Yangchuanosaurus

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order:Saurischia

Family: Metriacanthosauridae

Genus: Yangchuanosaurus

Species: Yangchuanosaurus shangyouensis

Introduction

Living about 155 million years ago in Jurassic China,Yangchuanosaurus was the largest and most fearsome dinosaurian predator in its ecosystem, filling the same ecological niche as its close American relative, Allosaurus. Like other carnivorous species, Yangchuanosaurus hunted the many sauropod and stegosaur species that lived in the prehistoric forests of China, such as the back-plated Tuojiangosaurus multispinus, Chialingosaurus kuani, and Chungkingosaurus jiangbeiensis.

The supercontinent Pangaea drifted apart during the Jurassic period, separating into Gondwana and Laurasia and creating the narrow Central Atlantic Ocean. By the Late Jurassic period (163–145 million years ago), sea levels were much higher than during the Triassic (251–201 million years ago) and had flooded vast areas to form shallow continental seas. No polar ice caps were present and the warm climate helped spread the temperate and tropical forests and rich river-valleys of China that were populated by Yangchuanosaurus and its prey.

Classification

Paleontologists use two major systems for classifying organisms. Linnaean taxonomy uses overall physical similarity to place organisms into different groups. By contrast, cladistics seeks to create groups of organisms based on evolutionary relationships; these groups are called “clades.” Cladistic analysis focuses on important key traits and attempts to trace the inheritance of a trait among descendants of a common ancestor.

Dinosaur species were first divided into two distinct groups by paleontologist Harry Seeley: Ornithischia, or bird-hipped dinosaurs, and Saurischia, or lizard-hipped dinosaurs. Although it may seem illogical, it is from the lizard-hipped dinosaurs that today's avian dinosaurs (birds) evolved. This is an interesting example of convergent evolution in which the backward pointing pubis seen in ornithischians and modern day birds actually evolved separately. However, by the first decades of the twenty-first century, some scientists had begun challenging this classification division.

Under the Linnaean classification system, Yangchuanosaurus belongs to the Saurischia order, which is comprised of two major groups—the large herbivorous sauropods and the predominately carnivorous theropods. Yangchuanosaurus is also part of the Theropoda suborder, a group of bipedal, generally carnivorous dinosaurs united by developments of the skull, teeth, and a few other skeletal features. The bipedal theropods are very diverse and among the oldest known dinosaurs, first appearing during the Late Triassic period about 230 million years ago. (Although theropod dinosaurs have historically been defined as carnivorous, further research has indicated some of these species may have had more varied, even herbivorous, diets.)

Using a cladistic analysis, Yangchuanosaurus falls under the Theropoda (“beast-footed”) clade, as it is bipedal. Theropods are an extremely diverse group of dinosaurs that first appeared during the late Triassic period about 230 million years ago. The largest and most advanced theropod clade, Tetanurae, first appeared in the fossil record during the Early or Middle Jurassic period (201–163 million years ago). Tetanurae is composed of the Megalosauroidea (sometimes also referred to as Spinosauroidea), Carnosauria, which includes Yangchuanosaurus, and Coelurosauria, which includes Tyrannosaurus.

Carnosaurs include the allosauroids (comprised of four families: Metriacanthosauridae (also sometimes called Sinraptoridae), Allosauridae, Carcharodontosauridae, and Neovenatoridae), which are generally characterized by their large heads and ornamental skull-crests, the most well-known of which is Allosaurus, a close North American relative of the Asian Yangchuanosaurus.

The Yangchuanosaurus genus was first established by Dong Zhiming in 1977. Historically, classification stated there were three separate Yangchuanosaurus species: Y. shangyouensis, Y. magnus, and Y. hepingensis. Upon further analysis, Y. hepingensis was typically reassigned as Sinraptor hepingensis. It was additionally argued in a 2012 paper that previously recovered material should belong to Yangchuanosaurus as another species, Y. zigongensis.

Anatomy

Although it was slightly smaller in stature, Yangchuanosaurus and its more famous relative Allosaurus were very similar in body structure and morphology. Both possessed a large head, short neck, long muscular hind legs, and massive tail. It is likely, in fact, that these two species occupied the same ecological and predator niche on their separate homelands of Asia and North America.

Yangchuanosaurus was the largest Asian predator during the Late Jurassic, possibly weighing upwards of 3,000 kilograms (6,600 lb) depending on the species. Like all lizard-hipped saurischian dinosaurs, Yangchuanosaurus possessed a backward-facing pubis bone and very large and long muscular hind legs, which ended with three-clawed toes on each foot. Its forearms were significantly shorter and each hand possessed three sharply clawed fingers. As is seen in almost all theropods, its greatly reduced forearms meant that Yangchuanosaurus was a bipedal dinosaur, with its long tail, almost half its 9-meter (29.5-ft) body, providing support and balance.

Yangchuanosaurus had a large and ridged skull, measuring more than 80 centimeters (31.5 in) long and 50 centimeters (20 in) high, with a knob-like structure on the end of its nose. Anatomical adaptations such as the deep sinuses found in many of its skull bones meant that Yangchuanosaurus's head was light and hollow, which greatly improved its ability to smell, breathe, and hunt efficiently. It also possessed large, serrated, dagger-like teeth, which it used to feed on other dinosaurs.

Intelligence

The most often used technique to measure and compare intelligence in animal species is the encephalization quotient (EQ), calculated as a body size to brain size ratio. With the exception of the troodontids (EQ of 5.8) and the dromaeosaurids (EQ of 5.7), all species of dinosaur fall below an EQ of 2.0. (In comparison, humans have an EQ of 7.44 and dolphins measure a 5.31.)

Carnivorousness in dinosaurs usually indicates higher intelligence, and the majority of theropod carnivores were more intelligent than their herbivorous relatives. This is required to perform more complex behavior such as hunting. With an EQ between 1.0 and 1.9, Yangchuanosaurus ranks highly on the dinosaur intelligence scale. Although this ranking is low compared to modern-day animals, recent neurological research has indicated that at least some dinosaurs may be of higher intelligence than indicated by their current EQ levels.

Reproduction and Population

Allosaurid species were a very successful group of dinosaurs that appeared to exceed other carnivores in number, including the megalosaurs and ceratosaurs species present during the Late Jurassic. Allosaurs were widely distributed across many geographical regions during the Jurassic and Cretaceous periods, including Asia, North America, Europe, and Africa.

Most dinosaurs are thought to be oviparous, including Yangchuanosaurus, laying up to several eggs in a clutch. Egg-laying methods appeared to differ, however, ranging from laying eggs in nests, burying eggs underground, and perhaps even incubating eggs by brooding. Post hatching parental care behavior is considered possible, even probable, for some dinosaurs, but evidence remains inconclusive for many species.

Diet

Yangchuanosaurus was the largest carnivore found in Late Jurassic Asia, and was a formidable predator with a large jaw, long curved teeth, and sharp slashing claws. It obtained energy to power its large body by consuming other dinosaur species, particularly herbivore species such as the medium-sized plate-backed stegosaurs and the long-necked sauropods. Fossil evidence has even indicated that Yangchuanosaurus may have been strong enough to hunt sauropods as large as the impressive Mamenchisaurus species.

Anatomical adaptations, particularly the deep sinus spaces found in many of its skull bones, meant that Yangchuanosaurus's head was light and hollow. This significantly improved its olfactory senses and its ability to breathe efficiently. Smelling prey from long distances, as well the ability to run much faster and further, greatly improved its hunting capacities and perhaps made it even more of a threat than its American contemporary, Allosaurus. Yangchuanosaurus's forward-facing eyes also provided excellent sight with which to hunt, and its long tail helped maintain balance during the chase.

As with later predators, such as Tyrannosaurus, there is some debate as to whether Yangchuanosaurus was an active hunting predator or an opportunistic carrion (animal carcass) scavenger. However, the majority of paleontologists suggest that it was probably both—hunting when required and scavenging if the opportunity presented itself.

Behavior

The discovery of well-preserved skeletons and partial remains of Yangchuanosaurus has provided paleontologists with the opportunity to describe the morphology and genus of this species in detail and has helped highlight the possible behavior and probable living conditions of this carnosaur species.

It is considered likely that Yangchuanosaurus would have exhibited similar behavior to its close relative Allosaurus, prowling and hunting in the lush Asian forests its prey inhabited. Many paleontologists believe that allosaurid species, such as Yangchuanosaurus, may even have hunted in packs.

Fossil evidence of a healed wound in a Yangchuanosaurus scapula bone provides evidence of conflict behavior. Chinese scientists believe that a species with a club-like tail, such as Mamenchisaurus hochuanensis, may have inflicted such wounds. This paints a picture of conflict between carnivorous predator, Yangchuanosaurus, and large sauropod herbivore, Mamenchisaurus. It indicates probable parental protection or defensive behavior from Mamenchisaurus and suggests that Yangchuanosaurus was large or strong enough to hunt this species.

Habitat and Other Life Forms

The Jurassic was a period generally characterized by much higher sea levels, higher atmospheric CO₂ concentrations, and warmer temperatures than seen today. The landscape was covered with low-lying gymnosperms such as ferns, horsetails, cycads, and mosses, and tall trees such as Ginkgoes and conifers. Angiosperms did not appear until 140 million years ago during the start of the Cretaceous period and the landscape was absent of grass, which did not evolve until very late in the Cretaceous period (and possibly later).

Asia was home to a diverse collection of dinosaur species during the Late Jurassic period. Yangchuanosaurus inhabited the temperate and tropical forests and rich river-valleys of what is now central China, preying on the herbivores that fed on the lush green vegetation prevalent at the time. They lived alongside many other Asian dinosaurs, including the herbivorous and vastly long-necked sauropods Omeisaurus and Mamenchisaurus, back-plated dinosaurs such as Tuojiangosaurus multispinus, Chialingosaurus kuani, and Chungkingosaurus jiangbeiensis, and the tiny herbivore Xiaosaurus.

Research

In 1977, an almost complete dinosaur skeleton was unearthed during the construction of a new dam in Yangchuan County of the Sichuan Province. The skeleton was the most complete specimen found in China, lacking only its forelimbs and parts of its backbone, and was collected from the dark-red mudstones of the Upper Shaximiao Formation of Sichuan. The remains were examined and named Yangchuanosaurus shangyouensis in 1978 by Dong Zhiming, a paleontologist from Beijing's Institute of Vertebrate Paleontology and Paleoanthropology.

In 1983, a second, larger species was described and named Yangchuanosaurus magnus by Dong. Also recovered from the Upper Shaximiao Formation of Sichuan, this species shared many characteristics with the original Yangchuanosaurus species but was larger in size. What was at the time considered a third and smallest species was discovered in 1985 by local farmers in Heping, Sichuan. The remains were excavated from the red mudstones of the Upper Shaximiao Formation and were described and named Yangchuanosaurus hepingensis in 1992 by Gao Yuhui from the Zigong Dinosaur Museum.

Although species located in the Upper Shaximiao Formation, such as Yangchuanosaurus, have been traditionally classified as living during the Late Jurassic, invertebrate and plant fossil evidence has suggested that the Upper Shaximiao Formation might be classified as Late-Middle Jurassic in age. If this is the case, Yangchuanosaurus may actually be older than has traditionally been accepted. Interest in the dinosaur continued into the first decades of the twenty-first century, with a 2017 Orlando, Florida, exhibit featuring a Yangchuanosaurus skeleton as part of its effort to educate the public, particularly children, about dinosaurs.

Bibliography

Brusatte, Steve. Field Guide to Dinosaurs. Quercus Books, 2009.

Carlton, Robert L. A Concise Dictionary of Paleontology. 2nd ed., Springer, 2019.

Carpenter, Kenneth, editor. The Carnivorous Dinosaurs. Indiana UP, 2005.

Carpenter, Kenneth. Eggs, Nests, and Baby Dinosaurs. Indiana UP, 1999.

Carrano, Matthew T., et al. "The Phylogeny of Tetanurae (Dinosauria: Theropoda)." Journal of Systematic Palaeontology, vol. 10, no. 2, 2012, pp. 211–300.

Fastovsky, David E., and David B. Weishampel. Dinosaurs: A Concise Natural History. 3rd ed., Cambridge UP, 2016.

Fastovsky, David E., and David B. Weishampel. Evolution and Extinction of the Dinosaurs. 2nd ed., Cambridge UP, 2007.

Hopson, James A. “Relative Brain Size and Behavior in Archosaurian Reptiles.” Annual Review of Ecology and Systematics, vol. 8, 1977, pp. 429–48.

Jerison, H. J. “Brain Evolution and Dinosaur Brains.” The American Naturalist, vol. 103, no. 934, 1969, pp. 575–88.

Lucas, Spencer G. Dinosaurs: The Textbook. 6th ed., Columbia UP, 2016.

Martin, Anthony J. Introduction to the Study of Dinosaurs. 2nd ed., Blackwell, 2006.

Norell, Mark, et al. Discovering Dinosaurs: Evolution, Extinction, and the Lessons of Prehistory. Updated and revised ed., U of California P, 2000.

Parker, Steve. Dinosaurus: The Complete Guide to Dinosaurs. 2nd ed., Firefly Books, 2016.

Tuttle, Brittani. "Extreme Dinosaurs: The Exhibition Now Open on International Drive." Attractions Magazine, 12 Oct. 2017, attractionsmagazine.com/extreme-dinosaurs-international-drive/. Accessed 22 May 2020.

UC Museum of Paleontology, University of California, Berkeley, 2020, www.ucmp.berkeley.edu/. Accessed 22 May 2020.

Weishampel, David B., et al., editors. The Dinosauria. 2nd ed., U of California P, 2004.

Willis, K. J., and J. C. McElwain. The Evolution of Plants. 2nd ed., Oxford UP, 2014.

"Yangchuanosaurus." Natural History Museum, www.nhm.ac.uk/discover/dino-directory/yangchuanosaurus.html. Accessed 22 May 2020.

"Yangchuanosaurus shangyouensis." Australian Museum, 11 Jan. 2018, australianmuseum.net.au/learn/dinosaurs/fact-sheets/yangchuanosaurus-shangyouensis/. Accessed 22 May 2020.