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About 460 million years ago, a sea scorpion about the size of an adult human swam around in the prehistoric waters that covered modern-day Iowa, likely dining on bivalves and squishy eel-like creatures, a new study finds.
The ancient sea scorpions are eurypterids, a type of arthropod that is closely related to modern arachnids and horseshoe crabs. The findings — which include at least 20 specimens — are the oldest eurypterid fossils on record by about 9 million years, said study lead researcher James Lamsdell, a postdoctoral associate of paleontology at Yale University.
The findings are also the largest known eurypterids from the Ordovician period, which began approximately 488 million years ago and ended 443.7 million years ago. The sea creatures measured up to 5.6 feet (1.7 meters) long.
Researchers dubbed the newfound species Pentecopterus decorahensis, named for Greek warships (penteconter) and the Greek word for wings (pterus) because the sea scorpion was likely a top predator that sped through the water, the researchers said. The species name also honors the Iowa city of Decorah, where the fossils were uncovered.
"The best way to describe this animal is bizarre," Lamsdell told Live Science. "For a long time, I had trouble being sure that this was one species because there are so many strange things about it."
Paddle-shaped limbs
An analysis showed that P. decorahensis had specialized limbs that developed as it aged. Its rear limbs are shaped like paddles with joints that appear to be locked in, suggesting that the predator used them as paddles to swim or dig, the researchers said.
Its second and third pairs of limbs were likely angled forward, which suggests they helped the ancient arthropod grab prey. Moreover, the three back pairs of limbs are shorter than the front pair, indicating that P. decorahensis walked on six legs instead of eight.
Interestingly, juveniles had different spines on their legs than adults did.
"It looks like the juveniles would have behaved more like horseshoe crabs, sort of walked around on the seafloor, grubbing in the mud, just eating worms or whatever they could find," Lamsdell said.
With age, their back legs shrank and probably helped the eurypterids balance while swimming. The front legs grew, as did the sharp spines growing on them, "and they could have been used for catching larger prey," Lamsdell said.