Jeholosaurus

Jeholosaurus is a genus of ornithischian dinosaur from the Early Cretaceous Period. It is thought to have been a herbivorous small ornithopod, probably a basal member of the Euornithopoda.

History of discovery
The first two Jeholosaurus specimens were found in 2000 at Lujiatun near Beipiao City, Liaoning Province, China, and named and described the same year by Xu Xing, Wang Xioalin and You Hailu. The type and only known species is Jeholosaurus shangyuanensis. The generic name Jeholosaurus means "lizard from Jehol"; Jehol is the name of a historical province situated in western Liaoning and northern Hebei. The specific name refers to the geographical area of Shangyuan where the fossil site is located.[1]

The type specimen of Jeholosaurus, on which the genus is based, is IVPP V 12529. It was found in a layer of the early Yixian Formation, dating from the early Aptian, about a 125 million years old. The layers consist of fluvial sandstone interspersed with tuff and it is thought that an enormous volcanic eruption occurred burying everything within a fifty to sixty mile radius. The holotype contains a compressed skull and a partial postcranial skeleton. The second specimen, IVPP V 12530, was referred to the species. It consists of a better preserved, perhaps smaller, skull and some neck vertebrae.[1] Both specimens represent juvenile or at least subadult individuals.

Size
Jeholosaurus was a small bipedal herbivore. Because the specimens are juvenile it is hard to ascertain the adult size.[2] The holotype is 71.1 centimetres (28.0 in) long with a 35.6 centimetres (14.0 in) long tail, nearly half the length of the complete animal. The length of the skull is 6.3 centimetres (2.5 in) and the lower jaws of the mandible are 5.9 centimetres (2.3 in) each. The forelimbs are 25.4 centimetres (10.0 in) long and the hindlimbs are 33 centimetres (13 in). The femur is 9 centimetres (3.5 in) long and the tibia is 10.7 centimetres (4.2 in).

Anatomy
Both skulls are incomplete and had to be partially restored. The referred specimens shows an extremely large orbit compared to the skull size, with a length of 40% of the total skull length. Its snout is short, also comprising 40% of the skull length. Both traits are indicative of a juvenile individual.

The premaxilla has six teeth and the maxilla has at least thirteen teeth. Although the back, maxillary, teeth are fan-shaped like those of a plant eater, the front premaxillary teeth are narrower and longer, more like a carnivorous dinosaur. This may mean that Jeholosaurus was omnivorous, eating both plants and animals. The deeply inset ventral margin in the maxilla suggests fleshy cheeks may have been present. The nasals have large foramina dorsolaterally and a midline fossa. No palpebrae are preserved. Though cervical vertebrae and caudal vertebrae have been preserved, their exact number is unknown. The femur is bowed and has an anterior trochanter slightly lower than the greater trochanter and a third as narrow as the latter. They are separated by a shallow cleft. Between the condyles of the lower femur the (anterior) extensor groove is absent. The fourth trochanter is pendant and located in a rather high position. The foot has four metatarsals. The longest is metatarsal III with a length of 5.5 centimetres (2.2 in); it is placed more anteriorly than the other metatarsals. Metatarsal I is more posterior and its upper part is transversely reduced to a splint.[1]

Some distinguishing traits of Jeholosaurus include: enlarged laterodorsal nasal foramina; a quadratojugal fenestra more than 25% maximum quadratojugal length; a quadratojugal less than 30% of skull height; a predentary with almost 150% of premaxillary body length; a dentary extending posteriorly almost to the posterior border of the angular; and the claw of the third toe being longer than the other third toe phalanges.