Scientists have revealed the oldest known scorpion—and arachnid—on Earth: a mysterious species more than 430 million years old uncovered near Waukesha, Wisconsin, about 29 kilometers west of Milwaukee.
“Anything that pushes the origins of arachnids back further is significant,” wrote paleontologist Jason Dunlop, curator of arachnids at Berlin’s Natural History Museum, in an email to Science. That’s because arachnids are the second most diverse group of animals after insects, and thus could shed light on the origin of spiders, ticks, mites, and modern-day scorpions.
About 450 million years ago, the Waukesha region used to be a warm, shallow ocean. Over time, low oxygen and high salinity preserved the fossils of the animals that used to roam there.
Researchers first unearthed what would turn out to be the oldest scorpion fossils in the early 1980s. But they didn’t know what they had found, and they filed most of the fossils away in drawers at the University of Wisconsin Geology Museum. Even decades later, “We didn’t know we had scorpions,” says Andrew Wendruff, a paleontologist at Otterbein University.
Wendruff and his team began to work with the Waukesha fossils around 2016, while Wendruff was finishing his Ph.D. After going through the entire collection, which includes mainly arthropods and worms, the researchers noticed what looked like two scorpions. The creatures sported seven sections on their thorax—or belly plates—says Paul Selden, a paleontologist at the University of Kansas, Lawrence, who was not involved in the research. Younger scorpion fossils have only six such plates, and today’s scorpions have five.
The scientists also noted that the internal anatomy of the ancient animals was well preserved, which is rare for fossils of this age. When they compared the anatomy of the two fossils to modern-day scorpions, they found striking similarities in the circulatory and respiratory structures. “This suggests that parts of the internal anatomy of scorpions have not changed much in nearly 440 million years,” Dunlop says. The researchers know the ages of fossils ages thanks to other well-dated animal fossils at the site.
One of the big remaining questions is whether P. venator lived in water or on land. Arachnids were among the first animals to become terrestrial, but scientist don’t know whether a single common ancestor came onto land and then branched into the different groups of arachnids we know today, of whether some groups made landfall independently. “There’s been a lot of controversy [over] whether these early scorpions were aquatic or not,” Selden says.
Wendruff and his colleagues argue that because the internal structure of P. venator is so similar to that of modern scorpions, it is very likely it could have lived on land and breathed air. However, because the specimens were found among other marine fossils in a shallow marine deposit near the shoreline, it’s also possible it was aquatic. Neither of the two fossils showed any evidence of ancient gills or lungs, or any other anatomical structure that could decisively reveal its ancient home. “Unfortunately, there’s really no evidence at all to swing it one way or the other,” Selden says.
Wendruff and his team hypothesize that P. venator lived in water but was able to venture onto land, as modern horseshoe crabs do to mate and spawn. According to Dunlop, it is also possible that early scorpions came to land in pursuit of their prey, mainly primitive insects, millipedes, and other arachnids, which also began to appear in the fossil record during this period.
Older, well-preserved scorpion fossils could help settle the debate. “At some point, someone will find an older scorpion,” Wendruff says. “But right now, this is definitely the base of the of the scorpion tree of life.”