We tested whether symbiotic morphology in trilobites is an adaptation for dysoxia.
Olenimorphs, exemplars of symbiosis, occur in oxic and dysoxic environments.
Symbiotic morphology represents phylogeny, not an environmental adaptation.
Symbiotic trilobites may not have recognizable morphological adaptations.
Arthropods are vulnerable to dysoxia, perhaps driving Permian trilobite extinctions.