Prosomapoda is a clade of
euchelicerates including the groups
Xiphosura
Xiphosura (; , in reference to its sword-like telson) is an order of arthropods related to arachnids. They are more commonly known as horseshoe crabs (a name applied more specifically to the only extant family, Horseshoe crab, Limulidae). They fir ...
(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, ofte ...
(a group comprising
bunodids,
pseudoniscids,
chasmataspidids,
eurypterids
Eurypterids, often informally called sea scorpions, are a group of extinct marine 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 ...
and
arachnids
Arachnids are arthropods in the class Arachnida () of the subphylum Chelicerata. Arachnida includes, among others, spiders, scorpions, ticks, mites, pseudoscorpions, harvestmen, camel spiders, whip spiders and vinegaroons.
Adult arachnids ...
), 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, ...
'' and ''
Dibasterium
''Dibasterium'' is an extinct genus of euchelicerate, a group of chelicerate arthropods. Fossils of the single and type species, ''D. durgae'', have been discovered in the Coalbrookdale Formation of the Middle Silurian period (Homerian age) in H ...
''.
References
Middle Ordovician first appearances
{{Chelicerata-stub