Mollisonia is probably not an arachnid

In a recent publication in Current Biology, Strausfeld et al.¹ describe the neuroanatomy of the Cambrian arthropod Mollisonia symmetrica, in which they infer a recurved brain (cerebrum), a condition found in some modern arachnids. However, beyond the proposed neuroanatomical key trait, little of the fossil's body plan organization bears any resemblance to that of any arachnid. Strausfeld et al.¹ complement their description with a phylogenetic analysis that recovers Mollisonia as sister to Arachnida. This placement stands in contrast to a previous morphological analysis that recovered Mollisonia as a stem-group chelicerate², a position that has also been questioned^3,4. Moreover, the new analysis by Strausfeld et al.¹ failed to recover the monophyly of both Arachnopulmonata (including spiders and scorpions) and Tetrapulmonata (spiders and allied orders). Notably, the latter is one of the most robust chelicerate clades, supported by over four decades of morphological and molecular analyses^5,6.