Megalograptus was an Odrovician eurypterid that was about 4' long and had long, spiny "arms" that were probably for catching prey. Megalograptus could have preyed upon animals such as small orthocones, trilobites, early fish, conodonts, and possibly small echinoderms. It was probably prey for giant orthocones, such as Cameroceras and Endoceras. Megalograptus appeared in the Ordovician Period, and died out in the Ordovician mass extinction.
Megalograptus had an odd telson, or tail, with two semi-circular plates on the side and a spine in the middle. The spine probably did not carry venom, but I believe that it may have been used to make Megalograptus appear to be venomous, which would warn off predators. The two semi-circular plates on Megalograptus's telson may have been used together, as a paddle to help it swim. Megalograptus also had paddles on a pair of legs that came off of the head, which I believe helped it steer.
The first fossils of Megalograptus discovered were its appendages, so at first scientists thought the appendages were giant graptolite shells, which explains the name "Megalograptus," which means "great graptolite" or "big writing" (a lot of graptolite fossils resemble scribbles, lines, or writing). But Megalograptus was actually a eurypterid, or sea scorpion.
Megalograptus is thought to be rare. But since it is found around common fossils such as brachiopods, trilobites, echinoderms, crinoids, and other creatures, it may have been more common. Megalograptus might only seem rare because the exoskeleton of most Megalograptus did not fossilize.