Between 460 and 248 million years ago, there existed an order of fearsome creatures in the warm waters of Earth: Eurypterida or the sea scorpion. Though, somewhat more frighteningly, the name is a misnomer as sea scorpions lived in both salty seas and freshwater lakes. Of the many species of sea scorpion the largest found was Jaekelopterus at 8’2″ in length.
While Jaekelopterus was certainly enormous, the average species of sea scorpion were less than 8 inches (but thus faster and more agile). Smaller fossils have been found outside bodies of water (or where water used to be). This shows they were able to not only carry themselves across land, but also possibly amphibious (breathing through air and water). The larger species would have been much slower and trapped to the water, unable to lift their weight outside.
Exactly how significant they were during the period is unknown because, like scorpions, they molted. A fossil molt is very difficult to discern from a true fossil, which leaves the question: how many of these fossils were sea scorpions and how many fossils were just their empty exoskeletons?
The most frightening part of the sea scorpion, for most, is the barb on its tail. The long spine on their tail, called the telson, was likely used in predation or defense in some species. However, there hasn’t yet been conclusive evidence of venom in it. It could be the telson had no venom, but either way it’s a relief knowing they aren’t around any longer.