The pliosaurs of the Oxford Clay Formation represent an excellent opportunity for elucidating the problems associated •with secondary adaptation to life in water. As predaceous, marine reptiles, pliosaurs were wonderfully adapted to a marine environment. Streamlining reduced drag, hydrofoil limbs permitted motion through a dense, yet buoyant medium, and an enlarged head mounted on a shortened neck allowed large prey to be tackled. In addition to mechanical and morphological adaptations, physiological constraints, usually hidden from the palaeontologist, no doubt also exerted considerable evolutionary pressures. The exceptionally well preserved fauna of the Peterborough Member, Oxford Clay Formation offers a rare opportunity to infer physiological adaptations to a high electrolyte load in extinct marine reptiles. The Callovian (Uppermost Middle Jurassic; Ci59-i6i million years ago), Peterborough Member preserves an exceptionally rich fossil fauna comprising upwards of 160 benthic and nektonic invertebrates including belemnites, ammonites and naked cephalopods, 32 fish and ii non-pliosaurian marine reptiles. Five genera of pliosaur are currently recognised within the Peterborough Member deposits, representing an important component of Callovian seas. The pliosaurian skull is superficially crocodile-like with an elongated snout, external nostrils close to the orbits, a large, upper temporal fenestra for huge jaw muscles, and an array of teeth including anteriorly enlarged 'caniniforms' (Figure i). The large head, with massive jaw muscles to exert a powerful bite, sharp teeth and powerful neck made pliosaurs top predators in Mesozoic marine food webs. Reconstructions of the three best preserved Callovian pliosaurian predators, Peloneustes, Liopleurodon and Simolestes (Figure i), indicates a range of adult sizes and a variety of cranial morphologies. As the vertebrate skull is primarily built for prey apprehension and manipulation, housing the sense organs, teeth, jaws and jaw muscles, the differences in pliosaur skull morphology can be attributed to divergent feeding strategies. A variety of evidence including skull morphology, tooth form, preserved stomach contents, and comparison to modern analogues indicates Peloneustes (Figure la) was a piscivore with a narrow, elongate skull with widely spaced, relatively sharply pointed, teeth. Liopleurodon (Figure ib) mainly preyed upon large, hard-boned prey, evidenced by a slightly wider skull, and more robust, strongly ornamented and frequently broken teeth. Tooth breakage is a result of coming into contact with the hard bones of prey, although breakage often leaves the teeth sharply pointed. Simolestes (Figure ic) has long been an enigma. In the type specimen (NHM R33I9) the jaws are anteriorly expanded with the teeth splayed out all round, and a distinct constriction behind. A new specimen (PETMG Rzp6) indicates the teeth did not splay out in life, that much of the anterior expansion is due to crushing, and the constriction, present in both jaws, allows the large 'caniniform' teeth from the opposing jaw to pass. Simolestes possesses ahigher, wider skull with relatively larger orbits than either Liopeurodon or Peloneustes, and appears to have almost exclusively preyed upon softer bodied invertebrates. A diet of marine invertebrates invariably imparts a considerable salt load on the predator, compared to vertebrate prey, and unwanted electrolytes must be removed from the body to avoid metabolic complications. Mammals excrete salt from their bodies via the kidneys, but reptile kidneys are not as efficient as those of mammals. Extant marine reptiles and birds (which are descended from reptiles) have cephalic salt glands, to remove excessive electrolytes, hence marine turtles 'cry' salt tears when they come onto land to lay eggs, and marine iguanas sneeze out concentrated salt solution. Salt glands, as soft parts, are not normally fossilisable and usually leave no trace on the skull bones as clues to their presence, however, cephalic salt secreting glands may take up a considerable amount of space inside the skull, especially around the eyes. The skull of Simolestes would have had no more organs than Peloneustes or Liopleurodon, but with a relatively high, wide skull Simolestes has plenty of room in and around the enlarged orbits for cephalic salt glands. Thus exceptional preservation, including presumed stomach contents, and comparison to modern analogues offers a rare insight to the possible physiological problems associated with a diet of marine invertebrates in ancient marine predators. It is proposed the Callovian pliosaur Simolestes vorax possessed cephalic, possibly orbital, salt secreting glands. Additionally, it is suggested a combination of evidence for salt glands is sought in other, secondarily adapted, fossil, marine reptiles which are inferred to have consumed invertebrates.