SYMBIOTIC TRILOBITE MORPHOLOGY IN THE CAMBRIAN: PHYLOGENY VERSUS ENVIRONMENT
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.