Environmental structure of the graptolite morphospace

Graptolites were key elements of pelagic ecosystems during the Ordovician. Although their environmental distribution patterns are well understood, the relationship between their morphology and distribution has been scarcely addressed. This study analysed the relationship of the morphology of Early to Middle Ordovician graptoloids and their habitat distribution in a discrete-character morphospace. Optimal partitioning identified three morphogroups, indicating ecological differentiation among groups. Both habitat distribution and taxonomy significantly influenced the morphospace structure. A morpho-compositional gradient across habitats suggests the presence of environmental filters differentially affecting the morphogroups. The didymograptid morphogroup is tightly clustered near the centroid of the morphospace; therefore, variations in its relative abundance strongly influence the sum of variances of the assemblages. Didymograptid morphogroup’s disparity between assemblages does not differ significantly, but its abundance varied greatly across habitats, evidencing the presence of restricted conditions. The sigmagraptid and phyllograptid morphogroups include a greater diversity of morphologies, evidencing high interspecific competition. They recorded almost identical disparity in Epipelagic and Deep-water assemblages and encompassed similar morphological strategies between habitats, pointing to similar environmental pressures. We recognised decoupled gradients of taxonomic diversity and morphological disparity. These patterns are not due to the highly analytically time-averaged nature of our analysis, but palaeobiological.