This is one beautiful plesiosaur, Polycotylus latippinus.
The unique aspect of this specimen is that it’s the only pregnant plesiosaur found; the fore and hind limbs bracket a jumble of bones from a juvenile or embryonic Polycotylus. It’s thought to actually be a fetal plesiosaur, rather than an overstuffed cannibal plesiosaur, because 1) the smaller skeleton is still partially articulated, and it’s large enough that it is unlikely it could have been swallowed whole, 2) the two sets are of the same distinctive species, 3) the juvenile is incompletely ossified and doesn’t resemble a post-partum animal, 4) the bones aren’t chewed, etched by acids, or accompanied by gastroliths. I think we can now confidently say that plesiosaurs were viviparous, which is what everyone expected.
There are other surprising details. The fetus is huge relative to the parent, and there’s only one — so plesiosaurs had small brood sizes and invested heavily in their offspring.
The authors speculate beyond this a bit, but it’s all reasonable speculation. That degree of parental investment in fetal development makes it likely that there would have been extended maternal care after birth, and rather more tenuously, that they may also have lived in larger social groups. The authors suggest that their lifestyle may have resembled that of modern social marine mammals — picture a pod of dolphins, only long-necked and lizardy.
O’Keefe FR, Chiappe LM (2011) Viviparity and K-Selected Life History in a Mesozoic Marine Plesiosaur (Reptilia, Sauropterygia) Science 333 (6044): 870-873.
(Also on FtB)