs, especially the forelimbs. It had a row of small plates running down its back, likely used for display to members of its species, perhaps to attract mates, and it probably also had a pair of small spikes over its hips as well as a thagomizer (the set of spikes at the end of a stegosaur’s tail) which it could have swung at predators like Sinraptor or Yangchuanosaurus. Most prominent are the pair of enormous spikes over its shoulders which almost resemble the horns of an enormous cape buffalo, each one almost a meter in length. And in life, these spikes would have had a keratin covering making them up to 20% longer. These could have been display features, but if they were, they almost certainly served a double-purpose for defense. The giant spikes would have made an excellent predator deterrent, which it would have needed, as the Shaximiao Formation was not without its predators. The largest, an allosauroid called Yangchuanosaurus could have grown to be 11 meters (36 ft) long, with a 1.11 meter (3.6 ft) long skull. But even this monster theropod might have thought twice about attacking something with spikes almost as long its head!

With the exception of its shoulder spikes, Gigantspinosaurus was not a particularly large stegosaur, measuring about 4.2 meters (14 ft) long and weighing about 700 kilograms (1,500 lb), though the bones known from Gigantspinosaurus might be from a subadult, so it is possible that it could have grown larger. Gigantspinosaurus was an herbivore, and given its stout quadrupedal stance, and short neck, it was probably a low-browser/grazer, eating plants within just a few feet of the ground, like ferns and cycads. Other herbivores in its environment included fellow stegosaurs like Toujiangosaurus, which probably had similar diets, as well as the much larger sauropods including Omeisaurus and Mammenchisaurus which probably browsed on trees, well beyond the reach of the stegosaurs.

Extinction and Legacy: Stegosaurs like Gigantspinosaurus were highly successful in the Late Jurassic, but as the Jurassic transitioned into the Cretaceous, for reasons not fully understood, but possibly due to changes in vegetation, Stegosaurs declined sharply and were usurped by their cousins, the Akylosaurs. By the end of the Early Cretaceous, Stegosaurs had all disappeared. The bones of Gigantspinosaurus, as well as a skeletal reconstruction of the full animal, can be seen at the Zigong Dinosaur Museum in Zigong, Sichuan Province, China.