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Eurypterid Overview: Sea Scorpion Facts and Information

Eurypterids, or sea scorpions, were an extinct order of aquatic arthropods that lived during the Paleozoic Era. They appeared in the Ordovician, became especially abundant during the Silurian and Devonian, and disappeared during the end-Permian mass extinction. Although their nickname makes them sound like marine scorpions, they were not true scorpions. They were chelicerates, part of the broader group that includes horseshoe crabs, spiders, and scorpions.

Most eurypterids lived in shallow marine, brackish, or freshwater environments. Their bodies were made of many segments and jointed appendages, and some genera had paddle-like swimming legs. Many were small, but a few giant pterygotids grew to extraordinary sizes. These large forms were among the biggest arthropods to ever exist.

Complete eurypterid fossils are rare, but certain Silurian deposits in New York and Ontario preserve them exceptionally well. The most famous species is Eurypterus remipes, the New York State Fossil. Because many eurypterid species were originally named from fragments, the taxonomy of the group has been revised many times, and some older names are now considered doubtful or synonymous with better-known forms.

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Sea Scorpions vs. Real Scorpions

Sea scorpions were not true scorpions. The nickname comes from their overall appearance, especially the segmented body and tail-like telson seen in many genera. True scorpions are arachnids, while eurypterids were a separate extinct lineage of chelicerate arthropods.

True scorpions also appeared during the Silurian, and the earliest forms may have been aquatic or semi-aquatic. In some deposits, fossil scorpions occur alongside eurypterids, which shows that these two groups overlapped in Paleozoic ecosystems even though they were not the same animals.

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Fossil Occurrence of Eurypterids

Eurypterid fossils have a broad geographic distribution, but they are often preserved only as fragments. The best complete specimens generally come from unusual environments where molts or bodies accumulated and were rapidly buried. One of the most famous eurypterid-bearing units is the Upper Silurian Bertie Group of New York and Ontario.

Within the Bertie Group, several eurypterid-bearing areas are known, including the Otisville Basin, the Pittsford Pool, the Herkimer Pool, and the Buffalo Pool. The Herkimer and Buffalo pools are especially famous because they preserve abundant and detailed eurypterid fossils.

These deposits are often interpreted as shallow tidal flats, salt flats, or restricted coastal environments. Eurypterid molts and bodies accumulated in shallow depressions called windrows. To find specimens, collectors look for natural fractures running through these windrows and split the dolostone carefully to reveal the fossils.

Some of the finest eurypterid fossils in the world have come from Allan Langheinrich's Quarry, also known as Lang's Quarry, which sits on the Herkimer Pool.

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Eurypterid Body Morphology

Eurypterids have a distinct segmented body plan. The body is divided into three main regions: the prosoma, mesosoma, and metasoma. The prosoma is the head region and includes the eyes, mouthparts, and appendages. The mesosoma is the main body section made of articulated segments called tergites. The metasoma is the rear body section and ends in the telson, or tail.

The telson varies by genus. In some eurypterids it is pointed and stinger-like, while in others it is broad and paddle-shaped. Some species, such as Eurypterus remipes, had four pairs of walking legs, one pair of swimming legs, and a narrow telson. Other groups developed large grasping claws or specialized paddles.

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Eurypterid Trackways: Including One of the World's Largest Sea Scorpion Tracks

Although complete eurypterid body fossils are rare, fossil trackways provide another line of evidence for how these animals moved. Eurypterid trackways have been found in several regions, including Early Devonian tidal deposits in Germany, Carboniferous rocks of Scotland, and Paleozoic deposits in the United States.

The trackway shown here was excavated from a Carboniferous outcrop in Elk County, Pennsylvania. It was discovered in 1983 and is one of the largest eurypterid trackways known. The trackway has been assigned to Palmichnium kosinskiorum and may have been made by an animal approximately 7.5 feet long. It is now displayed at the Carnegie Museum of Natural History.

Eurypterid trackways usually show sets of paired leg impressions on both sides of a central tail drag or body trace. These tracks were probably made in very shallow water or on wet sediment along the margins of aquatic environments.

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Pterygotids: Giant Sea Scorpions, Predators, and Scavengers

Pterygotids were among the largest sea scorpions. Some reached more than 2 meters in length, and Jaekelopterus may have grown even larger. With robust mouthparts, large claws, forward-facing eyes, and enormous size, pterygotids were long imagined as top arthropod predators of Paleozoic waters.

Recent research has complicated that picture. Anderson and colleagues studied the eyes of giant pterygotid eurypterids and compared their visual capabilities with those of other eurypterids and modern arthropods. Their results suggested that giant pterygotids may have had poorer visual acuity than expected for active visual predators.

This does not mean they were harmless, but it suggests some giant forms may have been nocturnal scavengers or opportunistic feeders rather than fast pursuit predators. Like many extinct animals, their ecology was probably more complicated than the monster-like reconstructions suggest.

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Eurypterid Fossil Examples

Eurypterus remipes, Eurypterus lacustris, and Pterygotus sp.

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Eurypterus remipes (DeKay, 1825)

Eurypterus remipes is one of the more common eurypterids found in New York and is the official New York State Fossil.

Formation: Bertie Dolostone of the Fiddler's Green Formation, Phelps Waterlime
Age: Upper Silurian, about 410 million years old
Location: Herkimer County, New York
Size: 3 3/8 inches (86 mm)
Trip: New York Eurypterid Trip

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Formation: Bertie Dolostone of the Fiddler's Green Formation, Phelps Waterlime
Age: Upper Silurian, about 410 million years old
Location: Herkimer County, New York
Size: 3 1/8 inches (79 mm)
Trip: New York Eurypterid Trip

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Formation: Bertie Dolostone of the Fiddler's Green Formation, Phelps Waterlime
Age: Upper Silurian, about 410 million years old
Location: Herkimer County, New York
Size: 2 1/4 inches (57 mm)
Trip: New York Eurypterid Trip

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Formation: Bertie Dolostone of the Fiddler's Green Formation, Phelps Waterlime
Age: Upper Silurian, about 410 million years old
Location: Herkimer County, New York
Size: 3/4 inch (19 mm)
Trip: New York Eurypterid Trip

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Formation: Bertie Dolostone of the Fiddler's Green Formation, Phelps Waterlime
Age: Upper Silurian, about 410 million years old
Location: Herkimer County, New York
Size: 1 1/4 inches (32 mm)
Trip: New York Eurypterid Trip

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Eurypterus lacustris (Harlan)

Eurypterus lacustris may be synonymous with E. remipes or represent a closely related form or subspecies.

Formation: Bertie Dolostone of the Fiddler's Green Formation, Phelps Waterlime
Age: Upper Silurian, about 410 million years old
Location: Ridgemount (Walker Brothers) Quarry, Fort Erie, Ontario
Size: 1/2 inch (13 mm)
Trip: Ontario Trip

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Pterygotus sp.

Pterygotus looks different from many eurypterids. One notable difference is the telson: instead of a narrow stinger-like telson, it had a broader swimming-paddle-like form. Pterygotids were also among the largest eurypterids, with some species reaching giant sizes.

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Links to Eurypterid Sites

Langs Fossils
The Langheinrich Fossil Preserve is dedicated to the preservation and study of Eurypterus remipes and the eurypterid-bearing deposits of New York.

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References / Works Cited

Anderson, R. P., McCoy, V. E., McNamara, M. E., & Briggs, D. E. G. (2014). What big eyes you have: the ecological role of giant pterygotid eurypterids. Biology Letters, 10(7), 20140412. doi:10.1098/rsbl.2014.0412

Poschmann, M., & Braddy, S. J. (2010). Eurypterid trackways from Early Devonian tidal facies of Alken an der Mosel (Rheinisches Schiefergebirge, Germany). Palaeobiodiversity and Palaeoenvironments, 90, 111-124.

Waddington, J., Rudkin, D. M., & Dunlop, J. A. (2015). A new mid-Silurian aquatic scorpion—one step closer to land? Biology Letters, 11(1), 20140815. doi:10.1098/rsbl.2014.0815

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