It was one small step for arthropods, one giant leap for land animals. Scientists taking a closer look at ancient arthropod footprints say that sandstone quarries of southern Ontario, Canada, mark a momentous event in the history of life--the first known steps on dry land.
Five hundred million years ago, Earth's oceans teemed with life, including a number of large, armored arthropods. These many-legged ancestors of today's insects and millipedes scurried along the muddy bottom of a sea that covered much of eastern North America. Yet the rock and soil just a few meters past the surf remained an open frontier, a moonscape that supported nothing more complex than mats of algae.
Scientists have long assumed that the Ontario trackways were merely prints left behind by animals moving underwater. But upon closer inspection, a team led by Robert MacNaughton of the Geological Survey of Canada found evidence that the sandstone slabs started out as terra firma. The surfaces bear ripples from wind as well as warty marks characteristic of sand blown across a slightly damp surface. The tracks consist of mirror-image lines of dimples, some separated by a midline groove. This symmetry shows that the animals placed both limbs of an opposing pair on the ground at the same time. Such a gait is most common among aquatic animals, but the mounds of sand displaced by each step suggests that the creatures were not buoyed by water at the time.
The scientists suspect that the trackways were made by lobster-sized arthropods known as euthycarcinoids, animals that walked on eight legs and dragged a spiked tail. The trackways capture their forays into a dune field on the shore of an ancient sea, the team concludes in the May issue of Geology. That pushes back the date of the first beach footfall by as many as 40 million years.
Paleontologist and arthropod trackway expert Ken McNamara of the Western Australian Museum in Perth says that researchers had been mystified about why, according to earlier estimates, arthropods took so long to venture onto land. "They were so common back then, they're beautifully adapted to [being terrestrial], with armor, bodies that can support their weight on land, and internal gills. Maybe the reality is, they didn't take long after all."
Arthropod Trackways 
Introduction to Ichnology, the study of plant and animal traces