Xuanhanosaurus
Xuanhanosaurus is a lesser-known theropod dinosaur that lived during the Middle Jurassic period, approximately 174 to 163 million years ago, primarily in what is now mainland China. It was a carnivorous predator, likely hunting smaller dinosaurs using its distinctive long forearms and slashing claws. Although it was not the largest or most dangerous dinosaur, Xuanhanosaurus is estimated to have reached about 6 meters (19.5 feet) in length. Its classification places it within the family Metriacanthosauridae, and it shares anatomical features with other theropods like Allosaurus, hinting at a complex evolutionary lineage.
The available fossil evidence is limited, comprising mostly fragments of its spine and forelimbs, which makes it challenging to fully reconstruct its appearance and behavior. Nevertheless, it is thought that Xuanhanosaurus exhibited significant differences from more recent theropods, particularly in its forelimb structure. This dinosaur was part of a rich ecosystem characterized by lush forests and various herbivorous species, where it likely played a vital role as a predator. Despite the scarcity of its fossils, Xuanhanosaurus continues to intrigue paleontologists and offers insights into the diversity of Middle Jurassic dinosaurs.
Xuanhanosaurus
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Saurischia
Family: Metriacanthosauridae
Genus:Xuanhanosaurus
Species:Xuanhanosaurus qilixiaensis
Introduction
Very little is known about this fierce predator, as only a few partial remains of Xuanhanosaurus have ever been found. It lived during the Middle Jurassic period (174–163 million years ago), a time characterized by high sea levels and vast areas of shallow continental seas. No polar ice caps were present, and the climate was warmer than today, assisting the spread of the temperate and tropical forests and rich river-valleys that were favored by Xuanhanosaurus and its herbivore prey.
Although it does not rank among the biggest or deadliest of dinosaurs, Xuanhanosaurus was likely a fearsome predator, hunting small prey throughout what is now mainland China. Its most distinctive feature were its very long forearms, which were much longer relative to its body size than those of other theropod species.
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.
Linnaean classification places Xuanhanosaurus under the family Metriacanthosauridae, the members of which also belong to the Saurischia (meaning “lizard-hipped”) order, which comprises two major groups: the large herbivorous sauropods and the predominately carnivorous, bipedal theropods, under which Xuanhanosaurus is classified. Although theropod dinosaurs have historically been defined as carnivorous, recent research has indicated some of these species may have had more varied, even herbivorous, diets.
A cladistic analysis places Xuanhanosaurus in the Tetanurae (meaning “stiff-tailed”) clade. Further cladistic analysis places Xuanhanosaurus into the clade Metriacanthosauridae because of its neutral spine. Previously, paleontologists had classified Xuanhanosaurusas primitive member of the Megalosauridae family.
Anatomy
It is difficult to determine how this species truly looked, as only bones from the spine and front limb have ever been uncovered. Based on these limited findings, it appears this species may have been closely related to the more-recent Allosaurus, a large meat-eating theropod from the Late Jurassic (163–145 million years ago). As such, it is likely that Xuanhanosaurus followed the derived anatomical and morphological characteristics of medium- and large-sized theropod dinosaurs, such as seen in Tyrannosaurus and Allosaurus.
Xuanhanosaurus was approximately 6 meters (19.5 ft) long and 2 meters (6.5 ft) high. Based on the anatomy of related theropod species, such as Allosaurus, it is likely that Xuanhanosaurus possessed a large skull with long, sharp teeth, a short neck, slashing claws at the ends of its forearms, large, powerful hind limbs, and a long stiffened tail. One notable difference, however, were its very long forearms and very robust humerus (forearm) bones, which are significantly reduced in more-recent theropod species such as Tyrannosaurus rex.
Such a dramatic deviation from the usual features of a theropod dinosaur indicates that Xuanhanosaurus may have exhibited pronounced anatomical differences from other Middle Jurassic theropods whose remains have been found in Asia. A possible exception is the species Szechuanosaurus zigongensis, with which it appears to share the most similarities. It has been theorized that the elongated forearms of Xuanhanosaurus indicate possible quadrupedalism, but most paleontologists believe this species was bipedal like all other theropods.
Intelligence
University of Chicago paleontologist James A. Hopson used the ratio of brain-to-body mass, known as the encephalization quotient (EQ), to estimate the intelligence of dinosaurs. Most dinosaurs fell within the 0.5 to 2.0 range on a scale that extends to 8.0, the approximate value calculated for the human brain. This places most dinosaurs well below the EQ ratings of many modern mammals and birds.
Most carnivorous dinosaur species were more intelligent than their herbivorous cousins, as a larger brain was necessary to perform the more complex behavior of hunting prey. Xuanhanosaurus ranks highly compared to other dinosaurs, with an EQ falling somewhere between 1.0 and 1.9. Though this is low compared to most modern animal species, new techniques in the study of intelligence have suggested that dinosaurs may have been smarter than originally thought.
Reproduction and Population
Theropod species were widely distributed, and their remains have been found on almost all continents. Based on the very limited recovery of its fossils, however, it is impossible to estimate with any degree of accuracy the possible population and distribution of Xuanhanosaurus beyond Asia, where its only fossils have been found.
Currently, most dinosaur species are thought to have been oviparous, including the theropods and Xuanhanosaurus, possibly laying several eggs per clutch. Egg-laying methods appear to differ among species, with some dinosaurs laying eggs in nests, some burying them, and some possibly incubating them. Post-hatching parental care behavior is considered a possibility for some species, but evidence remains inconclusive for Xuanhanosaurus.
Diet
The majority of theropods, including Xuanhanosaurus, were carnivorous (meat-eating) species. Xuanhanosaurus would have hunted smaller-sized dinosaurs, very likely using its unusually long arms and slashing claws to catch and hold prey. Although no evidence currently exists regarding what animals it consumed, smaller or similarly sized dinosaurs that lived in the same region, possibly the medium-sized stegosaur Huayangosaurus or small herbivore Xiaosaurus, are likely.
Xuanhanosaurus may have used its teeth to “grip-and-rip” prey. Known as the “puncture-pull” hypothesis, this feeding method required the dinosaur's skull to withstand a tremendous bite force. Some theropod species show evidence that physical characteristics such as open skull sutures and thickened nasal bones acted like “shock-absorbers” during feeding. Until a skull is uncovered for Xuanhanosaurus, however, it is not known if the dinosaur possessed similar modifications for its carnivorous, bone-crunching diet.
Behavior
Very few Xuanhanosaurus remains have been discovered. Given this lack of fossil evidence, little is understood regarding its behavior. To date, no trackways, group sites, or nest sites have been unearthed for this species. This makes it difficult for paleontologists to determine whether the species exhibited social interaction (outside of reproductive behavior), exhibited group or solitary living and hunting behavior, or provided offspring with parental care.
Although there were bigger and faster dinosaurs that lived during the same time and habitat, its predator lifestyle necessitated that Xuanhanosaurus outrun (or outthink) its preferred herbivore prey.
Habitat and Other Life Forms
The Middle Jurassic period was distinguished by a warm and tropical climate. The northern and southern polar ice caps were absent, which meant higher sea levels and hotter temperatures than seen today. An abundance of lush green gymnosperms such as ferns, cycads, mosses, and large tree species such as Ginkgoes and conifers covered the landscape, though flowering plants (angiosperms) would not evolve for another 25 million years.
At the beginning of the Jurassic, all landmasses were joined together in the supercontinent Pangaea. By the Middle Jurassic, however, Pangaea had already commenced its separation into the continents of Laurasia and Gondwana, resulting in an increase of large flooded areas and distribution of temperate and tropical forests throughout the area that is now Asia, a favorable habitat for Xuanhanosaurus and many of its prey species.
Xuanhanosaurus shared the lush landscape with many herbivore species, including the stegosaur Huayangosaurus and the sauropods Protognathosaurus, Shunosaurus, Datousaurus, and Omeisaurus. It would also have hunted alongside carnivorous species such as Gasosaurus, a large tetanuran dinosaur whose remains have been found in the same Lower Shaximiao Formation as Xuanhanosaurus.
Research
Named after Xuanhan County, where its fossils were first found, Xuanhanosaurus qilixiaensis was discovered in 1984 by Dong Zhiming, a paleontologist from Beijing's Institute of Vertebrate Paleontology and Paleoanthropology. It has only been found in the Lower Shaximiao Formation of Sichuan, China, and its remains (despite being partial) were different enough from the other Middle Jurassic theropods to warrant the creation of a new genus.
Although research continues in the area where this species was located, no further remains have been found. The incompleteness of the recovered specimens, despite an impressively well-preserved forearm, has provided only limited understanding of this species.
Bibliography
Carpenter, Kenneth. Eggs, Nests, and Baby Dinosaurs: A Look at Dinosaur Reproduction. Indiana UP, 1999.
"Department of Paleobiology." National Museum of Natural History, Smithsonian, naturalhistory.si.edu/research/paleobiology. Accessed Apr. 2010.
"Dinobase." University of Bristol, dinobase.gly.bris.ac.uk. Accessed Apr. 2010.
Fastovsky, David E., and David B. Weishampel. Dinosaurs: A Concise Natural History. Cambridge UP, 2009.
Fastovsky David E., and David B. Weishampel. Evolution and Extinction of the Dinosaurs. Cambridge UP, 2005.
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.” American Naturalist, vol. 103, 1969, pp. 575–88.
Lucas, Spencer G. Dinosaurs: The Textbook. 4th ed., McGraw-Hill, 2004.
Martin, Anthony J. Introduction to the Study of Dinosaurs. 2nd ed., Blackwell, 2006.
"Paleoportal." UC Museum of Paleontology, University of California, Berkeley, ucmp.berkeley.edu. Accessed Apr. 2010.
Rauhut, Oliver W. M. Special Papers in Palaeontology, The Interrelationships and Evolution of Basal Theropod Dinosaurs. Wiley, 2003.
Parker, Steve. Dinosaurus: The Complete Guide to Dinosaurs. Firefly Books, 2009.
"Xuanhanosaurus." Fossilworks, fossilworks.org. Accessed 3 June 2020.
Weishampel, David B., et al. Dinosauria. University of California Press, 2007.
Willis, K. J., and J. C. McElwain. Evolution of Plants. Oxford UP, 2002.