Eurypterids: The Giant Sea Scorpions of the Paleozoic Era

Eurypterids, popularly known as sea scorpions, were a group of aquatic arthropods that lived from roughly 467 to 252 million years ago, spanning most of the Paleozoic Era. Some were small, but others were among the largest arthropods that have ever existed: Jaekelopterus rhenaniae may have reached around 2.5 metres in length. Their fossils, especially from New York State and Scotland, make them one of the best-known groups of ancient invertebrates.

What eurypterids were

Eurypterids belong to the chelicerates, the arthropod group that includes spiders, scorpions, mites and horseshoe crabs. Like their relatives, they had a body divided into a front section (the prosoma), carrying the eyes and six pairs of appendages, and a long, segmented rear section (the opisthosoma), ending in a spine or paddle-shaped tail.

Despite the name, they were not scorpions. More than 250 species have been described, and they were long thought to be distant relatives of true scorpions, though their exact place among chelicerates is still debated.

In many eurypterids the last pair of legs was broadened into paddles used for swimming. Others had spiny front limbs used to sweep or trap prey, and the pterygotids had large forward-facing pincers called chelicerae. They breathed using gill-like structures under the body, and some may also have been able to breathe air for short periods.

Origins and heyday

The oldest described eurypterid is Pentecopterus decorahensis, from the Winneshiek Shale of Iowa, about 467 million years old (Middle Ordovician). Described in 2015, it was already up to about 1.7 metres long, showing that the group was large and specialised from early on.

Eurypterids reached their greatest diversity in the Silurian and Early Devonian, about 440 to 395 million years ago. During this time many moved from the open sea into lagoons, estuaries and fresh water. Their fossils are often found with early jawless fish, which may have been among their prey.

The giants

The best-known giant is Jaekelopterus rhenaniae, from Early Devonian rocks in the Rhineland of Germany, about 390 million years old. In 2007 Markus Poschmann found a single claw (chelicera) about 36.4 centimetres long in a quarry near Prüm. In 2008 he and colleagues estimated that the animal had a body length of about 2.5 metres, with a likely range of 2.3 to 2.6 metres, not counting the claws.

Other large pterygotids include Acutiramus and Pterygotus, which are estimated to have reached roughly 1.6 to 2 metres. These animals were probably active predators, though some researchers have argued from the structure of their claws and eyes that certain pterygotids hunted smaller, softer prey rather than armoured fish.

Not all large eurypterids were swimmers. Hibbertopterus, a broad, heavy animal, may have fed by raking sediment for small prey. A trackway of a large eurypterid about 330 million years old, reported from Fife in Scotland in 2005, shows that such animals could walk on land.

Famous fossil sites

New York State. The Silurian Bertie Group of western and central New York has produced thousands of well-preserved eurypterids, especially Eurypterus remipes and Eurypterus lacustris. Eurypterus remipes was the first eurypterid scientifically described, by the naturalist James E. De Kay in 1825; he did not recognise it as a relative of scorpions. It was designated the New York state fossil in 1984.

Scotland. Silurian rocks at Lesmahagow in Lanarkshire and in the Pentland Hills near Edinburgh have yielded important eurypterids, including Slimonia and pterygotids. Scottish Carboniferous rocks have produced Hibbertopterus and the walking trackway.

Other regions. Eurypterids are also known from Estonia (Saaremaa), the Welsh Borderland, Germany, Ukraine, China, Australia and South America, showing that the group was nearly worldwide.

Myths and facts

A famous fossil from Argentina, described in 1980 as Megarachne servinei, was publicised as a spider with a leg span of about half a metre. When the specimen was re-examined in 2005 by Paul Selden and colleagues, it was identified as a eurypterid, not a spider.

It is also often said that eurypterids grew large because of high oxygen levels. This explanation is more often applied to Carboniferous giant insects such as Meganeura; the largest eurypterids lived in the Silurian and Devonian, and their size may relate more to predation, competition and their aquatic habitat.

Decline and extinction

Eurypterids declined during the Devonian, probably as jawed fish became more diverse and powerful predators. The Late Devonian extinction further reduced their numbers. Only a few groups, mostly slow-moving sweep-feeders in fresh and brackish water, survived through the Carboniferous and Permian.

The last eurypterids disappeared in the end-Permian mass extinction about 252 million years ago.

Life habits

Eurypterids show a wide range of lifestyles. Many, such as Eurypterus, were probably active swimmers that also walked along the bottom, using the paddle-shaped rear limbs for propulsion. They likely hunted small prey such as worms, other arthropods and soft-bodied animals, which they caught with spiny front legs.

The pterygotids, including Jaekelopterus, were adapted for hunting in open water. Their large, forward-facing claws carried rows of teeth, and their compound eyes were large. Studies of eye structure in some pterygotids suggest that not all of them had sharp vision, which has led to debate about whether certain species were fast predators or slower hunters active in murky water.

Another group, the stylonurines, had long walking legs and no swimming paddles. Some, such as Hibbertopterus and its relatives, had broad bodies and front legs fringed with spines, which they may have used to rake through mud or water for small food items, somewhat like a comb.

Like other arthropods, eurypterids grew by moulting their outer skeleton. Many fossils are actually moulted skins rather than dead animals, which helps explain why some sites contain so many specimens. At some localities, large numbers of moults have been interpreted as gatherings for moulting and possibly mating, similar to mass gatherings of modern horseshoe crabs, though this is not certain.

Male and female specimens can sometimes be told apart by the shape of a genital appendage on the underside of the body, and studies of this feature have given insights into how eurypterids reproduced.

Eurypterid fossils are usually found in rocks laid down in calm, often low-oxygen water, such as lagoons and tidal flats, where scavengers and currents were less likely to destroy them. This means the fossil record may over-represent animals from such settings, and our picture of eurypterid habitats is partly shaped by where preservation was favourable.

Timeline

• c. 467 million years ago: Pentecopterus lives in a Middle Ordovician estuary in Iowa.
• c. 440-395 million years ago: Eurypterids reach their greatest diversity.
• c. 390 million years ago: Jaekelopterus lives in the Rhineland region.
• c. 252 million years ago: The last eurypterids die out.
• 1825: James E. De Kay describes Eurypterus remipes from New York.
• 1984: Eurypterus remipes becomes New York’s state fossil.
• 2005: Megarachne is reidentified as a eurypterid.
• 2008: The giant claw of Jaekelopterus is described.

Why it matters

Eurypterids show how arthropods experimented with large size and predatory lifestyles in the early Paleozoic, and how some moved from sea to fresh water and even onto land. Their abundant fossils help geologists understand Silurian and Devonian environments, and their giants remain a vivid symbol of the strange worlds of ancient seas.