IT鈥橲 always good to have a bit of help to cross into the unknown. Some of the earliest creatures to crawl out of the ocean onto land half a billion years ago borrowed shells to carry a bit of the sea with them. This allowed them to survive in an otherwise hostile world, much like tanks of compressed air allow people to explore the deep ocean.
Palaeontologist James Hagadorn of Amherst College in Massachusetts has been studying fossilised tracks left in sandstone in central Wisconsin, dating back 490 to 510 million years. At the time, vast sandy tidal flats fluctuated between wet and dry zones, thrusting organisms into new environments.
Hagadorn had determined that several sets of tracks were made by Protichnites, an arthropod with many pairs of legs crawling across the sand and dragging its tail behind. But some of the tracks show odd markings along their left side, as if the animals had bent tails that dragged to one side.
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Hagadorn and Adolf Seilacher of Yale University now report that these tracks are very similar to the distinctive ones left by a hermit crab carrying a coiled shell. They conclude that the unusual tracks must belong to Protichnites that had partly inserted their tails into similar shells in order to carry them on land (Geology, vol 37, p 295).
Hauling shells would have given the critters an advantage. Trapped moisture protected them from drying out and helped keep their gills moist. The shells also shielded the animals from harsh ultraviolet light and protected them from changing temperatures.
Although the behaviour resembles that of a hermit crab, Hagadorn suspects these early explorers were the ancestors of a long-extinct group called sea scorpions, which had 6 to 13 pairs of legs. The tracks suggest the shells probably came from coiled molluscs, but other sources are possible. The hermit-like behaviour obviously had advantages, but the researchers say it is unclear how it came about. 鈥淲e have no idea how this originated or what led to it,鈥� Hagadorn says.
