Fossils of the actual remains of eurypterids (i.e. their carcasses) are relatively rare. Typical of large eurypterids is a lightweight build. "Pterygotus" was among the largest eurypterids. The fossil record of Ordovician eurypterids is quite poor. This sea came to be inhabited by invertebrates like brachiopods, conodonts, eurypterids, jellyfish, and trilobites. Horseshoe crabs were traditionally grouped with the extinct eurypterids (sea scorpions) as the Merostomata. The pterygotid eurypterids include many of the largest known eurypterids, such as "Pterygotus" and "Acutiramus". Giant eurypterids were not limited to the family Pterygotidae. Today only 11 species can be confidently identified as representing Ordovician eurypterids. In most Silurian fossil beds, eurypterine eurypterids account for 90% of all eurypterids present. They are not applicable to all eurypterids either; stylonurine eurypterids lack swimming leg paddles entirely. Like many other stylonurine eurypterids, "Hibbertopterus" fed through a method called sweep-feeding. The internal classification of eurypterids within the Eurypterida is based mainly on eleven established characters. These were all freshwater animals, rendering the eurypterids extinct in marine environments. Until 1882 no eurypterids were known from before the Silurian.