See text Euthycarcinoidea are an enigmatic group of extinct, possibly amphibious arthropods that ranged from Cambrian to Triassic times.

[4] Euchelicerata Euthycarcinoidea Myriapoda Crustacea Hexapoda However, a 2020 study identified several characters, including compound eyes and various details of the preoral chamber, that suggested instead a position as the closest relatives of living myriapods.

[5] This would help to close the gap between the earliest body fossils of crown-group myriapods in the Silurian and molecular clock data suggesting a divergence from their closest relatives during the Ediacaran or Cambrian.

[9][10] It has been suggested that the biofilms and microbial mats that covered much of the vast tidal flats during the Cambrian Period in North America may have provided the nourishment that lured these arthropods onto the land.

[11] Fossil evidence also suggests the possibility that some euthycarcinoids came onto the land to lay and fertilize their eggs via amplexus, as do the modern horseshoe crabs.