Daliansaurus (meaning "Dalian reptile") is a genus of small troodontid theropod dinosaur, measuring approximately 1 metre (3 ft 3 in) long, from the Early Cretaceous of China. It contains a single species, D. liaoningensis, named in 2017 by Shen and colleagues from a nearly complete skeleton preserved in three dimensions. Daliansaurus is unusual in possessing an enlarged claw on the fourth digit of the foot, in addition to the "sickle claw" found on the second digit of the feet of most paravians. It also has long metatarsal bones, and apparently possesses bird-like uncinate processes (a first among troodontids). In the Lujiatun Beds of the Yixian Formation, a volcanically-influenced region with a cold climate, Daliansaurus lived alongside its closest relatives - Sinovenator, Sinusonasus, and Mei, with which it forms the group Sinovenatorinae.
Daliansaurus is a small, lightly-built, and most likely feathered troodontid measuring approximately 1 metre (3 ft 3 in) in length,[1] similar in size to Sinovenator and Sinusonasus.[2] Typical of mature troodontids,[3][4] the neural arches are fused to the vertebral centra in the dorsal (trunk) vertebrae of the only known specimen. Overall, Daliansaurus is very similar to Sinovenator, Sinusonasus, and Mei, its closest relatives, but differs from them in subtle ways.[1]
The skull of Daliansaurus, which measures 13.8 centimetres (5.4 in) long, is proportionally larger than that of Mei. Daliansaurus also has a smaller nostril. The projection of the premaxilla above the nostril is flattened instead of rod-like like in Sinovenator, while another projection below the nostril is straight like Sinusonasus but unlike Sinovenator. There are two openings of the skull in front of the eyes: the teardrop-shaped premaxillary fenestra, and the large, oval antorbital fenestra. Like Mei, Sinovenator, and Sinornithoides but unlike Sinusonasus, the bottom edge of the lower jaw is straight and not convex. There are at least 21 tooth positions in the upper jaw and 24 in the lower; the latter is less than Sinovenator (27[5]) and other troodontids. Like Sinovenator, however, teeth towards the back of the mouth have small serrations on their rear edges. The four premaxillary teeth are not recurved but D-shaped.[1]
Daliansaurus has at least nine cervical vertebrae, probably thirteen dorsal vertebrae, five sacral vertebrae, and at least twenty-eight caudal vertebrae in the tail. Out of the sacral vertebrae, the fifth has the widest transverse processes in Daliansaurus, while the third is the widest in Sinovenator. The front margin of the process is also less strongly concave in Daliansaurus. The ribs of two of the dorsal vertebrae unusually appear to bear uncinate processes, in the form of curved and rod-like projections similar to those of Velociraptor,[6] Linheraptor,[7] and Heyuannia.[8] These structures are unprecedented among troodontids. Like Sinorthoides, the bottom end of the anterior caudal chevrons are long, plate-like, and directed backwards.[1]
The humerus of Daliansaurus is relatively long, with the deltopectoral crest taking up 24% of its length, similar to Sinovenator and Sinornithoides. There are three digits on the hand. The first metacarpal bone is strongly curved in the vertical plane, more so than Sinornithoides; meanwhile, the second metacarpal is shorter than the third, which the opposite is true in both Sinovenator and Sinusonasus. Further below, he first digit of the hand is the most robust, and the claw of that digit bears a distinctly deep groove on its side. Meanwhile, the claw of the third digit is the smallest.[1]