Postcranial osteology of the basally branching hadrosauroid dinosaur <i>Tanius sinensis</i> from the Upper Cretaceous Wangshi Group of Shandong, China

<i>Tanius sinensis</i> was one of the first dinosaur species to be named from China. It was established on a partial skeleton recovered by a joint Sino-Swedish expedition in 1923. The fossils were excavated from Upper Cretaceous strata of the Jiangjunding Formation (Wangshi Group) in Shandong Province, and although their discovery dates back almost 100 years, they have not been reassessed in detail since their initial description in 1929. This omission is critical because <i>T. sinensis</i> is now recognized as one of the stratigraphically youngest non-hadrosaurid hadrosauroid taxa. Here, we re-evaluate the postcranial osteology of <i>T. sinensis</i> as a prelude to an anatomical and phylogenetic revision of the species. We examined the holotype and all currently referred specimens of <i>T. sinensis</i> first-hand, and identified a unique postcranial character state combination incorporating tall dorsal neural spines, a reduced postacetabular ridge on the ilium, a fully enclosed flexor tunnel formed by the distal condyles of the femur, and a lunate proximal end on metatarsal III. Comparisons with other species of <i>Tanius</i> confirm that: (1) <i>T. chingkankouensis</i> is a nomen dubium erected on non-diagnostic composite material; (2) <i>T. laiyangensis</i> was established on indeterminate hadrosaurid remains that are not attributable to <i>Tanius</i>; and (3) the anecdotal assignments of <i>Bactrosaurus prynadai</i> and <i>Tsintaosaurus spinorhinus</i> to <i>Tanius</i> cannot be substantiated. Close inspection of the holotype caudal vertebra further reveals a possible healed bite trace consistent with a prey–predator interaction. Lastly, our calculated average body mass estimate for <i>T. sinensis</i> of between 2091–3533 kg suggests that it was one of the largest non-hadrosaurid hadrosauroids.