Liaoningosaurus

The holotype IVPP V12560 is an articulated skeleton measuring approximately 34 centimetres (13 in) in length. The specimen is unique among all known ankylosaur fossils in the retention of the external mandibular fenestra. Antorbital fenestrae may also be present. It has relatively large teeth, including teeth in the praemaxilla ( primitive or possibly juvenile trait). The points on the tooth crown were unusually long and sharp, giving each tooth a fork-like shape. It had long feet, long lower legs, and long, sharp claws on the hands and feet, unlike the blunt claws of other ankylosaurians. All of these were initially interpreted as juvenile features, given the specimen's small size and lack of fusion between the spine and hip bones.[2] However, information from a second specimen has led some scientists to conclude that the lack of hip fusion and long limbs and toes are in fact adaptations to a semiaquatic lifestyle. The large, fork-like teeth and sharp claws may have been adapted to catch fish and other small animals. Stomach contents recovered with the second L. paradoxus specimen show that fish may have formed part of the animal's diet.[1] This indicates that Liaoningosaurus could have been either carnivorous or at least omnivorous, making it the first ornithischian thought to have adapted to such a lifestyle, though this is also suspected in more basal ornithischians like heterodontosaurs[3] and now in pachycephalosaurids [4]

Another supposedly unique feature originally identified from the type specimen was the presence of armor plating on the belly. A large segment of what appeared to be the bone from a flat osteoderm covered the underside of the abdomen. This had a surface structure covered with small hexagonal and rhombic bumps. Such plates had never been found with other ankylosaurians, and it has been suggested that, if L. paradoxus was indeed semiaquatic, the belly plates may have protected it from predators attacking from below, as in modern turtles.[1] Smaller triangular osteoderms were found on the shoulder, including a small shoulder spike.[2] However, closer examination of the bumps seemed to suggest that they were not sculpted onto a single piece of bone, but instead resembled normal dinosaurian scales. The broken edges of the supposed plates, when studied in 2013 by paleontologist Victoria Arbour and colleagues, did not reveal any internal structure as would be expected from a piece of bone. Arbour and colleagues concluded that the "belly plates" were likely to be pieces of preserved skin.[5]

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. published a cladistic analysis in 2001, placing Liaoningosaurus as an early branch within the Nodosauridae,[2] while Vickaryous et al. provisionally assigned it to the Ankylosauria incertae sedis in 2004.[6] A second cladistic analysis performed by Thompson et al. in 2011 suggested that Liaoningosaurus is a very basal ankylosaurid.[7]

• Timeline of ankylosaur research