Mollisonia is probably not an arachnid.

Area of Science:

• Paleontology
• Arthropod Evolution
• Neuroanatomy

Background:

• The Cambrian fossil Mollisonia symmetrica's phylogenetic position is debated.
• Previous studies have suggested conflicting relationships with chelicerates and arachnids.

Purpose of the Study:

• To describe the neuroanatomy of Mollisonia symmetrica.
• To determine the phylogenetic placement of Mollisonia symmetrica within arthropods.

Main Methods:

• Neuroanatomical analysis of Mollisonia symmetrica.
• Phylogenetic analysis incorporating morphological and molecular data.

Main Results:

• Mollisonia symmetrica exhibits a recurved brain, a trait shared with some arachnids.
• Phylogenetic analysis places Mollisonia symmetrica as sister to Arachnida, contradicting previous classifications.
• The study failed to recover the monophyly of key arachnid clades (Arachnopulmonata and Tetrapulmonata).

Conclusions:

• While Mollisonia symmetrica shares a neuroanatomical feature with arachnids, its overall morphology and phylogenetic position suggest it is not a direct arachnid ancestor.
• The findings challenge established arachnid phylogeny and highlight the complexity of early arthropod evolution.