The first sea creatures to venture onto land may have been temporary visitors who protected themselves on the dangerous trip with borrowed armor, according to a new study. Fossil tracks discovered on an ancient Cambrian-period beachhead suggest an intrepid group of aquatic scorpion-like creatures commandeered empty mollusk shells, much like modern day hermit crabs. Researchers think they used the shells as protection against the harsh dry air, and stole ashore under cover of darkness to graze on mats of algae exposed during low tide [Discovery News].
Much scientific attention has focused on the water-to-land transition that vertebrates made between 385 million and 376 million years ago…. 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 [Science News], notes lead researcher James Hagadorn. The tracks he studied, which date from about 500 million years ago, appear to have been made by a many-legged arthropod distantly related to scorpions and horseshoe crabs.
In the new study, published in the journal Geology, Hagadorn and a colleague came to the conclusion that the early arthropod borrowed a shell based on the tracks it left in the primordial sand. Some of the tracks show odd markings along their left side, as if the animals had bent tails that dragged to one side. [The researchers] now report that these tracks are very similar to the distinctive ones left by a hermit crab carrying a coiled shell [New Scientist]. Carrying a shell onto land could have kept the adventurous arthropod from drying out, as trapped seawater inside the shell would have kept its gills moist. It would have also shielded the critter from temperature fluctuations and ultraviolet light.
Hagadorn admits the notion of innovative hermit arthropods crawling in the open 500 million years ago is speculative. And all he and [his colleague Adolf] Seilacher have to go on right now are a series of tracks — no body fossils have been discovered. “I am confident the animal that made these tracks was an arthropod. I am not at all confident it was a hermit arthropod,” Hagadorn said. “It’s the best idea we’ve got, though. It’s kind of like solving a crime without the murder weapon or DNA evidence” [Discovery News].
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Image: James W. Hagadorn