| Agilisaurus | |
|---|---|
 | |
| Mounted skeleton at theZigong Dinosaur Museum | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | †Ornithischia |
| Clade: | †Neornithischia |
| Genus: | †Agilisaurus Peng,1990 |
| Species: | †A. louderbacki |
| Binomial name | |
| †Agilisaurus louderbacki Peng, 1990[1] | |
Agilisaurus (/ˌædʒɪlɪˈsɔːrəs/; 'agile lizard') is agenus ofornithischiandinosaur from the MiddleJurassic Period of what is now easternAsia. The only named species isA. louderbacki, recovered from the LowerShaximiao Formation ofSichuan,China. It was about 1.2–1.7 m (3.9–5.6 ft) in length, 60 cm (2.0 ft) in hip height, and 40 kg (88 lb) in weight.[2] 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 otherornithopods), presumed to make them more resistant to wear, was previously thought to be exclusive tohadrosaurs.[3] This is also the case forChangchunsaurus from theCretaceous Period.[3]
There is one namedspecies (A. louderbacki), named after Dr.George Louderback, anAmericangeologist and the first to recognize dinosaurfossils from theSichuan Province ofChina in 1915. The holotype, which is the only known specimen, was discovered in 1984, during the construction of theZigong Dinosaur Museum. Both genus andtype species were named by ChinesepaleontologistPeng Guangzhou in very brief fashion in 1990,[1] then described in further detail by Peng in 1992.[4]
The name is derived from theLatin "agilis" meaning 'agile' and theGreek "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 theSichuan Basin in1915.[5]
A single complete skeleton ofA. 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 theircounterparts on the right side.[1][6]
This skeleton was actually discovered during the construction of theZigong Dinosaur Museum, in which it is now housed.[7] This museum features many dinosaurs recovered from the famousDashanpu Quarry[8] outside the city ofZigong, in the Chinese province of Sichuan, includingAgilisaurus, as well asXuanhanosaurus,[9]Shunosaurus,[10] andHuayangosaurus.[11] This quarry preserves sediment from theLower Shaximiao Formation (sometimes called "Xiashaximiao") which ranges from theBathonian throughCallovian stages of theMiddle Jurassic Period, or from about 168 to 161 million years ago.[12]
Itstibia (lower leg bone), at 20.7 cm (8.1 in) in length, was slightly longer than itsfemur (upper leg bone), at 19.9 cm (7.8 in) in length, which indicates that it was an extremely fastbipedal runner, using its long tail for balance, although it may have walked on all fours when browsing for food.[6] It was a smallherbivore, 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, somepaleontologists think it may have been aburrow dweller.[6]
Despite its completeness,Agilisaurus has been placed in many different positions in the ornithischian family tree. It was originally placed in the familyFabrosauridae, which is no longer considered valid by most paleontologists.[1]
Several recent studies, includingcladistic analyses, findAgilisaurus to be the mostbasal member of the groupEuornithopoda, which includes all ornithopods morederived than the family Heterodontosauridae.[13][14]
However, heterodontosaurs are not universally considered to be ornithopods and have been considered more closely related to the suborderMarginocephalia, which includesceratopsians andpachycephalosaurs. In one recent cladistic analysis,Agilisaurus was found in a position basal to heterodontosaurs in the branch leading toMarginocephalia.[15]
Agilisaurus has been recovered in other positions as well, including as an ornithischian basal to both ornithopods and marginocephalians.[14][15]
In his more thorough 1992 description, Peng added a new species to the genusAgilisaurus.[6] This species had previously been known asYandusaurus multidens. Because this species did not belong in the genusYandusaurus and due to similarities withA. louderbacki, it was assigned the nameAgilisaurus multidens.[6]
Other scientists were not convinced that this species belonged to eitherYandusaurus orAgilisaurus, and in 2005, it was once again reassigned, this time to its own newly created genus. It is now known asHexinlusaurus multidens.[16] Several studies agree that this species is slightly more derived thanAgilisaurus.[17][16]
Comparisons between thescleral rings ofAgilisaurus and modern birds and reptiles suggest that it may have beendiurnal, unlike larger herbivorous dinosaurs that were inferred to becathemeral,[18] being active throughout the day at short intervals.[19]
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