Millions of years ago, something nightmarish scuttled along the ocean floor, something that we have only recently found any evidence of. And its fossil is monstrous. An ancient, giant sea scorpion, measuring over 1.5 meters (5 feet) long. Its closest modern relatives are the arachnids, such as the house spider.

The fossil dates back 460 million years, which makes it the oldest known sea scorpion (eurypterid). It has been named Pentecopterus decorahensis. Penteconter is an ancient Greek warship, and was chosen as a name because the shape of the sea beast resembles the outline of the ship. It seems fitting that a creature that could strike fear in the hearts of humans now is named after a ship that struck fear in the hearts of ancient soldiers. 

This incredible specimen has been identified using more than 150 fossil fragments. These samples were found in Iowa, U.S., in the upper layer of the Winneshiek Shale. This is where a layer of sand, 27 meters (89 feet) thick, has filled an ancient meteorite impact crater and is now covered by the Upper Iowa River. Lead author of the study James Lamsdell from Yale University told IFLScience that the fossils were "most likely the remains of shed exoskeletons, where the eurypterids came into this sheltered environment to molt." You can see the results in BMC Revolutionary Biology.

In fact, Lamsdell thinks that this shedding explains some of the odder features of the eurypterid. "The elongated front of the head probably increased the space available to the animal to escape the old exoskeleton during molting, when the carapace would separate from the plates on the underside of the head; this would have been important because of the massively enlarged raptorial limbs that it used to capture prey."

Some of the larger body segments indicate that the scorpion's body might have even been 1.7 meters (5 foot 7 inches) long. An average male American man is only a little taller than this at 1.77 meters (5 foot 10 inches) in height. This sea monster may just have been as big as a man!

^{A foot from the sea scorpion specimen. Line is 1cm. James Lamsdell/ Yale University.} 

Some of the fossil's features have helped the scientists postulate what their different functions might have been. Archeology is a subtle art, based on deductions, so these are all assumptions.

For example, the rearmost limbs have a large paddle with a substantial surface area. The joints of this limb appear to be locked, reducing the flexibility of the limb. It is possible that this sea scorpion used these limbs for either swimming or digging.

The second and third pair of limbs are speculated to be angled forwards. This angling would suggest that the underwater beast used them for tasks in front of its face, unlike the paddles. For example, catching prey instead of movement. 

The length of the legs also suggests how the sea scorpion may have walked. It had eight legs, but the back three pairs are shorter than the front pair. This implies that they may have scuttled around on six legs, instead of eight. 

One feature that is noticeably absent is a stinger on the end of its tail. Lamsdell explained to IFLScience, "no sea scorpions had stingers, in fact. Instead, their tails were used for balance and sometimes to help with swimming."

^{Hairs on the sea scorpion sample. Line is 1cm. James Lamsdell/ Yale University.} 

All these possibilities were facilitated by the exceptional preservation of the fossil. You can even see individual bristles called setae on the fossil, as well as details like scales and follicles. 

The rearmost limbs are covered in these setae and they are arranged similarly to modern swimming crabs, which are used to increase the surface area of their paddle to make swimming easier. However, the researchers note that these follicles are smaller than those on modern crabs, which indicates that they may be used for sensing more than as a swimming aid. 

There are also spines on some of the limbs, similar to those found on horseshoe crabs, that are assumed to assist with processing food.