Fossil Anthropods

Ancient Anthropods Used Borrowed Homes   Live Science - August 23, 2009  

Hermit crabs, a lineage some 200 million years old, may not have been the first to salvage mollusk shells for self-protection. Primitive arthropods were among the earliest animals to venture onto land - 500 million years ago - and they too recycled shells, according to new research.

James W. Hagadorn of Amherst College in Massachusetts and Adolf Seilacher of Yale University analyzed Cambrian-period fossil tracks left on a sand flat by ancient arthropods in what is now central Wisconsin. The small imprints are in remarkably good shape: a microbial mat permeating the beach probably saved them from obliteration by waves.

The tracks resemble those of present-day hermit crabs, complete with intermittent, off-center impressions‹hallmarks of a borrowed shell touching the ground with each step. But the marks suggest that compared with hermit crabs, the ancient trekkers wore their shells upside down. So whereas hermit crabs¹ rear ends curl under toward their bellies, the early arthropods¹ tails must have curled upward, like scorpions¹.

Also in contrast with hermit crabs, the borrowed shells seem to have been too small to cover all. They may have just protected the gills from desiccation, thus overcoming a critical hurdle in the move from sea to land.

After all, guarding against land predators was pointless: there were none. Indeed, the winding tracks suggest a leisurely excursion. The arthropods probably came ashore at low tide to graze upon the same microbial mat that recorded their tracks so well.










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