Megalograptus was a large Eurypterid, otherwise known as Sea Scorpion, of the Ordovician Period, around 470-440 Million Years Ago. It was around 1,2 metres long, with some of the greatest probably reaching nearer to 1.5 metres long. They were marine predators, half way up the food chain - eating smaller Arthropods such as Trilobites, and the tiny and primitive armoured Agathan (Jawless) Fish of the day such as Astraspis; but in turn being eaten by the Giant Orthocone Nautiloid Cephalopod Molluscs such as the 10-12 metre long monster, Cameroceras.

Its name is Greek for 'Big Writing', as its fossils were originally mistaken for Graptolites, very different organisms that lived at the same time but were much more 'plant like' (the claws and spines of the Sea Scorpion, being confused at first with the thecae covered stipes of the Graptolites) Megalograptus was a common genus in its time.