Polycotylus latipinnis was the first short-necked plesiosaur to be recognised in North America (Carpenter 1996), and the first polycotylid to be described and named (Cope 1869). It was established in the same volume that coined the name Elasmosaurus and contained the infamous ‘head on the wrong end’ reconstruction (Cope 1869).
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.
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.
An exciting new paper published this week in the journal Science (Vol. 333, p.870-873) provides the first direct evidence for live birth in plesiosaurs, and may have implications for plesiosaur behaviour (O’Keefe and Chiappe 2011).
