Arthropods came ashore in shells: gear may have kept gills wet during transition onto land.

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, researchers report in the April Geology. 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 around 380 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 of Amherst College in Massachusetts.

Hagadorn and Adolf Seilacher of Yale University studied arthropod trackways found in 500-million-year-old sandstone in central Wisconsin. Some of the trackways include impressions scraped into the sand on either side of the creature's footprints. If a trailing appendage like a tail had made those impressions, the scrapes would be similar to the tire tracks made by a truck-drawn trailer: extending farther to the right as the creature turned left, and more to the left as the creature turned right, Hagadorn says.

But in the Wisconsin trackways, the scrapes swung farther to the left of the footprints regardless of which direction the creature was turning. The team speculates that the asymmetric impressions were made by a shell that the creature carried on its back like modern-day hermit crabs do. That shell would have provided a humid chamber so the creatures could extend their foraging time on land.

"This is a pretty neat study," says Anthony Martin of Emory University in Atlanta. "That Cambrian arthropods had some sort of behavioral adaptation for coping with times out of water is not so surprising, but for them to be using shells like hermit crabs nearly 500 million years ago is amazing."