Agilisaurus

Agilisaurus, meaning "agile lizard," is a genus of ornithischian dinosaur that lived during the Middle Jurassic in what is now eastern Asia. The only named species is *A. louderbacki*, which was found in the Lower Shaximiao Formation in Sichuan, China. It measured about 1.2 to 1.7 meters long, stood 60 centimeters tall at the hip, and weighed around 40 kilograms. Its leaf-shaped teeth were well-suited for a tough, plant-based diet. Notably, the wavy enamel on its teeth—thought to increase wear resistance—was once believed to be unique to hadrosaurs, but it is also present in this genus and other ornithopods, as well as in the Cretaceous dinosaur *Changchunsaurus*.

The species name honors Dr. George Louderback, an American geologist who first identified dinosaur fossils in Sichuan Province in 1915. The only known specimen, the holotype, was discovered in 1984 during construction of the Zigong Dinosaur Museum. Chinese paleontologist Peng Guangzhou named the genus and species briefly in 1990, then provided a more detailed description in 1992. The name combines Latin "agilis" (agile) and Greek "sauros" (lizard), referring to its lightweight skeleton and long legs, which suggest quick movement. The skeleton is remarkably complete, missing only a few parts of the left forelimb and hind limb, which can be reconstructed from the right side. It is housed in the Zigong Dinosaur Museum, which also displays other dinosaurs from the Dashanpu Quarry, including *Xuanhanosaurus*, *Shunosaurus*, and *Huayangosaurus*. The quarry preserves sediment from the Lower Shaximiao Formation, dating to the Bathonian through Callovian stages of the Middle Jurassic, about 168 to 161 million years ago.

The tibia, at 20.7 centimeters, was slightly longer than the femur, at 19.9 centimeters, indicating it was a fast bipedal runner, using its long tail for balance, though it may have walked on all fours while browsing. Like all ornithischians, it had a beak-like structure on both jaws for cropping plants. Because its tail lacks stiffening bony struts, some paleontologists think it may have lived in burrows.

Despite its completeness, *Agilisaurus* has been placed in various positions in the ornithischian family tree. It was originally assigned to Fabrosauridae, a family now considered invalid by most experts. Recent cladistic analyses often find it as the most basal member of Euornithopoda