Considerable controversy surrounds the taxonomy and phylogenetic relationships of Maastrichtian basal neornithischian taxa from North America. Discovery of previously unrecognized cranial material from the paratype specimen of Thescelosaurus neglectus (USNM 7758), along with the examination of two new specimens that preserve nearly complete skulls and mandibles, allows for reevaluation of specimens previously referred to Thescelosaurus that preserve cranial material, including the holotypes of Bugenasaura infernalis (SDSM 7210) and Parksosaurus warreni (ROM 804). A phylogenetic analysis was conducted that included as terminals the holotypes of B. infernalis and P. warreni, the type series of T. neglectus, and six specimens previously referred to Thescelosaurus. This analysis is the first to recover a clade containing all basal neornithischian taxa from the Cretaceous of North America, within which P. warreni is recovered as the sister taxon to a Thescelosaurus clade whose monophyly is supported by five cranial autapomorphies. The results of this analysis support: (1) the synonymization of Bugenasaura with Thescelosaurus; (2) the removal of ROM 804 from Thescelosaurus; and (3) the validity of Thescelosaurus garbanii and its referral to Thescelosaurus despite its fragmentary nature. Currently, Parksosaurus and Thescelosaurus are the only basal neornithischian taxa definitively known from Maastrichtian age sediments of North America, while other basal neornithischian taxa proposed to demonstrate fossorial behavior (i.e., Orodromeus, Oryctodromeus, and Zephyrosaurus) are known from Campanian and older sediments. This temporal segregation may support previous hypotheses of an environmental or ecological shift during the latest Cretaceous in North America.
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