Merostomoidea is a

paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...

group of fossil arthropods. They have been described as a transitional form between

trilobites Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the ...

and true

chelicerates The subphylum Chelicerata (from New Latin, , ) constitutes one of the major subdivisions of the phylum Arthropoda. It contains the sea spiders, horseshoe crabs, and arachnids (including harvestmen, scorpions, spiders, solifuges, ticks, and mite ...

Sources

* https://web.archive.org/web/20101221002449/http://palaeos.com/Invertebrates/Arthropods/Arachnomorpha.html#Merostomoidea Arthropod classes Prehistoric protostome classes Cambrian arthropods {{paleo-arthropod-stub