One fossil preserves an adult with a single large fetus inside of it, indicating that Polycotylus gave live birth, an unusual adaptation among reptiles.

Polycotylus is thought to be a basal polycotylid because it has more vertebrae in its neck (a feature that links it with long-necked ancestors) and its humerus has a more primitive shape.

The long ischia of the pelvis are a distinguishing feature of Polycotylus, as are thick teeth with striations on their surfaces, a narrow pterygoid bone on the palate and a low sagittal crest on top of the skull.

Polycotylus and other polycotylids superficially resemble pliosaurs like Liopleurodon and Peloneustes because they have short necks, large heads, and other proportions that differ from true plesiosaurs.

Another newly described polycotylid called Eopolycotylus from Glen Canyon, Utah, was found to be the closest relative of Polycotylus.

Dolichorhynchops Trinacromerum Polycotylus Eopolycotylus A fossil of P. latippinis catalogued LACM 129639 includes an adult individual with a single fetus inside it.

F. Robin O'Keefe, one of the describers of LACM 129639, suggested that the social lives of plesiosaurs were "more similar to those of modern dolphins than other reptiles.

[4] Examination of the fetus of Polycotylus indicates that while in the womb, plesiosaurs sacrificed fetal bone strength for accelerated growth rates.