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SALT GLANDS IN THE FOSSIL CROCODILE METRIORHYNCHUS

The Metriorhynchidae, marine mesosuchian crocodiles from the Jurassic and early Cretaceous, must have possessed an extra-renal mechanism for processing salt just like all living marine reptiles (including some birds). Metriorhynchus superciliosus, a species known from all over Europe, was presumably no different-its other morphological specializations (enlarged paddle-like appendages, fin on the tail) demonstrate that it was well adapted to a marine lifestyle. However, unlike living marine mammals that excrete salt via their kidneys, the comparatively inefficient reptilian renal system necessitates extra-renal salt glands (SchmidtNielsen and Fange, 1958). All extant marine reptiles and many marine birds have evolved cranial salt glands to remove excessive electrolytes (Schmidt-Nielsen and Fange, 1958; Bentley, 1976; Dunson, 1976; Minnich, 1982)-in marine turtles, for example, salt is excreted via lacrimal glands located in the corner of the eye (Dunson, 1976; Reina et al., 2002). The skull of Metriorhynchus superciliosus is anatomically well known (Andrews, 1913; Wenz, 1968, 1970; Adams-Tresman, 1987) but as yet no direct fossil evidence for the position and shape of extra-renal salt glands has been found (Fernindez and Gasparini, 2000). The presence of a pocket in the antorbital region of metriorhynchid skulls has been noted by several authors (Witmer, 1997), including Fernandez and Gasparini (2000) who described a natural cast of the gland itself in this area of the snout in the metriorhynchid Geosaurus araucanensis (Fernandez and Gasparini, 2000; figs. 2, 4). Consistent with FernAndez and Gasparini's (2000) study of Geosaurus, we present the first direct fossil evidence for salt gland morphology in Metriorhynchus superciliosus. We describe two well-preserved skulls of M. superciliosus, from the mid-late Jurassic Oxford Clay Formation of Huntingdonshire, England. Not only do these skulls preserve a welldefined space that could have accommodated a nasal salt gland, but they also preserve a duct leading from this region into the antorbital fenestra. The presence of this duct shows that the nasal gland discharged via the antorbital fenestra, corroborating its function for excretion, most likely for salt. We discuss the presence of a salt gland in Metriorhynchus and comment on its physiological significance.