Eurypterids, often informally called sea scorpions, are a group of extinct marine arthropods that form the order Eurypterida. The earliest known eurypterids date to the Tremadocian stage of the Ordovician period, 480 million years ago. The group is likely to have appeared first during the Late Cambrian period. With approximately 250 species, the Eurypterida is the most diverse Paleozoic chelicerate order. Following their appearance during the Ordovician, eurypterids became major components of marine faunas during the Silurian, from which the majority of eurypterid species have been described. The Silurian genus Eurypterus accounts for more than 90% of all known eurypterid specimens. Though the group continued to diversify during the subsequent Devonian period, the eurypterids were heavily affected by the Late Devonian extinction event. They declined in numbers and diversity during the Permian period, though their time of extinction remains contentious, argued to be either the early Middle Permian, or the Permian–Triassic extinction event 251.9million years ago.
Although popularly called "sea scorpions", only the earliest eurypterids were marine; many later forms lived in brackish or fresh water, and they were not true scorpions. Some studies suggest that a dual respiratory system was present, which would have allowed for short periods of time in terrestrial environments. The name Eurypterida comes from Ancient Greek εὐρύς (eurús), meaning "wide, broad", and πτερόν (pterón), meaning "wing", referring to the pair of wide swimming appendages present in many members of the group.
The eurypterid order includes the largest known arthropods ever to have lived. The largest, Jaekelopterus, reached in length. Eurypterids were not uniformly large and most species were less than long; the smallest eurypterid, Alkenopterus, was only long. Eurypterid fossils have been recovered from every continent. A majority of fossils are from fossil sites in North America and Europe because the group lived primarily in the waters around and within the ancient supercontinent of Laurasia. Only a handful of eurypterid groups spread beyond the confines of Euramerica and a few genera, such as Adelophthalmus and Pterygotus, achieved a cosmopolitan distribution with fossils being found worldwide.
Description
Like all other arthropods, eurypterids possessed segmented bodies and jointed appendages (limbs) covered in a cuticle composed of proteins and chitin. As in other chelicerates, the body was divided into two tagmata (sections); the frontal prosoma (head) and posterior opisthosoma (abdomen). The prosoma was covered by a carapace (sometimes called the "prosomal shield") on which both compound eyes and the ocelli (simple eye-like sensory organs) were located.
The prosoma also bore six pairs of appendages which are usually referred to as appendage pairs I to VI. The first pair of appendages, the only pair placed before the mouth, is called the chelicerae (homologous to the fangs of spiders). They were equipped with small pincers used to manipulate food fragments and push them into the mouth. In one lineage, the Pterygotidae, the chelicerae were large and long, with strong, well-developed teeth on specialised chelae (claws). The subsequent pairs of appendages, numbers II to VI, possessed gnathobases (or "tooth-plates") on the coxae (limb segments) used for feeding. These appendages were generally walking legs that were cylindrical in shape and were covered in spines in some species. In most lineages, the limbs tended to get larger the farther back they were. In the Eurypterina suborder, the larger of the two eurypterid suborders, the sixth pair of appendages was also modified into a swimming paddle to aid in traversing aquatic environments.
The opisthosoma comprised 12 segments and the telson, the posteriormost division of the body, which in most species took the form of a blade-like shape. In some lineages, notably the Pterygotioidea, the Hibbertopteridae and the Mycteroptidae, the telson was flattened and may have been used as a rudder while swimming. Some genera within the superfamily Carcinosomatoidea, notably Eusarcana, had a telson similar to that of true scorpions and may even have been capable of using it to inject venom like them. The coxae of the sixth pair of appendages were overlaid by a plate that is referred to as the metastoma, originally derived from a complete exoskeleton segment. The opisthosoma itself can be divided either into a "mesosoma" (comprising segments 1 to 6) and "metasoma" (comprising segments 7 to 12) or into a "preabdomen" (generally comprising segments 1 to 7) and "postabdomen" (generally comprising segments 8 to 12).
The underside of the opisthosoma was covered in structures evolved from modified opisthosomal appendages. Throughout the opisthosoma, these structures formed plate-like structures termed ( in German). These created a branchial chamber (gill tract) between preceding and the ventral surface of the opisthosoma itself, which contained the respiratory organs. The second to sixth opisthosomal segments also contained oval or triangular organs that have been interpreted as organs that aid in respiration. These organs, termed or "gill tracts", would potentially have aided eurypterids to breathe air above water, while , similar to organs in modern horseshoe crabs, would cover the parts that serve for underwater respiration.
The appendages of opisthosomal segments 1 and 2 (the seventh and eighth segments overall) were fused into a structure termed the genital operculum, occupying most of the underside of the opisthosomal segment 2. Near the anterior margin of this structure, the genital appendage (also called the or the median abdominal appendage) protruded. This appendage, often preserved very prominently, has consistently been interpreted as part of the reproductive system and occurs in two recognized types, assumed to correspond to male and female.
Biology
Size
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.