It was first proposed by Clarke and Ruedemann in 1912, that eurypterids were originally marine creatures that invaded shallow, brackish lagoons at the estuaries of rivers, and then became adapted for the life in fresh water and coal forming swamps.  It is thought that they even made excursions onto land.  These palaeontological adaptations can be deduced from eurypterid remains found in association with other fauna, living at that time.  Eurypterid remains have been found in palaeozoic sediments from around the world.  Some excellent preservation means that the morphology of some species is particularly well known, unfortunately others are known by poorly preserved material, which has obvious implications when trying to classify the remains.
 
 
 
 

There are several suggestions for the ecology of eurypterids:

1.  River model.  Eurypterids occupied a fluvial habitat.

2.  Transition model. The eurypterid habitat changed through time.  In the Ordovician they inhabited marine environments, in the Silurian broader salinity regimes were occupied and by the Upper Palaeozoic they were based in brackish to freshwater settings.

3.  Euryhaline model. Eurypterids moved from different settings - brackish, marine and freshwater.  THey could tolerate fluctuating salinity levels.

4.  Facies model.  Eurypterids were divided into three biofacies;  the C/P phase (marine), the E phase (transitional marine), and the H/D/S phase (brackish-freshwater).
 
 

Mass death assemblages, where many eurypterid remains have been found, has been reinterpreted by the 'mass-mate-moult' hypothesis of Simon Braddy.  This suggests that eurypterids move to a near shore setting in order to mate and subsequently moult.  It is easy to see why this has been interpreted as death assemblage, and it can account for the presence of eurypterids in sediments such as the Bertie Waterlime.  Comparisons to extant taxa, such as limulus appear to confirm the hypothesis.