Liaoningosaurus

Liaoningosaurus is a very unusual ankylosaur from the Lower Cretaceous. The holotype IVPP V12560 (Institute of Vertebrate Paleontology and Paleoanthropology, Beijing), an essentially complete juvenile skeleton compressed on a slab, was collected from the Yixian Formation (Aptian stage) of Liaoxi, Yixian County, Liaoning Province, People's Republic of China.

The type species Liaoningosaurus paradoxus was named by Xu Xing, Wang Xiaolin and You Hailu in 2001. The generic name refers to Liaoning. The specific name refers to the confusing mix of nodosaurid and ankylosaurid features shown by the specimen.

The articulated skeleton measures approximately 34 cm in length, giving it the distinction of being the smallest-known ankylosaur specimen. The specimen is unique among all known ankylosaur fossils in the retention of the external mandibular fenestra. Antorbital fenestrae may also be present. Both are likely juvenile traits, just as the relatively large teeth, the retention of teeth in the praemaxilla, long feet, long lower legs and the possession of long sharp claws on the hand and feet. A real feature unique to the taxon, or autapomorphy might be the trapezoid form of the breast bone

Another new feature is the presence of ventral armor plating, two very large paired osteoderms covering the entire abdomen and having a surface structure sculpted with small hexagonal and rhombic elevations. Such plates had never been found with other ankylosaurians, of which the belly armor is poorly known, however. Smaller triangular osteoderms were found on the shoulder, the scapula showing the onset of a shoulder spike.

Determining the relations of Liaoningosaurus is difficult because of the young age of the individual specimen and the presence of traits both typical of ankylosaurids and nodosaurids. Xu et al. (2001) published a cladistic analysis, placing Liaoningosaurus basal within the Nodosauridae, while Vickaryous et al. (2004) have provisionally assigned it to the Ankylosauria incertae sedis. A new cladistic analysis performed by Thompson et al., 2011 suggests that Liaoningosaurus is a very basal ankylosaurid.