Mollisonia
Mollisonia is an extinct genus of Cambrian marine arthropod. Four species have been described from North America and China. Studies suggest it is a basal member of Chelicerata, a group which includes horseshoe crabs and arachnids.
Description
The genus is characterized by a capsule-like dorsal exoskeleton, which was divided into a subequal size of cephalon and pygidium with 7 thoracic segments in between. Below the cephalon was a pair of huge compound eyes, followed by a series of walking legs and gnathobasic limbs, each have 3 pairs in total. As a Cambrian arthropod, the genus is significant by bearing several traits of now-surviving chelicerates, such as pincer-like mouthparts and fused ring-like cephalic nerves within their head, as well as a series of multilayered book gills underneath their trunk appendages.Mollisonia may have been a benthic predator, using its anterior chelicerae and posterior gnathobasic limbs to devour prey items while using the 6 legs to walk around the sea floor. The gill-bearing trunk appendages may have been solely for breathing. This functional differentiation is closer to euchelicerates than the basal chelicerate genera of Habeliida.
Distribution and taxonomy
Three species are known from the Burgess Shale, with 21 specimens are known from the Greater Phyllopod bed, where they comprise less than 0.1% of the community as of 2006. The genus is also known from Langston Formation, and Wheeler Shale of North America, as well as Chengjiang Biota of China. Remains possibly attributable to the genus are also known from the Ordovician Fezouata Formation of Morocco and Bøggild Fjord Formation in Greenland.The taxonomic affinity of this genus was enigmatic until the discovery of its chelicerate features in 2019. Subsequent studies suggest it is a basal chelicerate, closer to crown-group Euchelicerata than members of Habeliida. It is also suggested to be closely related to Corcorania, Urokodia, and Thelxiope, which together form the order Mollisoniida. A 2025 study suggested that Mollisonia was deeply nested within crown chelicerates as a close relative of living arachnids, based on interpretations of its fossilised neural anatomy.
Cladogram after O’Flynn et al, 2023, placing Mollisonia as a stem-chelicerate:Cladogram after Strausfeld et al., showing Mollisonia as a stem-arachnid: