By Sid Perkins
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.
Much scientific attention has focused on the water-to-land transition that vertebrates made between 385 million and 376 million years ago (SN: 6/17/06, p. 379; 1/31/09, p. 30). But by that era, another group of creatures — arthropods, the group that today includes crustaceans, scorpions and insects — had been strolling around on land for more than 115 million years, notes James W. Hagadorn, a paleontologist at Amherst College in Massachusetts.
Ocean-dwelling arthropods would have been well equipped to survive on land, says Hagadorn. Their tough yet flexible exoskeleton would have kept the creatures from drying out quickly and provided gravity-defying strength to their legs. Despite these advantages, the creatures would have had to keep their gills moist during occasional forays on land, possibly for food, he adds. Now, he and Yale University paleontologist Adolf Seilacher suggest in the April Geology that some early arthropods sported shells to survive a lengthy excursion onshore.