There have been many differing interpretations of plesiosaur neck posture and function. Indeed, the function of the long plesiosaur neck is still controversial and unresolved today (Martill et al. 1994). It likely served as a mechanism for approaching prey, such as a school of fish, without being detected (Massare, 1988).
Contrary to early work (Zarnik, 1925, see figure below), the plesiosaur neck did not have the flexibility required to coil up and strike prey in the manner of some snakes and pleurodiran turtles (Pough et al. 1996). Tall neural spines inhibited vertical (up and down) flexibility and large processes on the anterior and posterior surfaces of the vertebrae (prezygapophyses and postzygapophyses) inhibited horizontal (side to side) flexibility. The neck was therefore actually rather inflexible so plesiosaurs could also not adopt the often depicted swan-like pose (Storrs, 1993).