Paleobiology and Paleoecology
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 (6.5 in) 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 (9.8 to 13 ft) 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.
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