Polycotylids have been traditionally classified as pliosaurs because of their short necks and relatively large heads. However, cladistic analyses that take into account a broader suite of anatomical characters have revealed that they are probably plesiosauroids, and therefore more closely related to the long-necked elasmosaurids and cryptoclidids than to pliosaurs.
All polycotylids share a number of features including an elongate rostrum and short postorbital region. They had a worldwide distribution in the Cretaceous Period.
- Brancasaurus (according to Ketchum and Benson, 2011)
Edgarosaurus is a basal polycotylid (Druckenmiller, 2002) from the Thermopolis Shale
Thililua has 30 cervical vertebrae. Each cervical has a longitudinal ridge on its lateral surfaces, this is a convergent feature with elasmosaurids. The skull of Thililua is relatively large with a short postorbital region and elongate rostrum. The type specimen (MHNGr.PA.11710) consists of a complete skull, entire cervical and pectoral series, and the first four …