Mollisoniida is an extinct order of

chelicerates The subphylum Chelicerata (from Neo-Latin, , ) constitutes one of the major subdivisions of the phylum Arthropoda. Chelicerates include the sea spiders, horseshoe crabs, and arachnids (including harvestmen, scorpions, spiders, solifuges, tic ...

, living from the mid-Cambrian to

Early Ordovician The Early Ordovician is the first epoch (geology), epoch of the Ordovician period, corresponding to the Lower Ordovician series (stratigraphy), series of the Ordovician system. It began after the Cambrian Stage 10, Age 10 of the Furongian epoch o ...

The clade is united by the presence of various characteristics. These include an elongated dorsal exoskeleton, seven articulating tergites and similarly sized cephalic and pygidial shields, in addition to only having three pairs of walking legs and the rest of the limbs being used as gills (which likely places them nearest to Euchelicerata, due to sharing similar arrangements of limbs). The clade is relatively diverse, containing three (possibly four) genera. '' Mollisonia'' and '' Thelxiope'' are both known from four species extending from Cambrian to Ordovician, with ''Thelxiope'' being very spiny and often with a shortened body. ''Mollisonia'', on the other hand, barely has any spines at all, and (with the exception of ''Mollisonia plenovenatrix'' and ''Mollisonia sinica'', which are similarly shaped to ''Thelxiope'') has a relatively elongated body. '' Corcorania'' is purely Ordovician, with an elongate body, a small pygidial shield, and three large spines on its cephalic shield. '' Esmeraldacaris'' is also purely Ordovician, although its position in the clade is uncertain. However, it seems to be similar in shape to the more compact ''Mollisonia'' species, alongside having an equally sized cephalic and pygidial shield and seven tergites, therefore a mollisoniid affinity is most likely for it. '' Urokodia'' was formerly included in this clade as a basal member (due to having 14 tergites instead of the standard seven), until a 2024 study reclassified it as the basalmost member of

Artiopoda Artiopoda is a clade of extinct arthropods that includes trilobites and their close relatives. It was erected by Hou and Bergström in 1997 to encompass a wide diversity of arthropods that would traditionally have been assigned to the Trilobitomor ...

. This clade may be related to

Pycnogonida Sea spiders are marine arthropods of the class Pycnogonida, hence they are also called pycnogonids (; named after ''Pycnogonum'', the type genus; with the suffix '). The class includes the only now-living order Pantopoda ( ‘all feet’), alongs ...

, as they share several features such as a generic seventh pair of prosomal limbs, an absence of the labrum and hypostome, and at least two distinct post-cheliceral cephalic appendage pairs.

References

{{Taxonbar, from=Q19839527 Chelicerate orders Prehistoric chelicerates Cambrian Series 2 first appearances Early Ordovician extinctions Fossil taxa described in 2020