Agilisaurus (AJ-ih-lih-SAWR-us; “Agile lizard”) is a genus of small, plant-eating ornithischian dinosaur that lived during the Middle Jurassic period, approximately 168 to 161 million years ago. Its fossils were discovered in what is now Sichuan Province, China. The dinosaur was named by paleontologist Peng Guangzhao in 1990. The name Agilisaurus was chosen because its light build and long legs suggest it was a very quick and agile runner, traits that would have helped it escape from predators. Remarkably complete skeletons of Agilisaurus have been found, giving scientists a good understanding of what this early ornithischian looked like.
Description and Classification
Agilisaurus was a relatively small dinosaur, measuring about 1.2 to 1.7 meters (4 to 5.5 feet) in length and weighing an estimated 12 to 40 kilograms (26 to 88 pounds). It walked on its two long hind legs, using its shorter arms primarily for grasping food or for support. A long tail stretched out behind its body, providing balance as it moved. Like other ornithischian dinosaurs, Agilisaurus had a beak at the front of its mouth for snipping off plant material, and rows of teeth further back for chewing.
In terms of classification, Agilisaurus belongs to the order Ornithischia, one of the two major groups of dinosaurs, characterized by their bird-like hip structure. It is considered an early (or basal) member of the Neornithischia group. This large group includes later, more advanced ornithopods like Iguanodon and the “duck-billed” hadrosaurs, as well as the horned dinosaurs (ceratopsians such as Triceratops). Some scientists place Agilisaurus in a group similar to the hypsilophodontids, known for their small size and speedy nature, although the exact relationships of these early ornithischians are still being studied. Its well-preserved fossils make Agilisaurus an important dinosaur for understanding the early evolution of these varied plant-eaters.
Distinguishing Features
Agilisaurus possessed several features that help paleontologists identify it and understand its lifestyle:
- A lightweight skeleton and long hind limbs, especially the lower leg bones (tibia and fibula), indicating it was built for fast running.
- A horny beak at the tip of its snout, used for cropping leaves, ferns, and other vegetation.
- Leaf-shaped cheek teeth, designed for grinding tough plant material.
- Relatively short forelimbs with five-fingered hands, which were likely not used for walking but could have helped with feeding.
- A long, stiffened tail that acted as a counterbalance, allowing for quick turns and agile movements.
Paleoenvironment and Diet
Agilisaurus lived in what is now the Sichuan Province of China during the Middle Jurassic. The specific rock formation where its fossils are found, the Lower Shaximiao Formation (also known as the Dashanpu Formation), reveals a warm and humid environment. This area was a dynamic floodplain, with numerous rivers, lakes, and lush forests composed of conifers, cycads, and ferns. Agilisaurus shared this environment with a variety of other dinosaurs. These included giant long-necked sauropods like Shunosaurus and Omeisaurus, meat-eating theropods such as Gasosaurus, and early armored dinosaurs like the stegosaur Huayangosaurus.
As a herbivore, Agilisaurus would have fed on the abundant low-growing plants available in its habitat, such as ferns, horsetails, and young cycads. Its beaked mouth was well-suited for nipping off plant parts, and its cheek teeth helped to break them down before swallowing. Its speed and agility were likely its primary defense against the predators of its time.
Significance and Ongoing Research
The discovery of Agilisaurus has been very important for paleontology because it is one of the most complete and well-preserved early ornithischian dinosaurs known from the Middle Jurassic. This period was a crucial time for the evolution of ornithischians, as they began to diversify into the many forms seen later in the Mesozoic Era. The detailed fossils of Agilisaurus provide valuable information about the anatomy and biology of these early plant-eaters.
Ongoing research continues to focus on Agilisaurus‘s exact place within the ornithischian family tree. Scientists compare its features with those of other early ornithischians, like Lesothosaurus from Africa or other basal neornithischians, to better understand how this diverse group of dinosaurs evolved. Studies also explore its locomotion, diet, and growth, helping to paint a clearer picture of life during the Middle Jurassic in Asia and the early adaptations that led to the success of later plant-eating dinosaurs.