Size , Pentecopterus decorahensis, Acutiramus macrophthalmus, A. bohemicus
, Carcinosoma punctatum, and Jaekelopterus rhenaniae'' Eurypterids were highly variable in size, depending on factors such as lifestyle, living environment and
taxonomic affinity. Sizes around are common in most eurypterid groups. The smallest eurypterid,
Alkenopterus burglahrensis, measured just in length. The largest eurypterid, and the largest known arthropod ever to have lived, is
Jaekelopterus rhenaniae. A chelicera from the
Emsian Klerf Formation of Willwerath,
Germany measured in length, but is missing a quarter of its length, suggesting that the full chelicera would have been long. If the proportions between body length and chelicerae match those of its closest relatives, where the ratio between claw size and body length is relatively consistent, the specimen of
Jaekelopterus that possessed the chelicera in question would have measured between , an average , in length. With the chelicerae extended, another meter (3.28 ft) would be added to this length. This estimate exceeds the maximum body size of all other known giant arthropods by almost half a meter (1.64 ft) even if the extended chelicerae are not included. Two other eurypterids have also been estimated to have reached lengths of 2.5 metres;
Erettopterus grandis (closely related to
Jaekelopterus) and
Hibbertopterus wittebergensis, but
E. grandis is very fragmentary and the
H. wittenbergensis size estimate is based on trackway evidence, not fossil remains. The family of
Jaekelopterus, the Pterygotidae, is noted for several unusually large species. Both
Acutiramus, whose largest member
A. bohemicus measured , and
Pterygotus, whose largest species
P. grandidentatus measured , were gigantic. Several different contributing factors to the large size of the pterygotids have been suggested, including courtship behaviour, predation and competition over environmental resources. Giant eurypterids were not limited to the family Pterygotidae. An isolated long fossil metastoma of the carcinosomatoid eurypterid
Carcinosoma punctatum indicates the animal would have reached a length of in life, rivalling the pterygotids in size. Another giant was
Pentecopterus decorahensis, a primitive carcinosomatoid, which is estimated to have reached lengths of . Typical of large eurypterids is a lightweight build. Factors such as locomotion, energy costs in
molting and respiration, as well as the actual physical properties of the
exoskeleton, limit the size that arthropods can reach. A lightweight construction significantly decreases the influence of these factors. Pterygotids were particularly lightweight, with most fossilized large body segments preserving as thin and unmineralized. Lightweight adaptations are present in other giant paleozoic arthropods as well, such as the giant millipede
Arthropleura, and are possibly vital for the evolution of giant size in arthropods. In addition to the lightweight giant eurypterids, some deep-bodied forms in the family Hibbertopteridae were also very large. A carapace from the Carboniferous of Scotland referred to the species
Hibbertoperus scouleri measures wide. As
Hibbertopterus was very wide compared to its length, the animal in question could possibly have measured just short of in length. More robust than the pterygotids, this giant
Hibbertopterus would possibly have rivalled the largest pterygotids in weight, if not surpassed them, and as such be among the heaviest arthropods.
Locomotion The two eurypterid suborders, Eurypterina and
Stylonurina, are distinguished primarily by the morphology of their final pair of appendages. In the Stylonurina, this appendage takes the form of a long and slender walking leg, while in the Eurypterina, the leg is modified and broadened into a swimming paddle. Other than the swimming paddle, the legs of many eurypterines were far too small to do much more than allow them to crawl across the
sea floor. In contrast, a number of stylonurines had elongated and powerful legs that might have allowed them to walk on land (similar to modern
crabs). A
fossil trackway was discovered in Carboniferous-aged fossil deposits of Scotland in 2005. It was attributed to the stylonurine eurypterid
Hibbertopterus due to a matching size (the trackmaker was estimated to have been about long) and inferred leg anatomy. It is the largest terrestrial trackway—measuring long and averaging in width—made by an arthropod found thus far. It is the first record of land locomotion by a eurypterid. The trackway provides evidence that some eurypterids could survive in terrestrial environments, at least for short periods of time, and reveals information about the stylonurine gait. In
Hibbertopterus, as in most eurypterids, the pairs of appendages are different in size (referred to as a heteropodous limb condition). These differently sized pairs would have moved in phase, and the short stride length indicates that
Hibbertopterus crawled with an exceptionally slow speed, at least on land. The large telson was dragged along the ground and left a large central groove behind the animal. Slopes in the tracks at random intervals suggest that the motion was jerky. The gait of smaller stylonurines, such as
Parastylonurus, was probably faster and more precise. The functionality of the eurypterine swimming paddles varied from group to group. In the
Eurypteroidea, the paddles were similar in shape to oars. The condition of the joints in their appendages ensured their paddles could only be moved in near-horizontal planes, not upwards or downwards. Some other groups, such as the Pterygotioidea, would not have possessed this condition and were probably able to swim faster. Most eurypterines are generally agreed to have utilized a rowing type of propulsion similar to that of crabs and
water beetles. Larger individuals may have been capable of underwater flying (or
subaqueous flight) in which the motion and shape of the paddles are enough to generate
lift, similar to the swimming of
sea turtles and
sea lions. This type of movement has a relatively slower acceleration rate than the rowing type, especially since adults have proportionally smaller paddles than juveniles. However, since the larger sizes of adults mean a higher
drag coefficient, using this type of propulsion is more energy-efficient. of
Palmichnium kosinkiorum, containing the largest eurypterid footprints known Some eurypterines, such as
Mixopterus (as inferred from attributed fossil trackways), were not necessarily good swimmers. It likely kept mostly to the bottom, using its swimming paddles for occasional bursts of movements vertically, with the fourth and fifth pairs of appendages positioned backwards to produce minor movement forwards. While walking, it probably used a gait like that of most modern insects. The weight of its long abdomen would have been balanced by two heavy and specialized frontal appendages, and the
center of gravity might have been adjustable by raising and positioning the tail. Preserved fossilized eurypterid trackways tend to be large and heteropodous and often have an associated telson drag mark along the mid-line (as with the Scottish
Hibbertopterus track). Such trackways have been discovered on every continent except for South America. In some places where eurypterid fossil remains are otherwise rare, such as in
South Africa and the rest of the former supercontinent
Gondwana, the discoveries of trackways both predate and outnumber eurypterid body fossils. Eurypterid trackways have been referred to several ichnogenera, most notably
Palmichnium (defined as a series of four tracks often with an associated drag mark in the mid-line), wherein the holotype of the ichnospecies
P. kosinkiorum preserves the largest eurypterid footprints known to date with the found tracks each being about in diameter. Other eurypterid ichnogenera include
Merostomichnites (though it is likely that many specimens actually represent trackways of crustaceans) and
Arcuites (which preserves grooves made by the swimming appendages).
Respiration e present in the
posterior legs of modern
isopods, such as
Oniscus (pictured). In eurypterids, the respiratory organs were located on the ventral body wall (the underside of the opisthosoma). , evolved from opisthosomal appendages, covered the underside and created a gill chamber where the "gill tracts" were located. Depending on the species, the eurypterid gill tract was either triangular or oval in shape and was possibly raised into a cushion-like state. The surface of this gill tract bore several
spinules (small spines), which resulted in an enlarged surface area. It was composed of spongy tissue due to many
invaginations in the structure. Though the is referred to as a "gill tract", it may not necessarily have functioned as actual gills. In other animals, gills are used for oxygen uptake from water and are outgrowths of the body wall. Despite eurypterids clearly being primarily aquatic animals that almost certainly evolved underwater (some eurypterids, such as the pterygotids, would even have been physically unable to walk on land), it is unlikely the "gill tract" contained functional gills when comparing the organ to gills in other invertebrates and even fish. Previous interpretations often identified the eurypterid "gills" as homologous with those of other groups (hence the terminology), with gas exchange occurring within the spongy tract and a pattern of branchio-cardiac and dendritic veins (as in related groups) carrying oxygenated blood into the body. The primary analogy used in previous studies has been horseshoe crabs, though their gill structure and that of eurypterids are remarkably different. In horseshoe crabs, the gills are more complex and composed of many lamellae (plates) which give a larger surface area used for gas exchange. In addition, the gill tract of eurypterids is proportionally much too small to support them if it is analogous to the gills of other groups. To be functional gills, they would have to have been highly efficient and would have required a highly efficient circulatory system. It is considered unlikely, however, that these factors would be enough to explain the large discrepancy between gill tract size and body size. It has been suggested instead that the "gill tract" was an organ for breathing air, perhaps actually being a
lung,
plastron or a
pseudotrachea. Plastrons are organs that some arthropods evolved secondarily to breathe air underwater. This is considered an unlikely explanation since eurypterids had evolved in water from the start and they would not have organs evolved from air-breathing organs present. In addition, plastrons are generally exposed on outer parts of the body while the eurypterid gill tract is located behind the . Instead, among arthropod respiratory organs, the eurypterid gill tracts most closely resemble the pseudotracheae found in modern
isopods. These organs, called pseudotracheae, because of some resemblance to the
tracheae (windpipes) of air-breathing organisms, are lung-like and present within the
pleopods (back legs) of isopods. The structure of the pseudotracheae has been compared to the spongy structure of the eurypterid gill tracts. It is possible the two organs functioned in the same way. Some researchers have suggested that eurypterids may have been adapted to an amphibious lifestyle, using the full gill tract structure as gills and the invaginations within it as pseudotrachea. This mode of life may not have been physiologically possible, however, since water pressure would have forced water into the invaginations leading to
asphyxiation. Furthermore, most eurypterids would have been aquatic their entire lives. No matter how much time was spent on land, organs for respiration in underwater environments must have been present. True gills, expected to have been located within the branchial chamber within the , remain unknown in eurypterids.
Ontogeny l (left) and juvenile (right)
instars of
Strobilopterus (not to scale) Like all arthropods, eurypterids matured and grew through static developmental stages referred to as
instars. These instars were punctuated by periods during which eurypterids went through
ecdysis (molting of the cuticle) after which they underwent rapid and immediate growth. Some arthropods, such as insects and many crustaceans, undergo extreme changes over the course of maturing. Chelicerates, including eurypterids, are in general considered to be direct developers, undergoing no extreme changes after hatching (though extra body segments and extra limbs may be gained over the course of
ontogeny in some lineages, such as
xiphosurans and
sea spiders). Whether eurypterids were true direct developers (with hatchlings more or less being identical to adults) or hemianamorphic direct developers (with extra segments and limbs potentially being added during ontogeny) has been controversial in the past. Hemianamorphic direct development has been observed in many arthropod groups, such as
trilobites,
megacheirans, basal
crustaceans and basal
myriapods. True direct development has on occasion been referred to as a trait unique to
arachnids. There have been few studies on eurypterid ontogeny as there is a general lack of specimens in the fossil record that can confidently be stated to represent juveniles. It is possible that many eurypterid species thought to be distinct actually represent juvenile specimens of other species, with paleontologists rarely considering the influence of ontogeny when describing new species. Studies on a well-preserved fossil assemblage of eurypterids from the
Pragian-aged
Beartooth Butte Formation in
Cottonwood Canyon,
Wyoming, composed of multiple specimens of various developmental stages of eurypterids
Jaekelopterus and
Strobilopterus, revealed that eurypterid ontogeny was more or less parallel and similar to that of extinct and extant xiphosurans, with the largest exception being that eurypterids hatched with a full set of appendages and opisthosomal segments. Eurypterids were thus not hemianamorphic direct developers, but true direct developers like modern arachnids. The most frequently observed change occurring through ontogeny (except for some genera, such as
Eurypterus, which appear to have been static) is the metastoma becoming proportionally less wide. This ontogenetic change has been observed in members of several superfamilies, such as the Eurypteroidea, the Pterygotioidea and the
Moselopteroidea.
Feeding depicted hunting Birkenia'' No fossil gut contents from eurypterids are known, so direct evidence of their diet is lacking. The eurypterid biology is particularly suggestive of a carnivorous lifestyle. Not only were many large (in general, most predators tend to be larger than their prey), but they had
stereoscopic vision (the ability to perceive depth). The legs of many eurypterids were covered in thin spines, used both for locomotion and the gathering of food. In some groups, these spiny appendages became heavily specialized. In some eurypterids in the Carcinosomatoidea, forward-facing appendages were large and possessed enormously elongated spines (as in
Mixopterus and
Megalograptus). In
derived members of the Pterygotioidea, the appendages were completely without spines, but had specialized claws instead. Other eurypterids, lacking these specialized appendages, likely fed in a manner similar to modern horseshoe crabs, by grabbing and shredding food with their appendages before pushing it into their mouth using their chelicerae. Fossils preserving
digestive tracts have been reported from fossils of various eurypterids, among them
Carcinosoma,
Acutiramus and
Eurypterus. Though a potential anal opening has been reported from the telson of a specimen of
Buffalopterus, it is more likely that the
anus was opened through the thin cuticle between the last segment before the telson and the telson itself, as in modern horseshoe crabs. Eurypterid coprolites discovered in deposits of Ordovician age in Ohio containing fragments of a trilobite and eurypterid
Megalograptus ohioensis in association with full specimens of the same eurypterid species have been suggested to represent evidence of
cannibalism. Similar coprolites referred to the species
Lanarkopterus dolichoschelus from the Ordovician of Ohio contain fragments of
jawless fish and fragments of smaller specimens of
Lanarkopterus itself. Though
apex predatory roles would have been limited to the very largest eurypterids, smaller eurypterids were likely formidable predators in their own right just like their larger relatives.
Reproductive biology As in many other entirely extinct groups, understanding and researching the reproduction and sexual dimorphism of eurypterids is difficult, as they are only known from fossilized shells and carapaces. In some cases, there might not be enough apparent differences to separate the sexes based on morphology alone. Sometimes two sexes of the same species have been interpreted as two different species, as was the case with two species of
Drepanopterus (
D. bembycoides and
D. lobatus). The eurypterid prosoma is made up of the first six exoskeleton segments fused together into a larger structure. The seventh segment (thus the first opisthosomal segment) is referred to as the
metastoma and the eighth segment (distinctly plate-like) is called the
operculum and contains the genital aperature. The underside of this segment is occupied by the genital operculum, a structure originally evolved from ancestral seventh and eighth pair of appendages. In its center, as in modern horseshoe crabs, is a genital appendage. This appendage, an elongated rod with an internal duct, is found in two distinct morphs, generally referred to as "type A" and "type B". These genital appendages are often preserved prominently in fossils and have been the subject of various interpretations of eurypterid reproduction and sexual dimorphism. Type A appendages are generally longer than those of type B. In some genera they are divided into different numbers of sections, such as in
Eurypterus where the type A appendage is divided into three but the type B appendage into only two. Such division of the genital appendage is common in eurypterids, but the number is not universal; for instance, the appendages of both types in the family Pterygotidae are undivided. The type A appendage is also armed with two curved spines called (lit. 'fork' in Latin). The presence of in the type B appendage is also possible and the structure may represent the unfused tips of the appendages. Located between the
dorsal and ventral surfaces of the associated with the type A appendages is a set of organs traditionally described as either "tubular organs" or "horn organs". These organs are most often interpreted as
spermathecae (organs for storing
sperm), though this function is yet to be proven conclusively. In arthropods, spermathecae are used to store the
spermatophore received from males. This would imply that the type A appendage is the female morph and the type B appendage is the male. Further evidence for the type A appendages representing the female morph of genital appendages comes in their more complex construction (a general trend for female arthropod genitalia). It is possible that the greater length of the type A appendage means that it was used as an
ovipositor (used to deposit eggs). The different types of genital appendages are not necessarily the only feature that distinguishes between the sexes of eurypterids. Depending on the genus and species in question, other features such as size, the amount of ornamentation, and the proportional width of the body can be the result of sexual dimorphism. In general, eurypterids with type B appendages (males) appear to have been proportionally wider than eurypterids with type A appendages (females) of the same genera. The primary function of the long, assumed female, type A appendages was likely to take up spermatophore from the substrate into the
reproductive tract rather than to serve as an ovipositor, as arthropod ovipositors are generally longer than eurypterid type A appendages. By rotating the sides of the operculum, it would have been possible to lower the appendage from the body. Due to the way different plates overlay at its location, the appendage would have been impossible to move without muscular contractions moving around the operculum. It would have been kept in place when not it use. The on the type A appendages may have aided in breaking open the spermatophore to release the free sperm inside for uptake. The "horn organs," possibly spermathecae, are thought to have been connected directly to the appendage via tracts, but these supposed tracts remain unpreserved in available fossil material. Type B appendages, assumed male, would have produced, stored and perhaps shaped spermatophore in a heart-shaped structure on the dorsal surface of the appendage. A broad genital opening would have allowed large amounts of spermatophore to be released at once. The long associated with type B appendages, perhaps capable of being lowered like the type A appendage, could have been used to detect whether a substrate was suitable for spermatophore deposition. ==Evolutionary history==