Tuojiangosaurus

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Ornithischia

Family: Stegosauridae

Genus:Tuojiangosaurus

Species:Tuojiangosaurus multispinus

Introduction

Tuojiangosaurus lived during the Late Jurassic period, which occurred between 163 and 145 million years ago. It showed significant similarities to Kentrosaurusaethiopicus, a Late Jurassic stegosaur from Tanzania, and Lexovisaurus durobrivensis, a Middle to Late Jurassic (174–145 million years ago) stegosaur, the fossils of which have been found in Europe. Its most famous relative was, however, the North American Stegosaurus.

Tuojiangosaurus was the largest known Jurassic stegosaur of those found on the modern-day Asian continent. Like all stegosaur species, Tuojiangosaurus had the characteristic spikes on its tail and rows of alternating pointy plates running down its back. It is one of the better known Asian stegosaurs, although fewer specimens have been found than for the smaller and most primitive basal stegosaur, Huayangosaurus. However, the skeletal remains of Tuojiangosaurus collected in 1977 are considered to be among the finest preserved stegosaurids found in Asia.

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.

Under the Linnaean system, all armored, back-plated Thyreophora dinosaurs belong to the Ornithischia (or “bird-hipped”) order. Tuojiangosaurus also falls under the infraorder Stegosauria, the members of which are quadrupedal herbivores with rows of boney plates running along their backs. Other members of this infraorder include Kentrosaurus and Stegosaurus.

A cladistic analysis also places Tuojiangosaurus under the Thyreophora clade, the members of which feature dermal armor, and further under the Stegosauridae clade, due to its general shape, posture, and dermal armor.

Anatomy

Complete skulls have been recovered for stegosaur species, including Huayangosaurus, Stegosaurus, and Tuojiangosaurus, providing insight into the anatomy and morphology of stegosaur species. Evidence shows that the skull and dorsal spine structure of Tuojiangosaurus were most similar to Kentrosaurus of Late Jurassic East Africa.

Although stegosaurs generally possessed small heads (and brains), their snouts were extremely elongated and tapered to a narrow beak-like mouth. As with other stegosaur species, the wedge-shaped skull and narrow beaked jaw of Tuojiangosaurus was well-matched to its vegetarian diet. While all stegosaur species appear to possess spatula-shaped cheek teeth, the premaxillary teeth found in the basal species Huayangosaurus were later absent in the more-recent Tuojiangosaurus.

All bulky bodied stegosaur species, including Tuojiangosaurus, were four-legged herbivores possessing pillar-like limbs and short, broad feet with hoofed toes. With the exception of Huayangosaurus, all other stegosaur species possessed markedly shorter forelimbs than hind limbs. This is considered a shared, derived feature of these dinosaurs, as are the obviously arched backs required to accommodate the elongated hind legs.

Tuojiangosaurus possessed four long tail spikes at the tip of its tail. Characteristic of all stegosaurs, it also had a double row of approximately fifteen dermal triangular plates (osteoderms) running down the end two-thirds of its back and tail. Tuojiangosaurus also possessed a single, sharp parascapular spine over each shoulder. While these shoulder spikes were present in other species such as Huayangosaurus and Kentrosaurus, they were absent in the more famous Stegosaurus.

Intelligence

The most-often used technique to measure and compare intelligence in animal species is the encephalization quotient (EQ), which is based on a comparison of a creature's body-to-brain size to that of a similarly sized creature. Most species of dinosaur have an EQ of below 2.0. While this is low compared to most animal species today, new techniques in the study of intelligence have suggested that dinosaurs may have been smarter than currently credited.

Tuojiangosaurus had a relatively small-sized brain-to-body-mass ratio, giving it an EQ of between 0.5 and 0.6 and placing it on the lower end of dinosaur intelligence. Such low intelligence may have limited the cognitive capability and behavioral flexibility of this species, and also supports the theory that most herbivore dinosaurs were less intelligent than were their carnivorous cousins.

Reproduction and Population

Although stegosaur species did not show strong diversity, their fossils are widely distributed across many modern geographical regions, including North America (Stegosaurus), Africa (Kentrosaurus and Paranthodon), and China, where Tuojiangosaurus fossils have been found.

Most dinosaurs, including all stegosaur species, are thought to have been oviparous, laying up to several eggs in a clutch. While nesting sites have not been found for Tuojiangosaurus, evidence from other stegosaur species hints at probable parental care after the offspring hatched. Paleontologists believe that body armor on species such as stegosaurs would not have developed until after the young had hatched, allowing the young embryo to fit comfortably rolled up inside an egg.

While the process of fossilization destroys most of the elements required to accurately identify sex, current paleontological theory suggests some dinosaur species would have exhibited sexual dimorphism, such as female dinosaurs possessing a wider, more robust pelvic anatomy to allow for the passage of eggs.

Diet

The relatively low position of stegosaur heads suggests that this species was restricted to feeding on low vegetation. Some researchers believe that the large, long hind limbs seen in most stegosaur species, particularly Stegosaurus, may have allowed them to stand on their back legs to feed on higher foliage from trees such as Ginkgoes and conifers. Current evidence indicates, however, that Tuojiangosaurus lacked certain vertebrae structures present in Stegosaurus, and this would have restricted its rearing ability. As such, it is considered very unlikely that Tuojiangosaurus was capable of standing on its hind legs and feeding, limiting its diet to low-lying vegetation.

The dinosaur's toothless, beaked mouth and small ridged cheek-teeth were greatly suited to its preferred plant-based diet, as was their roomy gut. Tuojiangosaurus would likely have plucked, pulled, and chewed low-lying gymnosperms such as ferns, horsetails, cycads, and mosses. Much like today's large herbivores, a significant portion of its day would have been spent foraging, eating, and digesting vegetation to obtain sufficient nutrients and energy to power its massive bulk.

Behavior

Understanding behavior from such a vast distance of time is one of the difficulties faced by researchers. Paleontologists gain insight through observations of the behavior of extant animals that share a similar ecological niche to extinct species, and through fossilized remains, nesting-sites, bone-beds, and trackways.

As can be seen with many large herbivore species today, current paleontological evidence suggests that some dinosaur species exhibited herding behavior. Evidence for stegosaur herding behavior in North America is quite strong and indicates that these species lived in non-segregated herds consisting of both juveniles and adults. Extrapolating from this, scientists believe many stegosaur species, including Tuojiangosaurus, also exhibited this group behavior. Herding would have helped protect Tuojiangosaurus from efficient predators such as Yangchuanosaurus shangyouensis, a large carnivore that lived during the Late Jurassic.

One of the most controversial debates among paleontologists is whether the function of the dermal plates seen on stegosaur species is behavioral or physiological.

The most common and well-supported theories postulate that the plates served the following functions: defensive structures used to protect or defend against predators, reproductive structures used to attract mates, or thermoregulatory structures used to help control body temperature.

Habitat and Other Life Forms

Tuojiangosaurus was a large stegosaurian herbivore that lived during the Late Jurassic period between 163 and 145 million years ago, after the break-up of the supercontinent Pangaea into Gondwana and Laurasia. This continental drift also created the Central Atlantic Ocean, which was narrow enough to allow for dinosaur migration across periodic land bridges and may explain why species of stegosaur are found distributed in many different areas.

The Jurassic period (201–145 million years ago) was generally characterized by much higher sea levels, higher atmospheric CO₂ concentrations, and warmer temperatures than seen today. Tuojiangosaurus inhabited the temperate and tropical forests and rich river-valleys of what is now central China, feeding on the lush green vegetation prevalent at the time. The world's flowering plants (angiosperms) had not yet evolved, but the landscape was covered with low-lying gymnosperms such as ferns, horsetails, cycads, and mosses, as well as tall trees such as Ginkgoes and conifers.

Tuojiangosaurus lived alongside many other dinosaurs in these warm humid conditions, including herbivorous and long-necked sauropods such as Omeisaurus and Mamenchisaurus, back-plated dinosaurs such as Chialingosaurus and Chungkingosaurus, and carnivorous theropods such as Yangchuanosaurus.

Research

Tuojiangosaurus multispinus was first discovered in 1977 by Dong Zhiming, a paleontologist from Beijing's Institute of Vertebrate Paleontology and Paleoanthropology. It takes its name from a river in Sichuan, the Tuo Jiang. Described from two partial skeletons and cranial material collected from the Upper Shaximiao Formation of Sichuan in China, these skeletal remains are considered to be among the finest preserved Asian stegosaurids ever found.

Although Tuojiangosaurus has been traditionally classified as living during the Late Jurassic Period, recent invertebrate and plant fossil evidence has suggested that the Upper Shaximiao Formation, from where Tuojiangosaurus was discovered, may be Late Middle Jurassic in age. If this is the case, Tuojiangosaurus would actually be older than is currently accepted.

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