The broad paddle, Eurypterus (1825)

Phylum : Arthropoda
Class : Merostomata
Order : Eurypterida
Superfamily : Eurypteroidea
Family : Eurypteridae
Genus : Eurypterus
Species : E. cephalaspis, E. dekayi, E. flintstonensis, E. hankeni, E. henningsmoeni, E. laculats, E. lacustris, E. leopoldi, E. megalops, E. minor, E. ornatus, E. pittfordensis, E. quebecensis, E. remipes, E. tetragonophthalmus

  • Silurian (432 - 418 Ma)
  • 23 cm long (size)
  • Oceans worldwide (map)

Eurypterus belongs to the suborder Eurypterina, eurypterids in which the sixth appendage had developed a broad swimming paddle remarkably similar to modern-day swimming crabs. These appendages could only be moved in near-horizontal planes (i.e. they can not be bent much upwards or downwards). They are generally thought to utilize a rowing type of locomotion. The paddles are almost vertically oriented on the backward and down stroke, pushing the animal forward and lifting it up. They are then oriented horizontally on the recovery stroke to slash through the water without pushing the animal back. This type of swimming is exhibited by crabs and water beetles.

However, larger individuals may have been capable of underwater flying (or subaqueous flight), in which the sinuous motions and shape of the paddles themselves acting as hydrofoils are enough to generate lift. This type is similar to that found in sea turtles and sea lions. It has a relatively slower acceleration rate than the rowing type, especially since adults have proportionally smaller paddles than juveniles. But since the larger sizes of adults mean a higher drag coefficient, using this type of propulsion is more energy-efficient.

Juveniles probably swam using the rowing type, the rapid acceleration afforded by this propulsion is more suited for quickly escaping predators. A small 16.5 cm Eurypterus could achieve two and a half body lengths per second immediately. Larger adults, meanwhile, probably swam with the subaqueous flight type. The maximum velocity of adults when cruising would have been 3 to 4 m per second, slightly faster than turtles and sea otters.

Eurypterus did not swim to hunt, rather they simply swam in order to move from one feeding site to another quickly. Most of the time they walked on the substrate with their legs (including their swimming leg). They were generalist species, equally likely to engage in predation or scavenging. They hunted small soft-bodied invertebrates like worms. They utilized the mass of spines on their front appendages to both kill and hold them while they used their chelicerae to rip off pieces small enough to swallow. Young individuals may also have fallen prey to cannibalism by larger adults.

Eurypterus were most probably marine animals, as their remains are mostly found in intertidal shallow environments. The concentrations of Eurypterus fossils in certain sites has been interpreted to be a result of mass mating and molting behavior. Juveniles were likely to have inhabited nearshore hypersaline environments, safer from predators, and moved to deeper waters as they grew older and larger. Adults that reach sexual maturity would then migrate en masse to shore areas in order to mate, lay eggs, and molt. Activities that would have made them more vulnerable to predators. This could also explain why a the vast majority of fossils found in such sites are molts and not of actual animals. The same behavior can be seen in modern horseshoe crabs.


Examinations of the respiratory systems of Eurypterus have led many paleontologists to conclude that it was capable of breathing air and walking on land for a short amount of time. Eurypterus had two types of respiratory systems. Its main organs for breathing were the book gills inside the segments of the mesosoma. These structures were supported by semicircular ‘ribs’ and were probably attached near the center of the body, similar to the gills of modern horseshoe crabs. They were protected under platelike appendages (which actually formed the apparent 'belly’ of Eurypterus) known as Blatfüsse. These gills may have also played a role in osmoregulation.

The second system are the Kiemenplatten, also referred to as gill-tracts. These oval-shaped areas within the body wall of the preabdomen. Their surfaces are covered with numerous small spines arranged into hexagonal 'rosettes’. These areas were vascularized, hence the conclusion that they were secondary breathing organs.

The function of the book gills are usually interpreted to be for aquatic breathing, while the Kiemenplatten are supplementary for temporary breathing on land. However, some authors have argued that the two systems alone could not have supported an organism the size of Eurypterus. Both structures might actually have been for breathing air and the true gills (for underwater breathing) of Eurypterus have yet to be discovered.Eurypterus, however, were undoubtedly primarily aquatic.


Juvenile Eurypterus differed from adults in several ways. Their carapaces were narrower and longer (parabolic) in contrast to the trapezoidal carapaces of adults. The eyes are aligned almost laterally but move to a more anterior location during growth. The preabdomen also lengthened, increasing the overall length of the ophisthosoma. The swimming legs also became narrower and the telsons shorter and broader (though in E. tetragonophthalmus and E. henningsmoeni the telsons changed from being angular in juveniles to larger and more rounded in adults). All these changes are believed to be a result of the respiratory and reproductive requirements of adults.