Did "Hermit" Sea Creature Hide Under Borrowed Shells in First Forays Onto Land?

By Eliza Strickland | April 13, 2009 1:35 pm

tracks CambrianThe 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].

Related Content:
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DISCOVER: A Secret History of Life on Land tracks the evolutionary history of invertebrates

Image: James W. Hagadorn

CATEGORIZED UNDER: Living World
MORE ABOUT: evolution, fossils, ocean
  • Kenneth C Gass

    New fossil and experimental evidence has now been announced that validates Hagadorn’s own scepticism about the hermit theory. Joseph Collette, James Hagadorn and I published our findings in the May issue of the Journal of Paleontology. The new evidence indicates that the animal responsible for making the shingled imprints was capable of doing so with its own tail, without the help of a mollusk shell. Instead of being the oldest fossil evidence of hermit behavior, these fossils now seem more likely to be the oldest evidence of mating behavior in the fossil record. The peculiar, shingled imprints may have been produced by the female’s tail held to the side to prevent its interference with fertilization. It is suggested that the male of the pair produced the observed, repeated “notches” with its own tail as he fertilized the eggs while clasped onto the female, much in the same way as modern horseshoe crabs mate. The new evidence explains these unusual fossils without the difficulties of the hermit theory, such as the lack of fossil evidence that a shell capable of producing the shingled imprints had evolved by that time.

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