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Earth Notes: Arthropod Tracks

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Anne Miller
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Arthropod tracks in the Grand Canyon

The Grand Canyon is full of footprints, not all of them made by people or four-footed animals. Divots in solid rock show where ancient arthropods walked, long ago.

Modern arthropods include scorpions, crabs, spiders, centipedes, and all kinds of insects. They have no backbone and a hard exoskeleton. They’re among the first creatures to evolve on Earth. That happened during the Cambrian Explosion, more than five hundred million years ago. The land was barren and empty at the time, but the oceans were full of life.

Many arthropods left no imprint of their bodies when they died, so it’s difficult to learn about them. That’s why “trace fossils” are important. The Bright Angel Shale at the bottom of the Grand Canyon is full of tracks, burrows, and other trace fossils that reveal how the earliest creatures lived their lives.

One arthropod with a long, inflexible body and at least fifty legs left tiny footprints in the rock. Like a semitruck on a narrow street, it had trouble making turns. The tracks show a 5-point turn where it tried to reverse direction. That was the clue paleontologists needed to identify it as a rare arthropod that could grow up to a foot long. Only two specimens exist, both discovered in Canada. No sign of it had been found in the Grand Canyon before. The arthropod may have been walking along the ocean floor in search of food when it found itself in a tight corner.

The creature has long since gone extinct. Lucky for science, the mark it made on the world remains.

Melissa joined KNAU's team in 2015 to report on science, health, and the environment. Her work has appeared nationally on NPR and been featured on Science Friday. She grew up in Tucson, Arizona, where she fell in love with the ecology and geology of the Sonoran desert.
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