| Named By: | Miller in 1974 |
| Time Period: | Ordovician |
| Location: | USA |
| Size: | About 1.2 meters long from the head to the tip of the telson (final segment of the tail) |
| Diet: | Carnivore |
| Fossil(s): | Many individuals, but often of partial remains |
| Classification: | | Arthropoda | Merostomata | Eurypterida | Megalograptoidea | Megalograptidae | |
Megalograptus (Greek for "big writing") is a 4-foot-long (1.2 m) Ordovician eurypterid (sea scorpion), and was among the earliest known genera (and was a member of the family Megalograptidae). The generic name is derived from the fact that its first fossils were of its very spiny legs, which were mistaken for massive graptolites. It lived from 460 to 445 mya. Megalograptus preyed on fish, trilobites, other sea scorpions, and smaller orthocones, using the spines on their claws to feel for their prey hiding in the sand and mud. Adult Cameroceras, along with the bigger sea scorpion species, preyed on it. Megalograptus did not have a stinger, but may have curled its tail and sword-like telson forward as a threat pose, much like a scorpion.