The Silurian Period (444 - 419 mya) was the time of the great sea scorpion arthropods. Other marine arthropods such as trilobites had existed right across the globe in huge numbers for millions of years - with some possibly even moving up onto dry land - but now the Eurypterids became the dominant lifeform.
It was once thought that the Devonian Period was the time of the great animal transition onto land and it still appears that this was the period of major colonisation of land by plant and animal life. However more evidence keeps on surfacing to indicate that arthropods may have already begun the transition as early as the Silurian period. Track marks (ichnofossils) have been discovered that point to some kind of movement across land at this time. Some anatomical features also suggest that arthropods such as Eurypterus may have developed dual methods of capturing oxygen from the environment. This could indicate an ability to move between water and land.