Agilisaurus (; 'agile lizard') is a genus of dinosaur from the Middle Jurassic Period of what is now eastern Asia. The only named species is A. louderbacki, recovered from the Lower Shaximiao Formation of Sichuan, China. It was about in length, in hip height, and in weight.[Paul, G.S. (2010). The Princeton Field Guide to Dinosaurs. Princeton University Press] It has leaf-shaped teeth that were well-adapted to its abrasive, plant-based diet. Most surprisingly, the wavy enamel of the teeth of this genus (and all other Ornithopoda), presumed to make them more resistant to wear, was previously thought to be exclusive to Hadrosauridae.[ This is also the case for Changchunsaurus from the Cretaceous Period.][Jun Chen, Aaron R. H. LeBlanc, Liyong Jin, Timothy Huang, Robert R. Reisz. Tooth development, histology, and enamel microstructure in Changchunsaurus parvus: Implications for dental evolution in ornithopod dinosaurs. PLOS ONE, 2018; 13 (11): e0205206 DOI: 10.1371/journal.pone.0205206]
Discovery and history
There is one named species ( A. louderbacki), named after Dr. George Louderback, an United States geologist and the first to recognize dinosaur fossils from the Sichuan Province of China in 1915. The holotype, which is the only known specimen, was discovered in 1984, during the construction of the Zigong Dinosaur Museum. Both genus and type species were named by Chinese paleontologist Peng Guangzhou in very brief fashion in 1990,
The name is derived from the Latin " agilis" meaning 'agile' and the Ancient Greek " sauros" meaning 'lizard', and refers to the agility suggested by its lightweight skeleton and long legs.
The species epithet, louderbacki, honors the late U.S. geologist Dr. George D. Louderback the first to discover fossils within the Sichuan Basin in 1915.
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.[
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This skeleton was actually discovered during the construction of the Zigong Dinosaur Museum, in which it is now housed.[Li, K; Liu, J.; Yang, C.; Hu, F. (2011). "Dinosaur assemblages from the Middle Jurassic Shaximiao Formation and Chuanjie Formation in the Sichuan-Yunnan Basin, China". Volumina Jurassica. 9 (9): 21–42.] outside the city of Zigong, in the Chinese province of Sichuan, including Agilisaurus, as well as Xuanhanosaurus,[Dong, Z. (1984). "A new theropod dinosaur from the Middle Jurassic of Sichuan Basin". Vertebrata PalAsiatica 22(3):213-218] Shunosaurus,[Dong, Z., Zhou, S. & Zhang, Y. 1983. Dinosaurs. Palaeontologica Sinica, New Series C 162(23): 1-136] and Huayangosaurus.[Dong, Z., Tang, Z. and Zhou, S.W. (1982). "Note (in Chinese). Vertebrata PalAsiatica 20 (1) :83-87] This quarry preserves sediment from the Lower Shaximiao Formation (sometimes called "Xiashaximiao") which ranges from the Bathonian through Callovian stages of the Middle Jurassic Period, or from about 168 to 161 million years ago.[See for a detailed geologic timescale Gradstein et al. (2004)]
Description
Its tibia (lower leg bone), at in length, was slightly longer than its femur (upper leg bone), at in length, 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, 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.][
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Classification
Despite its completeness, Agilisaurus has been placed in many different positions in the ornithischian family tree. It was originally placed in the family Fabrosauridae, which is no longer considered valid by most paleontologists.[
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Several recent studies, including cladistic analyses, find Agilisaurus to be the most basal member of the group Euornithopoda, which includes all ornithopods more Synapomorphy than the family Heterodontosauridae.
However, heterodontosaurs are not universally considered to be ornithopods and have been considered more closely related to the suborder Marginocephalia, which includes and . In one recent cladistic analysis, Agilisaurus was found in a position basal to heterodontosaurs in the branch leading to Marginocephalia.[
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Agilisaurus has been recovered in other positions as well, including as an ornithischian basal to both ornithopods and marginocephalians.
In his more thorough 1992 description, Peng added a new species to the genus Agilisaurus.[ This species had previously been known as Yandusaurus multidens. Because this species did not belong in the genus Yandusaurus and due to similarities with A. louderbacki, it was assigned the name Agilisaurus multidens.][
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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 multidens.[Norman, D.B., Sues, H-D., Witmer, L.M., & Coria, R.A. (2004). "Basal Ornithopoda". In: Weishampel, D.B., Dodson, P., & Osmolska, H. (eds.) The Dinosauria (2nd Edition). Berkeley: University of California Press. pp. 393–412.][
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Paleobiology
Comparisons between the sclerotic ring of Agilisaurus and modern birds and reptiles suggest that it may have been Diurnality, unlike larger herbivorous dinosaurs that were inferred to be Cathemerality,
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