Some of the first creatures to leave the ocean and venture onto land may have done so by carrying a bit of the sea with them: Fossil trackways left on ancient tidal flats 500 million years ago hint that some ocean-dwelling arthropods, like today’s hermit crabs, hauled out onto land wearing shells. Those shells would have protected the creatures’ delicate gills from drying out and may also have held small reservoirs of seawater.
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