If there’s any animal that in my mind is the hallmark of the Silurian, it’s the eurypterids. The name means “broad wing,” for the paddle-like appendages they used to swim. They’re called sea scorpions – and they do look sort of like scorpions – and like scorpions they are arthropods, the group that includes insects, crabs, and trilobites. But they are most closely related to modern horseshoe crabs and ultimately to spiders rather than scorpions.
Eurypterids were predators, and with the largest reaching a length of two and a half meters – more than eight feet, the largest arthropods that ever lived – they were probably at the top of the food chain during the Silurian Period. A more typical size would be 20 centimeters, or 8 inches, but they were still formidable attackers in shallow waters, both in the oceans and in lakes.
More than 240 species of eurypterids have been described, and all are extinct. The group got started in the late Ordovician, managed to survive the end Ordovician extinction, and proliferated during the Silurian. They are so common in Silurian rocks of New York that they became the state fossil of New York in 1984.
Eurypterids had strong legs – strong enough to possibly scuttle around on land, like crabs today, but these were fundamentally aquatic critters. Many varieties had a long, sharp, pointed tail, called a telson, which might or might not have been used to inject venom into victims. There’s no hard evidence for that.
Eurypterids survived until the extinction at the end of the Permian, giving them a range of about 210 million years. They declined in diversity after the Silurian.
—Richard I. Gibson
Reconstruction from Ernst Haeckel's Kunstformen der Natur (1904; public domain).
Photo of one of the largest eurypterids