Mollisoniida is an extinct order of chelicerates, living from the mid-Cambrian to Early Ordovician.

These include an elongated dorsal exoskeleton, seven articulating tergites and similarly sized cephalic and pygidial shields,[1] 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).

[4] Esmeraldacaris is also purely Ordovician,[5] 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.

[6] This clade may be related to Pycnogonida,[citation needed] 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.