All About the Pterygotus Eurypterida: The Sea Scorpion

All About the Eurypterid

Photo: CoreyFord via Getty Images

These ancient creatures resemble swimming versions of living scorpions. The Eurypterida are an extinct Paleozoic group of chelicerate arthropods with over 200 fossil species. They were magnificent animals, though extremely rare as fossils. The largest, such as Pterygotus, could reach 2 meters or more in length, but most species were no longer than 20 cm.

Some forms, such as Mixopterus, resembled scorpions and may have even been ancestral to scorpions. There are about two dozen eurypterid families known.

Description and Appearance

All About the Eurypterid

Photo: CoreyFord via Getty Images

The pterygotid eurypterids are classified into ecological groups based on morphology and visual acuity. The number of lenses in arthropod compound eyes and the interommatidial angle is the primary methods for determining visual acuity. Because it is low in modern active arthropod predators, the IOA is especially important because it can be used to distinguish different ecological roles in arthropods.

Pterygotus anglicus had extremely high visual acuity, as evidenced by a low IOA and many lenses in their compound eyes. Pterygotus' chelicerae were enlarged and robust, with a curved free ramus and denticles of varying lengths and sizes, all of which correspond to solid puncturing and grasping abilities in extant scorpions and crustaceans. Pterygotus' IOA values match those of high-level and active modern predatory arthropods, indicating that they were visual and functional predators.

Cheliceral claws were found on all known pterygotids. The chelicerae's first joint, where it connects to the epistoma (a plate located on the "head"), would have been capable of twisting the entire appendage. This led researchers to conclude that the chelicerae's function would not have been solely or even primarily for defence but rather to capture and convey food to the mouth.

Sea scorpions were also the only eurypterids that could travel across open oceans. The pterygotid eurypterids were the largest arthropods to ever exist. They reached total lengths of more than 8 feet (about 2.5 meters) (about 2.5 meters). With their unique and formidable claws and binocular vision that provided depth perception, the pterygotids would attack and slice into prey such as primitive fish or the ancestors of squids.

Most eurypterids are small enough to fit in your arms. However, one specimen in the Division's collections belonged to a fully grown eurypterid.


Cannibalism was common among sea scorpions, who ate any smaller members of their species they came across. They would have devoured any fish or other aquatic animals that were smaller than themselves. They had large claws with sharp teeth to grab their prey quickly. The scorpions had a firm grip and would have been able to hold even the most slippery prey.

Their morphology suggests that they ate a variety of foods. When captured, prey would need to be cut into smaller pieces to fit into the mouth; eurypterid mouths were even less adapted to devour large pieces than modern crab mouths are. Cutting, transporting, or grasping anything, were tasks for the chelicerae. This is because the eurypterid walking appendages were incapable of doing anything. According to the feeding process observed in modern arthropods with chelicerae, one claw would hold the prey. At the same time, the other cut off pieces and transported them to the mouth with continuous and simple movements.


Understanding sexual dimorphism in extinct organisms is one of many issues that paleontologists face when reconstructing organisms. The researchers are used to classify creatures according to their bones, shells, or carapaces. In some cases, the two sexes of an organism have been interpreted as two distinct species coexisting; in others, there is an insufficient difference to distinguish different sexes. Even when they can be distinguished, it is difficult to determine which is male and female.

The eurypterids are a particularly challenging example of this. As previously stated, there are two types of genital appendages found in any Eurypterid population. According to the original hypothesis, type A is the male sexual organ, with "clasping organs" on the prosomal appendages. In contrast, type B is the female part for depositing eggs and fitting neatly over the male clasper.

However, research indicates that eurypterid reproduction occurred via spermatophore (sperm package) transfer on the substrate. In this case, the type A appendage is a female who can retrieve a spermatophore for storage in her spermathecae ('horn organs'). The 'scimitar lobes' (male) are thought to be clasping structures.


Only the first sea scorpions were marine, despite their name. The majority of them resided in salty water, sheltered lagoons, etc. Many species lived in shallow aquatic environments, and some may have been amphibious, emerging onto land during their life cycle. They may have been able to breathe in both water and air.

Despite their name, sea scorpions did not live solely in the sea. The giants were only found in what is now Germany, but other subspecies were seen worldwide. Smaller sea scorpions would occasionally leave the water to shed their skins and mate. The larger types would have stayed in the water because their legs were too weak to support their bodies on the shore.


Eurypterid fossils have been found on all continents and preserved so well that their exterior structure is the best known of all extinct creatures. 

New York State and southern Ontario, Canada, are currently producing excellent fossils in Silurian rock. Despite being relatively rare, the fossils are well-known for their exceptional preservation. People looking for eurypterid fossils frequently visit Ridgemount Quarry in Fort Erie, Ontario, Canada.

Eurypterids are related to horseshoe crabs and sea scorpions today. There are over two dozen eurypterid families recognized.

The International Union for Conservation of Nature's Red List of Threatened Species designated the American horseshoe crab as vulnerable in 2016 and the Japanese horseshoe crab as endangered in 2018.

Final Thoughts

The eurypterids have been dubbed "sea-scorpions" because to their long tails and spine-like appendages near the tip. They are, in reality, linked to scorpions and other arachnids. Have a feel of Gage Beasley’s Pterygotus Eurypterid Arthropod Soft Stuffed Plush Toy to familiarize yourself with all its features.

Gage Beasley's Pterygotus Eurpyterid Arthropod Soft Stuffed Plush Toy

Pterygotid eurypterids have traditionally been considered active, high-level visual predators. However, recent research on the cheliceral morphology of the pterygotid Acutiramus contradicts this theory.

Pterygotids evolved in response to a shift in ecology from generalized feeders to specialized predators. They have a distinct morphology, but their ecology varies greatly between genera, even though some were top predators.



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