Life on land is so challenging that many land vertebrates have taken the plunge and gone back to the sea. From porpoises to placodonts, pliosaurs to pinnipeds, the examples are legion. But life in the sea poses a unique problem for a returning land vertebrate — salt.

A marine vertebrate with a terrestrial ancestry has body fluids that are much less salty than sea water, so an unprotected vertebrate immersed in the sea runs the risk of dehydration. Water can be replaced by drinking sea water. This raises the saltiness of the body with respect to the sea, so there can be no net gain of water unless the salts are excreted in a solution at least as concentrated as that of sea water. The kidneys of marine reptiles, such as turtles, do not have this concentrating power: without other methods of voiding excess salt, a marine reptile cannot eat salty food or drink sea water without becoming dehydrated in the midst of plenty.

Marine life is simply unliveable unless an animal solves the salt problem. This 200-millimetre-long, 110-million-year-old fossil marine turtle, described by Ren Hirayama on page 705 of this issue (Nature 392, 705–708; 1998), has taken this lesson on board. The creature, which comes from the Lower Cretaceous Santana Formation of north-east Brazil, extends the fossil record of marine turtles back ten million years. The turtle is primitive in the sense that the bones in its wrists, ankles and digits have not become consolidated into rigid paddles. In other words, its feet would have looked more like those of freshwater terrapins than fully seagoing turtles.

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Figure 1

Its salt-excreting arrangements were, in contrast, far less haphazard. Marine turtles have lachrymal glands (each one larger, in some cases, than the brain) modified to excrete a concentrated salt solution. The skull of the Santana turtle shows evidence that it had enormous salt glands around the eyes. In which case, the evolution of salt-excreting glands preceded that of rigid paddles. These salt glands were the turtles' passport back to the sea.