Recent fieldwork in the Tropic Shale of southern Utah has resulted in the recovery of several specimens of short-necked plesiosaurs including both polycotylid plesiosauroids and pliosaurids. This report focuses on the former, whereas the latter are discussed in an accompanying paper. Although the Late Cretaceous Cenomanian/Turonian Stage boundary falls within the lower few meters of the Tropic Shale, all but one of the specimens were found in lower Turonian strata based on molluscan assemblages found in direct association with the skeletal material. The plesiosauroid family Polycotylidae is represented by at least three taxa: (1) the ‘typical’ late Cenomanian/early Turonian Trinacromerum; (2) Eopolycotylus rankini, gen. et sp. nov., a new genus and species that shares synapomorphies with the late Santonian/early Campanian Polycotylus latipinnus; and (3) Palmula quadratus, gen. et sp. nov., another new taxon that shares synapomorphies with unnamed taxa from the late Cenomanian of South Dakota and Japan. Cladistic analysis supports the division of the Polycotylidae into two new subfamilies, here designated the Polycotylinae and the Palmulainae. The Turonian plesiosaur fauna of the Tropic Shale, and consequently the large vertebrate fauna of the Cretaceous Western Interior Seaway, is considerably more diverse than previously realized, and there is no indication that this fauna suffered any negative consequences as a result of global scale oceanographic events, including marine extinctions, that transpired during late Cenomanian-early Turonian time.