Mollisoniida

Mollisoniida Temporal range: Cambrian Stage 3–Floian PreꞒ Ꞓ O S D C P T J K Pg N
Restoration of Mollisonia plenovenatrix
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Arthropoda
Clade:	Arachnomorpha
Subphylum:	Chelicerata
Order:	†Mollisoniida Lerosey-Aubrey et al, 2020[1]
Type species
†Mollisonia symmetrica Walcott, 1912
Genera
†Mollisonia †Thelxiope †Corcorania †Esmeraldacaris?

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

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,^[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). The clade is relatively diverse, containing three (possibly four) genera. Mollisonia and Thelxiope are both known from four species extending from Cambrian to Ordovician,^[2] 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^[3] 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.^[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. 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.^[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.