CUTICULAR MICROSTRUCTURE OF EXTINCT AND EXTANT MARINE ARTHROPODS

Fossilized marine arthropod cuticle microstructure, which includes cross-sectional and surficial features, has rarely been studied to assess the potential of macro- and microscopic morphology for the distinction of evolutionary relationships between taxonomic groups. If cuticle is well-preserved, these studies are important when distinguishing broken or disarticulated fossil arthropod remains, especially if macrostructures are missing. Previous work described decapod crustacean cuticle microstructures and their viability in phylogenetic analyses. The purpose of this study was to establish whether cuticle microstructure can be used not only for decapods but also for other extinct and extant marine arthropod groups. The current study was the first to create codeable characters across multiple marine arthropod higher taxa, including among crustaceans, decapods, isopods, mantis shrimp, ostracods, and branchiopods; horseshoe crabs and eurypterids; and trilobites, to incorporate cuticular features into a broader systematic or evolutionary study. For 22 marine taxa and 1 terrestrial taxon, 33 cross-sectional and surficial cuticle characters and character states were distinguished and normalized using scanning electron microphotographs, thin section photographs, and descriptions and images from the literature. A character data matrix was prepared and analyzed using a principal coordinate analysis (PCO) to determine whether higher systematic groups were similar based on polygonal morphospaces in the resulting scatter plot. In general, the PCO demonstrated that arthropod taxa that exhibited similar surficial and cross-sectional cuticle morphological features clustered together. This suggests that arthropod cuticle microstructure can help establish similarities between arthropod taxa and can provide useful characters to incorporate into systematic or evolutionary analyses of marine arthropods. Well-preserved, cross-sectional cuticle characters are especially useful for taxon identification and could be incorporated into phylogenetic or cladistic analyses in addition to molecular data or other morphological features to enhance the results of those studies.