Pterygotus: Silurian Sea Scorpion

Pterygotus was a gigantic form of eurypterid (sea scorpion) that lived during the Silurian Period. It was also the largest arthropod of all time, growing to astonishing lengths of nearly nine feet. The next largest arthropod to rival pterygotus was the giant proto-millipede arthropleura that lived during the Carboniferous Period. The largest pterygotus specimen was unearthed in New York, but other fossils have been found in Britain.

This arthropod was the top predator of the Silurian seas, and reached its apex at the end of the Silurian and into the early Devonian Period. It had a broad, tapered body that ended with a fan-like tail and probably swam through the oceans in an undulating motion like earlier anomalocarids. They had large compound eyes, gills for breathing dissolved oxygen underwater, as well as four sets of legs, a pair of "flippers," and a pair of pincers at the front of the head. These pincers were unlike modern scorpion pincers (which are the second row of appendages), but served a similar purpose of grasping prey and drawing it in toward the mouth. Small barbs on these large claws would also help in holding onto slippery prey like early fish or cephalopods. Pterygotus probably would settle in a certain area and wait for prey to pass by it before striking. Evidence for this can be seen in the positioning of the arthropod's eyes which face out to the side rather than forward. This would have given pterygotis good peripheral vision for discerning nearby movement but poor binocular vision for chasing and zeroing-in on prey items.

Pterygotus and other eurypterids were probably able to attain their large size in the Silurian due to the high presence of dissolved oxygen in the water as well as a lack of large, vertebrate predators. Fish at this point were still evolving, and the majority did not even have a jaw. It wasn't until this evolutionary milestone proceeded that fish were able to consume larger quantities of food and thus reach larger sizes. In the meantime, the arthropods were the masters of the oceans, forming the top tier of food chains that were beginning to diversify to include all manners of detrivores (most early fish) and intermediate carnivores.

The glory days of pterygotus and other giant eurypterids came to an end in the Devonian Period. During this period, fish would evolve to become the dominant predators in the seas and the giant arthropods could not deal with competition coming from massive, four ton carnivorous fish.