List of Eurypterids - Body Structure

Body Structure

Eurypterids have been formally described as follows:

"Small to very large merostomes with elongate lanceolate, rarely trilobed body; prosoma of moderate size; opisthosoma with 12 moveable segments and styliform to spatulate telson, with division commonly into 7-segmented preabdomen and 5-segmented postabdomen; prosomal appendages 6, comprising 3-jointed chelicerae, walking legs, the last pair commonly transformed into swimming legs. Mouth central, bordered posteriorly by endostoma and metastoma. Operculum with median genital appendage, abdominal appendages plate-shaped with nonlaminate gills. Ordovician-Permian."

The typical eurypterid had a large, flat, semicircular carapace, followed by a jointed section, and finally a tapering, flexible tail, most ending with a long spine at the end (Pterygotus, though, had a large flat tail, possibly with a smaller spine). Behind the head of the eurypterids were twelve body segments. These segments are formed by a dorsal plate called tergite, and a ventral plate called sternite. The tail, known as the telson, is spiked in most eurypterids like in modern scorpions and in some species it may have been used to inject venom, but so far there is no certain evidence any eurypterids were venomous. Most eurypterids have paddles toward the end of the carapace and beyond, which were used to propel themselves through water. The suborder Stylonurina have walking legs instead of paddles. Some argue that the paddles were also used for digging. It is possible that it was used for both. Underneath, in addition to the pair of swimming appendages the creature had 4 pairs of jointed legs for walking, and two claws at the front, chelicerae, which were enlarged in pterygotids. The walking legs had odd hairs, similar to modern day crabs. Other features, common to ancient and modern arthropods of this type, include one pair of compound eyes and a pair of smaller eyes called ocelli, between the other larger pair of eyes.

Many eurypterids had legs large and long enough to do more than allow them to crawl over the sea bottom; a number of species (particularly hibbertopterids) had large stout legs, and were probably capable of terrestrial locomotion (like land crabs today). Studies of what are believed to be their trackways indicate that eurypterids used in-phase, hexapodous (six-legged) and octopodous (eight-legged) gaits. Some species may have been amphibious, emerging onto land for at least part of their life cycle; they may have been capable of breathing both in water and in air. A predatory arthropod whose traces are known as Protichnites, found in Cambrian strata dating from 510 million years ago, is a possible stem group eurypterid, and is among the first evidence of animals on land.

Among the largest eurypterids are the hibbertopteroids, named after the British palaeontologist Dr. Samuel Hibbert, who described Hibbertopterus scouleri at the East Kirkton limestone quarry in Scotland in 1836. Fossil tracks (a form of trace fossil) were identified recently in East Lothian, Scotland, as made by a 1.6 metre long Hibbertopterus. The largest well-described genus of sea-scorpion was Pterygotus, an arthropod the size of a crocodile. Fossils of Pterygotus are relatively common although complete fossils are rare. At 2.1 metres (6 ft 11 in) long, they were until recently the largest known arthropod ever to have lived. Their fossils have been found worldwide, except in Antarctica. In 2007, a 46 cm (18 in) claw belonging to Jaekelopterus rhenaniae (a species originally described in 1914) was discovered by a group of paleontologists led by Simon Braddy at the University of Bristol, indicating that J. rhenaniae was 2.5 m (8 ft 2 in) in length, making it the largest arthropod ever found.