Zigongosaurus

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Saurischia

Family: Mamenchisauridae

Genus:Zigongosaurus

Species:Zigongosaurus fuxiensis

Introduction

Zigongosaurus is both a controversial and poorly known dinosaur, as paleontologists are divided over its exact classification. It is known to be a sauropod dinosaur with the characteristic long neck and tail, small head, and the elephantine, tree-trunk-like legs of other sauropods.

The fossilized remains of Zigongosaurus were found in geological strata that indicate it lived more than 150 million years ago during the Middle Jurassic period. The name Zigongosaurus is taken from the region of China in which it was originally discovered, which has contributed to confusion regarding its classification. Few remains of the creature have been found, and the skeletal fragments that have been located and excavated by paleontologists do not seem to have characteristics that unequivocally separate Zigongosaurus from other sauropods that have been unearthed in China.

The geographical range over which identifiable fossil remains of a particular creature are found often provides important clues that can be related directly to the geographical range of the territory inhabited by those animals when they were alive. Often, what researchers know about the movements of the continents (plate tectonics) and what they glean from fossil sites inform one another. For example, fossils found in both South America and Africa have been identified as being of the same creature. The matching shapes of the east and west coasts of the two continents, as well as data from the fields of plate tectonics and fossil identification, offer a convincing argument that the two regions were at one time part of the same land mass in which those creatures roamed freely. Conversely, with the current notions of where land masses were located roughly 150 million years ago, at the time when Zigongosaurus is known to have been roaming the earth, scientists surmise other locations at which fossil remains of Zigongosaurus may be found. That such remains have not yet been found presents something of a mystery to them.

Classification

As a member of the suborder Sauropodomorpha, Zigongosaurus was a quadrupedal herbivore having the long neck and tail typical of other sauropods. It is known to have lived during the Middle Jurassic period, a bit earlier than two very similar sauropods identified as Mamenchisaurus jingyanensis and Omeisaurus junghsiensis. Zigongosaurus fossil remains seem to possess traits and characteristics that are identifiable with each of these other two sauropods. However, uncertainty in classification of Zigongosaurus is due to a lack of definitive fossil remains.

Some paleontologists have concluded that Omeisaurus and Zigongosaurus are in fact one and the same and so should not be differentiated by name. Others disagree with this assessment and continue to regard Omeisaurus and Zigongosaurus as distinct species. Still others are of the mind that Zigongosaurus and Mamenchisaurus are the same, with Omeisaurus being either different or the same as well. Thus, five schools of thought currently exist regarding the classification of these three sauropods (all different, all the same, and three combinations of any two of them being the same), but the idea that Zigongosaurus, Omeisaurus, and Mamenchisaurus are the same species is growing in favor.

Despite their general similarities, the Sauropodomorpha vary greatly in their respective anatomies. This has resulted in the overlapping descriptions of several families of sauropods, from which only a small number stand out. In the case of Zigongosaurus, the remains that have been found—which include a nearly complete skeleton found in situ—have incompletely bifurcated neural spines but with simple chevrons, suggesting that it may belong rather to the camarasaurid (“chambered lizard”—referring to hollow chambers in the vertebrae) or opisthocoelicaudid (“rear cavity tail”—referring to the tail vertebrae) family instead of the cetiosaurids.

Anatomy

Sauropodomorpha, like Zigongosaurus, share a common characteristic anatomical form consisting of an elephantine body supported by four pillar-like legs, a greatly extended neck, and a counterbalancing tail. They also have the saurischian hip structure, with the legs positioned vertically beneath the body to provide the most effective support of its massive weight.

The neck structure of Zigongosaurus consists of only nineteen vertebrae, each augmented with rod-like extensions for extra support. This contributes to controversy over the manner in which Zigongosaurus carried the neck and head that made up nearly half of its total body length.

One school of thought holds that the reinforced structure and positioning of the shoulder blades mitigate against the animal having been able to raise up its neck. Zigongosaurus could not, therefore, have used its neck to graze the upper growth of trees. If the dinosaur's neck extended horizontally, like its tail, this position would have allowed it to graze by reaching into thick undergrowth from the edges of forests, although doing so would have allowed extreme vulnerability to attacks by predators of all kinds.

Another school of thought holds that the reinforcing, rod-like structures of the neck vertebrae had evolved to better support the neck in a nearly vertical position, allowing the creature to graze on fresh shoots and plant growth some 13 to 16 meters (40 to 50 feet) above the ground. Or perhaps Zigongosaurus used both stances, raising and lowering its neck like the boom of a construction crane. Its extreme neck length suggests that the vessels carrying blood to and from the head would have been extremely strong to accommodate the high blood pressures generated by raising and lowering its head.

A co-eval Chinese sauropod, the Shunosaurus, is the first sauropod that has been identified as having a tail club, similar to that of the Ankylosaurus. Some research suggests that Zigongosaurus may also have carried such a device.

Intelligence

Paleontologists and physiologists use the encephalization quotient (EQ), a relationship between the average mass of an animal's brain relative to that of other animals of similar overall body mass, to estimate its probable relative intelligence level. They achieve this with fossils, by using castings of the brain case when skulls are available, and estimates of the overall mass of the animal. The sauropods are characterized by very small heads atop very large, massive bodies. Accordingly, the brain volume of Zigongosaurus is miniscule in comparison to the mass of its body, producing an EQ of approximately 0.05. This value is common to all of the sauropods and is taken to indicate that they probably did little else than browse all day.

Reproduction and Population

The possibility of any reasonable estimate of the reproductive habits and population of Zigongosaurus is tied to the uncertainty of its identification in the fossil record. Even allowing all known Omeisaurus, Mamenchisaurus, and Zigongosaurus fossils found to date to be one and the same species, their general scarcity mitigates against such estimates; and because this equivalence is by no means certain, any population estimates and estimates of the creature's geographical range are equally uncertain.

Paleontologists surmise that Zigongosaurus relied on the same reproductive activities as other sauropods and thus were likely to have been oviparous rather than viviparous. In fact, fossil eggs have, been found in a linear distribution pattern rather than a circular or nested pattern, which seems to indicate that the sauropods deposited their eggs in trenches. Some researchers have suggested that this distribution is the result of sauropods depositing their eggs as they walked, though this would not have provided any protection for the eggs afterwards, and undoubtedly predators would have consumed them quickly. In the absence of fossil evidence, it is not possible to know with any degree of certainty what were the actual mating habits and reproductive means of the animal.

Diet

Zigongosaurus, like all other sauropods, was an herbivore. Given its size and dentition, it is likely that Zigongosaurus ate almost constantly. Its teeth and jaws were suited to strip leaves from the branches of trees and any smaller greenery, such as ferns and cycads that grew in profusion during the Jurassic period. As mentioned in the Anatomy section, Zigongosaurus may have carried its neck outstretched to graze low-growing plants as it walked from one location to another; at other times it may have carried its neck nearly upright to keep out of reach from dangerous predators while grazing higher-level leaves and shoots.

The rock formation in which the Zigongosaurus fossils have been found is also an effective indicator of the nature of the contemporary environment. Fine sedimentary rocks, mudstone, and some sandstone are formed from deposits left by moving water. These rock formations and the fossils they contain show that the climate was warm and moist during the Jurassic period. A variety of fast-growing and tender plants grew in profusion and would have provided ample food for creatures like Zigongosaurus and other herbivores.

Behavior

Zigongosaurus was a large creature. After growing to a certain size, it would have been relatively safe from attack by predators. Only a very large predator or several smaller ones would have been able to take down and kill a creature of its size.

Fossil evidence indicates that some sauropods moved in structured herds, with the larger, mature individuals on the outsides of the group enclosing the smaller members of the herd in a protective ring. (Present-day herd animals such as elephants exhibit this same behavior.) It is likely that Zigongosaurus, like other sauropods, also moved in herds. But as with other aspects of knowledge about Zigongosaurus, details about its behavior are obscured by the uncertainties of its actual classification.

Habitat and Other Life Forms

In the warm, moist environment of the Middle Jurassic period, plant life had gone through some major evolutionary changes, and the environment fairly exploded with thick jungles of rich plant growth. The fossil record reveals a world fairly covered with dense forests, marshy areas, shallow seas and lakes, and rainfall sufficient to produce massive floods and huge deposits of sediments. Those deposits eventually became the sedimentary rock beds from which fossils of Zigongosaurus and its contemporaries are taken today.

The forests contained many kinds of plants that produced seeds but not flowers, called “gymnosperms.” Undoubtedly, the profusion of plant growth provided food for creatures that had adapted to take advantage of this food supply at different levels. Numerous herbivores are known to have existed at the same time as Zigongosaurus, and their various anatomies indicated the level at which they probably fed. In a way, the Middle and Late Jurassic periods might be thought of as the Age of Sauropods: Mass fossil beds of sauropod bones found in different locations around the world indicate that they were very common during this time.

It is not entirely certain whether Zigongosaurus lived only during the Middle Jurassic period of 163 million to 145 million years ago. Other dinosaurs that have been found in China, which are known to have existed at about the same time as Zigongosaurus, include the herbivorous stegosaurids Huayangosaurus and Tuojiangosaurus and the carnivorous Jiangjunosaurus.

Research

Zigongosaurus was first found in the Lower Shaximiao Formation, a sedimentary rock structure near Zigong in Sichuan Province, China. It was first described in the literature in 1976, based on fossil remains that included fragments of the skull and jaw structure. Because of the fragmentary knowledge of identified dinosaur species from China, a clear taxonomical classification of the creature has not yet been made. Zigongosaurus was initially identified as distinct from other fossils known as Omeisaurus and Mamenchisaurus, but today researchers believe that all three are representatives of the same species. This situation has not yet been clarified because of the scarcity of representative fossils. Certain classification of Zigongosaurus will be achieved only after more fossil remains have been identified and a more complete picture of the Jurassic period in China has been developed.

Bibliography

Barlow, Connie. Ghosts of Evolution. Nonsensical Fruit, Missing Partners and Other Ecological Anachronisms. Basic Books, 2000.

Clark, Neil, and William Lindsay. Backpack Books 1,001 Facts About Dinosaurs. DK Publishing, 2002.

Cox, Barry, et al. The Simon & Schuster Encyclopedia of Dinosaurs and Prehistoric Creatures. Simon & Schuster, 1999.

Debus, Allen A., and Diane E. Debus. Dinosaur Memories: Dino-trekking for Beasts of Thunder, Fantastic Saurians, “Paleo-People,” “Dinosaurabilia” and Other “Prehistoria.” Authors Choice Press, 2002.

"Dinosaurs." National Park Service, 17 Oct. 2019, www.nps.gov/subjects/dinosaurs/index.htm. Accessed 3 Apr. 2020.

"Exhibits." The Royal Tyrrell Museum, tyrrellmuseum.com/whats‗on/exhibits. Accessed 3 Apr. 2020.

Fastovsky, David B, et al. Evolution and Extinction of the Dinosaurs. Cambridge University Press, 1996.

Hitt, Jack. “CSI: Jurassic.” Discover, Sept. 2006, pp. 62–67.

Holtz, Thomas R. The Little Giant Book of Dinosaurs. Sterling Publishing, 2001.

Hou, L.-h., et all. “New Discovery of Sauropod Dinosaurs from Sichuan” (in Chinese). Vertebrata PalAsiatica, vol. 14, no. 3, 1976, pp. 160–65.

Leslie, Mitch. “The Strange Lives of Polar Dinosaurs.” Smithsonian, Dec. 2007, pp. 68–74.

Lucas, Spencer G. Chinese Fossil Vertebrates. Columbia UP, 2001.

"Mamenchisaurus." National History Museum, www.nhm.ac.uk/discover/dino-directory/mamenchisaurus.html. Accessed 3 Apr. 2020.

Novacek, Michael. Dinosaurs of the Flaming Cliffs. Anchor Books, 1996.

Rajewski, Genevieve. “Where Dinosaurs Roamed.” Smithsonian, May 2008, pp. 20–24.

Vickers Rich, Patricia et al. The Fossil Book: A Record of Prehistoric Life. Dover Publications, 1996.

Weishampel, David B., et al, editors. Dinosauria. 2nd ed., University of California Press, 2004.

Yeoman, Barry. “Schweitzer's Dangerous Discovery.” Discover, April 2006, pp. 37–41, 77.

Zhang, Y., and W. Chen. “Preliminary Research on the Classification of Sauropods from Sichuan Basin, China.” Continental Jurassic, Museum of Northern Arizona, vol. 60, 1996, pp. 97–107.

Zimmer, Carl. “Dinosaurs.” Discover, vol. 26, no. 4, 2005, p. 32. Canadian Reference Center, search.ebscohost.com/login.aspx?direct=true&db=rch&AN=16382092&site=eds-live. Accessed 3 Apr. 2020.