Liaoningosaurus
Liaoningosaurus is an unusual genus of ankylosaurian dinosaurs from the Early Cretaceous period of China. It contains a single species, Liaoningosaurus paradoxus, and is represented by two fossil specimens collected from the Yixian Formation (Aptian age) of Liaoning Province. "L. paradoxus" was unusual among advanced ornithischian dinosaurs in that it is speculated to have hunted or scavenged, with preserved gut contents showing that it may have eaten fish. Additionally, some features of its skeleton may suggest that it was partially aquatic. The type species "L. paradoxus" was named 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. Discovery. The holotype IVPP V12560 is an articulated skeleton measuring approximately in length. It was discovered in the Early Cretaceous Yixian Formation in Liaoning Province, China, and was formally described as "Liaoningosaurus paradoxus" in 2001 by Chinese paleontologists Xu Xing, Wang Xiaolin, and You Hai-lu. The generic name is after Liaoning Province, and the specific name in reference to a suite of highly unusual traits for an ankylosaur. It was originally interpreted as a juvenile in based on the unfused vertebral centra and neural arches . In 2016, paleontologist Ji Qiang suggested that the holotype was actually fully grown, and unusual characteristics are aquatic adaptions. If correct, this would make "Liaoningosaurus" the smallest known ornithischian dinosaur and the only known aquatic or semi-aquatic armoured dinosaur. In 2018, a microanatomical study by Chinese palaeontologist Wenjie Zheng instead concluded that the holotype was no older than 12 months. In 2019, Li Xiaobo and Robert R. Reisz investigated three more specimens and also concluded they represent juveniles. About 20 specimens of "Liaoningosaurus" have been collected from four different localities of the Yixian Formation, and one locality from the  nearby Jiufotang Formation. Many of them remain unstudied. Description. 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 by the original describers as juvenile  features, given the specimen's small size and lack of fusion between the spine and hip bones. Based on a second specimen, Ji and colleagues instead argued that these traits are adaptations to a semiaquatic lifestyle. The large, fork-like teeth and sharp claws would 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. This indicates that "Liaoningosaurus" could have been either carnivorous or at least omnivorous, making it the first non-herbivorous ornithischian ever discovered, though the  behavior has also been suspected in more basal ornithischians like heterodontosaurs and pachycephalosaurids Triangular osteoderms were found on the shoulder, including a small shoulder spike. Xu identified an expansive belly plate (osteoderm), unlike any other ankylosaurian; those with belly armor have unfused, isolated osteoderms instead of this shield-like structure. The osteoderm is dotted with hexagonal and rhombic bumps. Ji and colleagues believed the evolution of this plate converges with turtles to protect the underside when swimming. In 2013, paleontologist Victoria Arbour and colleagues analyzed the supposed osteoderm and interpreted it as normal dinosaur scales (that is, "Liaoningosaurus" did not  have a belly plate). Classification. Given the animal's unusual anatomy, Xu and colleagues had trouble identifying its relationship to other ankylosaurs. Their phylogenic analysis loosely placed it into Nodosauridae (as opposed to Ankylosauridae). They conceded this binary division of Ankylosauria may not be entirely sound, meaning "Liaoningosaurus" could represent some third lineage. In 2004, paleontologist Matthew Vickaryous and colleagues left it under "incertae sedis". A second cladistic analysis performed by Richard Thompson and colleagues in 2011 suggested that "Liaoningosaurus" is a basal ankylosaurid. In 2018, Zheng suggested that "Liaoningosaurus" could be the juvenile form of the much larger "Chuanqilong  discovered in 2014 from the Jiufotang Formation.