Anatomical and Physiological adaptations of the Plesiosaur in respects to extended high pressure diving

Question: How did Plesiosaurs hold their breath for long enough to hunt at great depths in the ocean?

*an attempt to explore the possible anatomical and physiological adaptations that the plesiosaur may have utilised to ensure homeostasis during extended deep sea dives to hunt.*

the saurian family Plesiosauria included both plesiosaurs and pliosaurs, included some of the largest marine reptiles to inhabit the planet between the early jurassic and late cretaceous periods. Ranging in size from 2 to 15, and weighing up to 25 tonnes.
The origin of the plesiosaur is uncertain, though it is believed that the plesiosaur evolved from an intermediate form of Nothosaur (a streamlined fish-eating land reptile that used its forelegs to swim) The plesiosauria family is divided into 2 superfamiles; plesiosauriodea and pliosauroidea, having important differences as well as similar features.
Fossils of the jurassic plesiosaurs (plesiosaurus and cryptoclidus) and pliosaurs (liopleurodon) have been found in Europe, while remains of the cretaceous plesiosaur (elasmosaurs) and pliosaur (kronosaurus) have been discovered in asia/north america and Queensland, Australia respectively.

Members of the plesiosauriodea superfamily are described as generally having long necks and small heads, with forelimbs generally larger than the hindlimbs. The last surviving plesiosaur- elasmosaurus- had a neck as long as both its body and tail combined.

The pliosauroidea superfamily are descibed as having shorter necks but much larger heads, with extremely powerful jaws. The pliosaur Kronosaurus was the largest of these, with jaws 3 meters long and more powerful than that of a tyrannosaurus.

Both superfamilies were efficient swimmers, but it is believed that their hunting styles were varied in accordance with their shape. it is believed that the plesiosauriodea gracefully "flew" through the water in search of prey, laying in wait, being highly maneouverable and able to turn on the spot instantaneously, in ambush of prey with their long necks. in contrast, pliosaurs were very agile long distance swimmers and would pursue much more active prey, hunting by the chase.

as plesiosaurs hunted at great depths, and for long periods of time, they would have had to develop specialised anatomical and physiological traits to withstand the immense pressure and energy debt that such diving put on their bodies.