Limusaurus

Limusaurus Temporal range: Oxfordian, 161–156 Ma PreЄ Є O S D C P T J K Pg N ↓
Holotype and second specimen exhibited in Tokyo (slab also contains a small crocodyliform)
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Clade:	Dinosauria
Order:	Saurischia
Suborder:	Theropoda
Family:	†Noasauridae
Subfamily:	†Elaphrosaurinae
Genus:	†Limusaurus Xu et al., 2009
Species:	†L. inextricabilis
Binomial name
Limusaurus inextricabilis Xu et al., 2009

Limusaurus (meaning "mud lizard") is a genus of theropod dinosaur from the Jurassic (Oxfordian stage) Upper Shishugou Formation in the Junggar Basin of western China. The genus contains a single species, L. inextricabilis. Limusaurus was a small, slender animal, about 1.7 metres (5 ft 7 in) in length, that had a long neck and legs but also highly reduced forelimbs. It underwent a drastic morphological transformation as it aged; while juveniles were toothed, these teeth were completely lost and replaced by a beak with age, corresponding to a shift in diet from omnivory to herbivory.

Limusaurus is the first definitively known ceratosaur from Eastern Asia; while originally considered to be the most basal members (i.e. phylogenetically closest to the origin) of the group Ceratosauria along with its closest relative, Elaphrosaurus, a 2016 analysis showed that they are in fact members of the Noasauridae, a group of similarly small and lightly-built abelisaurs. The pattern of digit reduction in Limusaurus has been used to support the contested hypothesis that the three-fingered hand of tetanuran theropods is the result of the loss of the first and fifth digits from the ancestral five-fingered theropod hand, which has implications for the evolution of birds. However, it is now considered to be irrelevant to the subject of digit homology.

Limusaurus was a small, slender animal with a long neck and legs (the latter being 1.8 times the length of the torso), measuring about 1.7 metres (5 ft 7 in) in length and 15 kilograms (33 lb) in weight. It had a short skull, which was roughly half the length of the femur. A beak covered the slightly convex premaxilla at the tip of the jaw, a feature that was not known in any non-coelurosaurian theropods until the discovery of Limusaurus. The opening, or fenestra, of the mandible was quite large, being 40% the length of the entire lower jaw. The eye socket was also quite large. Other bones with distinctive morphologies in the skull of Limusaurus are the nasal bones, which were ridged on each side, and were short and wide relative to the entire skull; the lacrimal bones, which sloped downwards sharply over the eyes; and the jugal bones, the branches of which were slender and rod-like.