A taxonomic and phylogenetic re-evaluation of Therizinosauria (Dinosauria: Maniraptora)

Therizinosaurians are among the most poorly understood dinosaurs. Their unusual morphology and fragmentary fossil record has precluded a synthetic understanding of the group since their remains were first discovered over 60 years ago. Although the clade was recently substantiated as a monophyletic group of maniraptoran theropods, little foundational work has been conducted at the species level. A recent plethora of therizinosaurian discoveries - including the most complete primitive and derived members recovered to date - permits an alpha taxonomic and phylogenetic re-evaluation of the clade. The phylogenetic analysis presented is the most comprehensive yet conducted for Therizinosauria, and provides a foundation for scrutinizing previous definitions of Therizinosauria, Therizinosauroidea and Therizinosauridae. Here, support is provided for the maintenance of all three taxa; however, Therizinosauria is redefined and Falcarius is excluded from Therizinosauroidea. In addition, the previously described therizinosauroids, Beipiaosaurus, Enigmosaurus, Suzhousaurus, Segnosaurus and Therizinosaurus, are rediagnosed and photodocumented. In contrast to other analyses, the ingroup topology recovered in this study suggests intermediate (therizinosauroid) status for Neimongosaurus, Erliansaurus and Enigmosaurus (based on relatively primitive pelvic morphology), despite the derived forelimb anatomy evident in the former two taxa. Here, the large-bodied taxa Nothronychus and Nanshiungosaurus brevispinus are recovered as therizinosaurids. This discrepancy indicates a relatively complex pattern of mosaic evolution, which may ultimately be found to correlate with body-size trends in the clade. This work also reviews the chronostratigraphic and biogeographic distribution of therizinosaurian taxa and putatively referred elements and finds no compelling evidence of the clade outside of Asia and North America, nor for the referral of therizinosaurian materials from Kazakhstan to cf. Neimongosaurus. Time calibration of ingroup relationships indicates a pre-Turonian dispersal event is needed to account for the presence of therizinosaurids in the Late Cretaceous of North America and Asia; this conclusion supports previous hypotheses of a Laurasian faunal interchange event during the Albian.