An interesting collection of fossils from Herkimer County, New York (but on display at the Denver Museum of Nature and Science):
Eurypterids, or “Sea Scorpions” are an extinct order of arthropods (related to the modern king crab) that reached their peak in the Silurian, from about 432 to 418 million years ago. They were formidable predators for their time, but like many species, went extinct in the great Permian mass extinction.
Most Eurypterids were under 10″ (25 cm) in length, but some were as long as a tall human and were the largest arthropods ever. Eurypterid fossils are found in a wide variety of places, but usually just as fragments. For well preserved full fossils, you need to go to New York state or Ontario (understandably then, Eurypterus remipes is the state fossil of New York). The fossils in this image came from the Fiddler’s Green Formation in Herkimer County, New York.