quality:correction, structural:expansion
Ontogeny

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.