Polycotylidae

Abstract: Publisher Summary This chapter presents comparative cranial anatomy of two North American cretaceous Plesiosaurs. The skulls of Elasmosaurus morgani and Dolichorhynchops osborni are compared as representatives of the Cretaceous plesiosaur families Elasmosauridae and Polycotylidae, respectively. Cranial features and the atlas-axis complex appear to be more stable evolutionarily than postcranial features. Similarities indicate that the short-necked Cretaceous polycotylids are the sister group to long-necked elasmosaurids. This implies that the short-necked polycotylids of the Cretaceous are not descended from the short-necked pliosaurs of the Jurassic. The chapter further elaborated the significance of similarities between Libonectes and Dolichorhynchops. The short neck has appeared independently at least twice in the Plesiosauria, and the term "pliosaur" to refer any short-necked plesiosaur should be abandoned to avoid any phyletic implications. Differences between Elasmosaurus morgani and Elasmosaurus platyurus demonstrate that the two species belong to a different genera and a new name is proposed for E. morgani.

Abstract: 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 ...