April 27, 2009
Evolutionary scientists have long hypothesized that at some point millions of years ago when the Earth was mostly covered in water, animals living in the oceans left their wet environment and adapted to life on land, paving the way for terrestrial creatures that would evolve years later. How exactly those first animals did so has stumped researchers for years—until now. Amherst geology professor Whitey Hagadorn and Dolf Seilacher, from Yale University’s Department of Geology and Geophysics, believe they have identified the trace fossils, or fossilized tracks, of scorpion-like arthropods that likely appropriated the shells of mollusks to help them make the transition from water to land. A study detailing the duo’s findings was published in the April issue of the journal Geology.
While examining some 500 million year-old marks on sandstone from an area of central Wisconsin that had once been a tidal flat, Hagadorn discovered a curious and unusual pattern that prompted further analysis. In addition to two parallel sets of footprints, the tiny trackways had what looked like the drag impression of a tail. Those latter marks, however, were obliquely segmented, pointed to the left—regardless of which way the creature turned—and not continuous. Such tracks aren’t consistent with the trace fossils of other animals known to exist at the time, so Hagadorn and Seilacher began thinking about what other types of creatures could produce the marks.
A clue came from a modern-day animal: the hermit crab. After studying the tracks of the common beach crustacean and comparing them to the trace fossils they found, Hagadorn and Seilacher believe that the tracks were created by an animal that, like a hermit crab, carried and dragged a coiled shell. But they also hypothesize that the creature’s tail was partly inserted into a shell that had been commandeered from another ocean-inhabiting animal.
“If these markings were produced by a scorpion-like arthropod carrying a shell, that would explain a lot,” said Hagadorn. “A shell would account for the intermittent, tail-like impressions, first and foremost. But it would explain how an animal that had once lived in the ocean would be able to survive on land as well.”
Scientists have yet to ascertain definitively how creatures breathing in wet environments would be able to live in a dry one, because most aquatic animals have to breathe through gills that require moisture to function, Hagadorn continued. What’s more, the ocean protects such creatures from ultraviolet rays, heat and other outside elements, and a new habitat on land would leave them vulnerable to a host of dangers. But a shell would serve as both a protective device and a humid chamber that would facilitate respiration outside of the water. “Because these critters would have been among the first to walk on land, they wouldn’t have had to worry about predators just yet,” he said. “That would enable them to move in between the water and tidal flat—although they probably came out of the ocean at night, like many intertidal hermit crabs.”
As for the reason they would leave the comforts of their home? Given the winding paths of their tracks, they were likely foraging for food, according to Hagadorn. “What this shows is that early animals may not have been as primitive as previously thought,” he added. “Using a shell involves a set of pretty complex behaviors, such as finding the shell in the first place, figuring out how to use it and changing to a new one as the creature outgrows it. To see these activities only 50 million years after the first animals appeared on Earth is just remarkable.”