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Agilisaurus is a genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 of ornithischia
Ornithischia or Predentata is an extinct order of beaked, herbivorous dinosaurs. The name ornithischia is derived from the Greek ornitheos meaning 'of a bird' and ischion meaning 'hip joint'...

n dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...

 from the Middle Jurassic Period of what is now eastern Asia. The name is derived from the Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 agilis meaning 'agile' and the Greek
Ancient Greek
Ancient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...

 sauros meaning 'lizard', and refers to the agility suggested by its lightweight skeleton and long legs. Its tibia
The tibia , shinbone, or shankbone is the larger and stronger of the two bones in the leg below the knee in vertebrates , and connects the knee with the ankle bones....

 (lower leg bone) was longer than its femur
The femur , or thigh bone, is the most proximal bone of the leg in tetrapod vertebrates capable of walking or jumping, such as most land mammals, birds, many reptiles such as lizards, and amphibians such as frogs. In vertebrates with four legs such as dogs and horses, the femur is found only in...

 (upper leg bone), which indicates that it was an extremely fast bipedal runner, using its long tail for balance, although it may have walked on all fours when browsing for food. It was a small herbivore
Herbivores are organisms that are anatomically and physiologically adapted to eat plant-based foods. Herbivory is a form of consumption in which an organism principally eats autotrophs such as plants, algae and photosynthesizing bacteria. More generally, organisms that feed on autotrophs in...

, about 1.2 meters (4 feet) in length, and like all ornithischians, it had a beak-like structure on the ends of both upper and lower jaws to help it crop plant material.

Discovery and history

There is one named species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 (A. louderbacki), named after Dr. George Louderback, an American
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

A geologist is a scientist who studies the solid and liquid matter that constitutes the Earth as well as the processes and history that has shaped it. Geologists usually engage in studying geology. Geologists, studying more of an applied science than a theoretical one, must approach Geology using...

 and the first to recognize dinosaur fossils from the Sichuan Province of China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...

 in 1915. Both genus and type species
Type species
In biological nomenclature, a type species is both a concept and a practical system which is used in the classification and nomenclature of animals and plants. The value of a "type species" lies in the fact that it makes clear what is meant by a particular genus name. A type species is the species...

 were named by Chinese paleontologist Peng Guangzhou in very brief fashion in 1990, then described in further detail by Peng in 1992.

A single complete skeleton of A. louderbacki is known to science, one of the most complete small ornithischian skeletons ever found. Only a few parts of its left fore limb and hind limb are missing, and those can be reconstructed from their counterparts on the right side.

This skeleton was actually discovered during the construction of the Zigong Dinosaur Museum
Zigong Dinosaur Museum
Located near the Prefecture-level city of Zigong, China, in the smaller township of Dashanpu, is the Zigong Dinosaur Museum. The museum sits on top of a large concentration of dinosaur fossils — both in terms of quantity but also of diversity. The museum claims the largest number of dinosaur...

, in which it is now housed. This museum features many dinosaurs recovered from the famous Dashanpu Quarry
Dashanpu Formation
The Dashanpu Formation is a Mid to Late Jurassic rock formation in China, most notable for the wealth of dinosaurs that have been excavated from the area...

 outside the city of Zigong
Zigong , ancient name Ziliujing and Gongjing, is a prefecture-level city and the third largest city in Sichuan Province, in southwest China.-Geography:...

, in the Chinese province of Sichuan, including Agilisaurus, as well as Xuanhanosaurus
Xuanhanosaurus is a genus of theropod dinosaur which lived during the Jurassic of China.The type species Xuanhanosaurus qilixiaensis was named by Dong Zhiming in 1984. The generic name refers to Xuanhan County in Sichuan. The specific name is derived from the town of Qilixia...

, Shunosaurus
Shunosaurus, meaning "Shu Lizard", is a genus of sauropod dinosaur from Middle Jurassic beds in Sichuan Province in China, 170 million years ago...

, and Huayangosaurus
Huayangosaurus is a genus of stegosaurian dinosaur from the Middle Jurassic of China. The name derives from "Huayang", an alternate name for Sichuan , and "saurus", meaning "lizard"...

. This quarry preserves sediment from the Lower Shaximiao Formation (sometimes called "Xiashaximiao") which ranges from the Bathonian
In the geologic timescale the Bathonian is an age or stage of the Middle Jurassic. It lasted from approximately 167.7 Ma to around 164.7 Ma...

 through Callovian
In the geologic timescale, the Callovian is an age or stage in the Middle Jurassic, lasting between 164.7 ± 4.0 Ma and 161.2 ± 4.0 Ma. It is the last stage of the Middle Jurassic, following the Bathonian and preceding the Oxfordian....

 stages of the Middle Jurassic
Middle Jurassic
The Middle Jurassic is the second epoch of the Jurassic Period. It lasted from 176-161 million years ago. In European lithostratigraphy, rocks of this Middle Jurassic age are called the Dogger....

 Period, or from about 168 to 161 million years ago.


Despite its completeness, Agilisaurus has been placed in many different positions in the ornithischian family tree. It was originally placed in the family Fabrosauridae
The Fabrosauridae was originally proposed by Galton , as a group of basal ornithischian dinosaurs from the Early to Middle Jurassic. However, recent studies indicate the Fabrosaurs are not a natural grouping of dinosaurs, and instead consist of unrelated genera.The proposed "Fabrosaurs" descended...

, which is no longer considered valid by most paleontologists (Peng 1990).

Several recent studies, including cladistic analyses, find Agilisaurus to be the most basal
Basal (phylogenetics)
In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...

 member of the group Euornithopoda, which includes all ornithopods more derived
In phylogenetics, a derived trait is a trait that is present in an organism, but was absent in the last common ancestor of the group being considered. This may also refer to structures that are not present in an organism, but were present in its ancestors, i.e. traits that have undergone secondary...

 than the family Heterodontosauridae (Weishampel et al. 2003; Norman et al. 2004).

However, heterodontosaurs are not universally considered to be ornithopods and have been considered more closely related to the suborder Marginocephalia
Marginocephalia is a clade of ornithischian dinosaurs that includes the thick-skulled pachycephalosaurids, and horned ceratopsians. They were all herbivores, walking on two or four legs, and are characterized by a bony ridge or frill the back of the skull...

, which includes ceratopsia
Ceratopsia or Ceratopia is a group of herbivorous, beaked dinosaurs which thrived in what are now North America, Europe, and Asia, during the Cretaceous Period, although ancestral forms lived earlier, in the Jurassic. The earliest known ceratopsian, Yinlong downsi, lived between 161.2 and 155.7...

ns and pachycephalosaurs. In one recent cladistic analysis, Agilisaurus was found in a position basal to heterodontosaurs in the branch leading to Marginocephalia (Xu et al. 2006).

Agilisaurus has been recovered in other positions as well, including as an ornithischian basal to both ornithopods and marginocephalians (Barrett et al. 2005; Butler 2005; Butler et al. 2008).

Second Species?

In his more thorough 1992 description, Peng added a new species to the genus Agilisaurus. This species had previously been known as Yandusaurus
Yandusaurus is a genus of herbivorous basal euornithopod dinosaur from the Bathonian age of China.-Discovery and naming:...

. Because this species did not belong in the genus Yandusaurus and due to similarities with A. louderbacki, it was assigned it the name Agilisaurus multidens.

Other scientists were not convinced that this species belonged to either Yandusaurus or Agilisaurus, and in 2005, it was once again reassigned, this time to its own newly-created genus. It is now known as Hexinlusaurus
Hexinlusaurus is a genus of basal ornithischian dinosaur from the Middle Jurassic of China. The holotype , consists of an almost complete, articulated skull and some postcranial material, collected from a terrestrial sandstone within the Lower Shaximiao Formation at the famous dinosaur-bearing...

(Barrett et al. 2005). Several studies agree that this species is slightly more derived than Agilisaurus (Norman et al. 2004; Barrett et al. 2005; Butler 2005). Both Yandusaurus and Hexinlusaurus were also found in the Dashanpu Quarry.


Comparisons between the scleral ring
Sclerotic ring
Sclerotic rings are rings of bone found in the eyes of several groups of vertebrate animals, except for mammals and crocodilians. They can be made up of single bones or small bones together. They are believed to have a role in supporting the eye, especially in animals whose eyes are not spherical,...

s of Agilisaurus and modern birds and reptiles suggest that it may have been diurnal, unlike larger herbivorous
Herbivores are organisms that are anatomically and physiologically adapted to eat plant-based foods. Herbivory is a form of consumption in which an organism principally eats autotrophs such as plants, algae and photosynthesizing bacteria. More generally, organisms that feed on autotrophs in...

 dinosaurs that were inferred to be cathemeral
A cathemeral organism is one that has sporadic and random intervals of activity during the day or night in which food is acquired, socializing with other organisms occurs, and any other activities necessary for livelihood are performed...

, active throughout the day at short intervals.

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