Fossil Detectives Close the Case on Prehistoric Spider Footprints

These are footprints left behind when a tarantula-size arachnid crawled over the sand of what’s now northern Arizona, 260 million years ago. We don’t know where it was going or what it was doing, but each time the critter pressed one of its eight feet into the ground, it left a tiny, cup-shaped imprint in the dewy grains.

The sand’s moisture helped preserve the imprints of the creature’s journey: As the sand dried out, the delicate prints solidified. Over millennia, layers of sand covered the arachnid’s footwork and hardened into rock. Then, in 1968, the rock and its footprints were pulled from the Arizona desert; it now lives in The Raymond M. Alf Museum of Paleontology, where visitors can stop by and see the footsteps of an ancient spider.

Well, it’s probably a spider. There are many clues that suggest as much, though the organism responsible for these tracks, which span 2.5 inches, is long gone.

“You just don’t have fossil spiders from this part of the world,” says Alf museum curator Andrew Farke. “They’re too squishy to preserve very well.”

Deciphering the traces left by organisms – whether footprints, tracks, trails, burrows – is the realm of a discipline known as ichnology. Basically, ichnology is what you would get if you mixed Sherlock Holmes with felonious fossils. Ichnologists look at the shapes of tracks, their distribution, and other clues to learn more about the animals and the environment at the time the tracks were made.

We didn’t always know what spider footprints looked like. When paleontologist Raymond Alf (the museum’s namesake) retrieved the fossil in 1968, he did some experimenting to determine whether the eight-legged footprints were the work of a spider or a scorpion or something else. “He got some spiders and inked up their little legs on an inkpad and had them run across paper,” Farke says. “In his opinion, these things were fairly close matches for a spider.”

Then, two decades ago, geologist Christa Sadler redid the experiment. She set up a 4-meter long sandy runway with a slight hill in the middle. She sprayed water on some of the sand and kept the rest dry. Then she released tarantulas and scorpions onto the runway. She varied the speeds at which they could traverse the course by introducing a headwind, and recorded how their footprints changed with speed, angle, and moisture. In the end, she reached the same conclusion about this fossil: It most closely resembles the footwork of a tarantula.

But there’s still a lot to learn about spider footprints — especially those belonging to prehistoric arachnids.

We checked in with Emory University ichnologist Anthony Martin, to see if modern fossil forensics has anything to tell us about this fossil.

He thinks Alf and Sadler were on the right track, so to speak. When most people look at this fossil, they might immediately think the tracks were made by something with four toes, but Martin’s trained eyes see something else: The unmistakable imprints of an eight-legged creature, one with an “out of phase, alternating” gait. In other words, the creature didn’t hop.

“The legs on each side of the spider are moving at different times,” he says. “This is a very typical pattern with terrestrial arthropods.”

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