It has leaf-shaped teeth that were well-adapted to their abrasive, plant-based diets. Most surprisingly, the wavy enamel of the teeth of A. louderbacki and all other ornithopods, presumably to make it more resistant to wear, was previously thought to be exclusive to the hadrosaurs.[3] This is also the case for Changchunsaurus parvus.[3]
The name is derived from the Latin "agilis" meaning 'agile' and the 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. geologistDr. George D. Louderback the first to discover fossils within the Sichuan Basinin1915.[5]
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.[1][6]
Its tibia (lower leg bone) 207.0 mm in length, was longer than its femur (upper leg bone) 199.0 mm 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.[6] It was a small herbivore, about 2 meters (6.5 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.[6]
Because it lacks tail-stiffening interwoven bony struts, some paleontologists think it may have been a burrow dweller.[6]
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.[1]
Several recent studies, including cladistic analyses, find Agilisaurus to be the most basal member of the group Euornithopoda, which includes all ornithopods more derived than the family Heterodontosauridae.[13][14]
However, heterodontosaurs are not universally considered to be ornithopods and have been considered more closely related to the suborder Marginocephalia, which includes ceratopsians and pachycephalosaurs. In one recent cladistic analysis, Agilisaurus was found in a position basal to heterodontosaurs in the branch leading to Marginocephalia.[15]
Agilisaurus has been recovered in other positions as well, including as an ornithischian basal to both ornithopods and marginocephalians.[14][15]
Size compared to a human
In his more thorough 1992 description, Peng added a new species to the genus Agilisaurus.[6] 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.[6]
Other scientists were not convinced that this species belonged to either YandusaurusorAgilisaurus, and in 2005, it was once again reassigned, this time to its own newly created genus. It is now known as Hexinlusaurus multidens.[16] Several studies agree that this species is slightly more derived than Agilisaurus.[17][16]
Comparisons between the scleral ringsofAgilisaurus and modern birds and reptiles suggest that it may have been diurnal, unlike larger herbivorous dinosaurs that were inferred to be cathemeral,[18] active throughout the day at short intervals.[19]
^ abcdPeng, G. (1990). "A new small ornithopod (Agilisaurus louderbacki gen. et sp. nov.) from Zigong, China". Newsletter of the Zigong Dinosaur Museum (in Chinese). 2: 19–27.
^Paul, G.S. (2010). The Princeton Field Guide to Dinosaurs. Princeton University Press
^ abJun 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
^Peng, G. (1992). "Jurassic ornithopod Agilisaurus louderbacki (Ornithopoda: Fabrosauridae) from Zigong, Sichuan, China". Vertebrata PalAsiatica (in Chinese). 30: 39–51.
^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.
^Dong, Z. (1984). "A new theropod dinosaur from the Middle Jurassic of Sichuan Basin". Vertebrata PalAsiatica 22(3):213-218
^Dong, Z., Zhou, S. & Zhang, Y. 1983. [Dinosaurs from the Jurassic of Sichuan]. Palaeontologica Sinica, New Series C 162(23): 1-136
^Dong, Z., Tang, Z. and Zhou, S.W. (1982). ["Note on the new Mid-Jurassic stegosaur from Sichuan Basin, China"] (in Chinese). Vertebrata PalAsiatica 20 (1) :83-87
^See for a detailed geologic timescale Gradstein et al. (2004)
^ abBarrett, P.M.; Butler, R. J.; Knoll, F. (2005). "Small-bodied ornithischian dinosaurs from the Middle Jurassic of Sichuan, China". Journal of Vertebrate Paleontology. 25 (4): 823–834. doi:10.1671/0272-4634(2005)025[0823:sodftm]2.0.co;2.
^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.
^Weishampel, D.B.; Jianu, C.-M.; Csiki, Z.; Norman, D.B. (2003). "Osteology and phylogeny of Zalmoxes (n.g.), an unusual euornithopod dinosaur from the latest Cretaceous of Romania". Journal of Systematic Palaeontology. 1 (2): 65–123. doi:10.1017/s1477201903001032. S2CID86339025.
