SYMBIOTIC TRILOBITE MORPHOLOGY IN THE CAMBRIAN: PHYLOGENY VERSUS ENVIRONMENT

The early to middle Cambrian represents a period of mostly warm, stratified oceans conducive to the formation of dysoxic (low-oxygen, <.01 ml/l O₂) conditions. These conditions may have driven evolutionary changes in benthic organisms such as trilobites. Previous work established a set of qualitative morphological characters associated with olenimorph trilobites (i.e., Hypermecaspis) that may infer symbiosis with bacteria as an adaptation for life in dysoxic habitats. The ubiquity of this symbiotic morphology in the Cambrian is not well known, with the exception of Elrathia kingii from the Wheeler Shale. Our study determined if symbiotic morphology was common, and whether this morphology occurred in oxic and dysoxic habitats within eight Cambrian trilobite-rich Lagerstätten. We also examined how symbiotic morphology varied between genera from three major trilobite orders.

Symbiotic morphology was converted into a ranked character matrix from which non-metric multidimensional scaling (NMDS) scores were generated for 67 Cambrian trilobite genera. The NMDS scores described overall fitness to dysoxic conditions with higher scores indicating high fitness for symbiosis, while lower scores indicated poor fitness. Trilobites scored most commonly in the moderately well-fit range, driven to more positive values by ptychopariids. Five ptychopariids scored > 0.700, similar to the iconic Ordovician Hypermecaspis (0.777), while Elrathia scored lower (0.535); highest values for corynexochids and redlichiids were 0.340 and 0.193, respectively. There was no significant morphological distinction between oxic and dysoxic habitats. Ptychopariids scored high, regardless of environment, while corynexochids and redlichiids, from predominately dysoxic and oxic environments, respectively, both scored low in fitness for symbiotic morphology. Therefore, phylogenetic bias may be the primary factor that explains symbiotic morphology: olenimorph morphology appears to describe general ptychopariid morphology, rather than a symbiotic adaptation for dysoxia. Yet, some ptychopariids scored similarly to Hypermecaspis, suggesting that symbiosis, if it exists, may be more pervasive among ptychopariids, but is generally rare within Cambrian assemblages.