sides of the centra of the neck vertebrae show a horizontal ridge. Apart from these autapomorphies, the undersides of the side processes of the back vertebrae are exceptionally concave. The spines at the shoulder are curved to the rear, long, flattened, narrow and pointed. Their underside shows a shallow trough. The front spine is the longest at 42.5 centimetres; to the rear the spines become gradually shorter and wider. A fourth spine, of about the same build but more forward-pointing, is present immediately behind the skull. In 2013 Sven Sachs and Jahn Hornung suggested a configuration in which there were five lateral neck spines, the new German spine having a morphology adapted to fit in the third position. Phylogeny Hylaeosaurus was the first ankylosaur discovered. Until well into the twentieth century its exact affinities would remain uncertain. In 1978 Coombs assigned it to the Nodosauridae within the Ankylosauria. This is still a usual classification, Hylaeosaurus being recovered as a basal nodosaurid in most exact cladistic analyses, sometimes more precisely as a member of the Polacanthinae, and thus being related to Gastonia and Polacanthus. However, in the 1990s, the polacanthines were sometimes seen as basal ankylosaurids, because they were mistakenly believed to have small tail-clubs. A more popular alternative today is that they formed a Polacanthidae, a basal group outside of the nodosaurids + ankylosaurids clade. A 2012 study finding Hylaeosaurus to be a basal nodosaurid but not a polacanthine is shown in this cladogram: See also Timeline of ankylosaur research References External links Paper Dinosaurs, 1824–1969, from Linda Hall Library. (illustration) Nodosaurids Valanginian life Early Cretaceous dinosaurs of Europe Cretaceous England Fossils of England Fossil taxa described in 1833 Taxa named by Gideon Mantell Ornithischian genera