Prosomapoda is a clade of
euchelicerates including the groups
Xiphosura (horseshoe crabs) and
Planaterga
Planaterga is a clade of prosomapod euchelicerates including several synziphosurid genera (mainly bunodids and pseudoniscids) and the group Dekatriata (which in turn includes arachnids, chasmataspidids and eurypterid
Eurypterids, often ...
(a group comprising
bunodids,
pseudoniscids,
chasmataspidids,
eurypterids
Eurypterids, often informally called sea scorpions, are a group of extinct arthropods that form the order Eurypterida. The earliest known eurypterids date to the Darriwilian stage of the Ordovician period 467.3 million years ago. The group is l ...
and
arachnids
Arachnida () is a Class (biology), class of joint-legged invertebrate animals (arthropods), in the subphylum Chelicerata. Arachnida includes, among others, spiders, scorpions, ticks, mites, pseudoscorpions, opiliones, harvestmen, Solifugae, came ...
), as well as several basal
synziphosurid genera.
The clade is defined by the lack of exopods (outer branches) of prosomal appendage II-V in the adult instar,
where in contrast the exopods of appendage II-V are well-developed in the non-prosomapod euchelicerates ''
Offacolus
''Offacolus'' is an extinct genus of euchelicerate, a group of chelicerate arthropods. Its only species, ''O. kingi'', has been found in deposits from the Silurian period (Homerian epoch) in the Wenlock Series Lagerstätte of Herefordshire, Eng ...
'' and ''
Dibasterium''.
References
Middle Ordovician first appearances
{{Chelicerata-stub